Sum grequency generation spectroscopy from first principles
NASA Astrophysics Data System (ADS)
Wan, Quan; Galli, Giulia
2015-03-01
Sum frequency generation (SFG) spectroscopy is widely used to study the structural and dynamical properties of surfaces and interfaces. Within the dipole approximation, SFG signals are solely determined by the surface, and bulk contributions vanish. However, the bulk portion of a material may contribute to SFG spectra through higher multipole excitations, e.g. quadrupole, which usually are difficult to separate in the measured spectra. Here we present a first principles theoretical framework, to compute SFG spectra of molecular solids and fluids. Within the dipole approximation, we computed the dipole and polarizability using maximally localized Wannier functions (MLWF) and density functional perturbation theory. We then extended our method to include quadrupole contributions, and we computed quadrupole moments and their derivatives using MLWF and a real-space correction scheme, and an electric enthalpy functional. We applied our approach to investigate the ice Ih surface and we present results obtained by both finite differences and ab initio molecular dynamics simulations. This work was supported by Grant DOE-BES DE-SC0008938.
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.
A first-principles theoretical approach to heterogeneous nanocatalysis
NASA Astrophysics Data System (ADS)
Negreiros, Fabio R.; Aprà, Edoardo; Barcaro, Giovanni; Sementa, Luca; Vajda, Stefan; Fortunelli, Alessandro
2012-02-01
A theoretical approach to heterogeneous catalysis by sub-nanometre supported metal clusters and alloys is presented and discussed. Its goal is to perform a computational sampling of the reaction paths in nanocatalysis via a global search in the phase space of structures and stoichiometry combined with filtering which takes into account the given experimental conditions (catalytically relevant temperature and reactant pressure), and corresponds to an incremental exploration of the disconnectivity diagram of the system. The approach is implemented and applied to the study of propylene partial oxidation by Ag3 supported on MgO(100). First-principles density-functional theory calculations coupled with a Reactive Global Optimization algorithm are performed, finding that: (1) the presence of an oxide support drastically changes the potential energy landscape of the system with respect to the gas phase, favoring configurations which interact positively with the electrostatic field generated by the surface; (2) the reaction energy barriers for the various mechanisms are crucial in the competition between thermodynamically and kinetically favored reaction products; (3) a topological database of structures and saddle points is produced which has general validity and can serve for future studies or for deriving general trends; (4) the MgO(100) surface captures some major features of the effect of an oxide support and appears to be a good model of a simple oxide substrate; (5) strong cooperative effects are found in the co-adsorption of O2 and other ligands on small metal clusters. The proposed approach appears as a viable route to advance the role of predictive computational science in the field of heterogeneous nanocatalysis.
A first-principles theoretical approach to heterogeneous nanocatalysis.
Negreiros, Fabio R; Aprà, Edoardo; Barcaro, Giovanni; Sementa, Luca; Vajda, Stefan; Fortunelli, Alessandro
2012-02-21
A theoretical approach to heterogeneous catalysis by sub-nanometre supported metal clusters and alloys is presented and discussed. Its goal is to perform a computational sampling of the reaction paths in nanocatalysis via a global search in the phase space of structures and stoichiometry combined with filtering which takes into account the given experimental conditions (catalytically relevant temperature and reactant pressure), and corresponds to an incremental exploration of the disconnectivity diagram of the system. The approach is implemented and applied to the study of propylene partial oxidation by Ag(3) supported on MgO(100). First-principles density-functional theory calculations coupled with a Reactive Global Optimization algorithm are performed, finding that: (1) the presence of an oxide support drastically changes the potential energy landscape of the system with respect to the gas phase, favoring configurations which interact positively with the electrostatic field generated by the surface; (2) the reaction energy barriers for the various mechanisms are crucial in the competition between thermodynamically and kinetically favored reaction products; (3) a topological database of structures and saddle points is produced which has general validity and can serve for future studies or for deriving general trends; (4) the MgO(100) surface captures some major features of the effect of an oxide support and appears to be a good model of a simple oxide substrate; (5) strong cooperative effects are found in the co-adsorption of O(2) and other ligands on small metal clusters. The proposed approach appears as a viable route to advance the role of predictive computational science in the field of heterogeneous nanocatalysis. PMID:22057595
Infrared Spectroscopy of Functionalized Graphene Sheets from First Principle Calculations
NASA Astrophysics Data System (ADS)
Zhang, Cui; Dabbs, Daniel; Aksay, Ilhan; Car, Roberto; Selloni, Annabella
2014-03-01
Detailed characterization of the structure of functionalized graphene sheets (FGSs) is an important and challenging task which could help to improve the performance of FGS materials for technological applications. We present here first principles calculations for the infrared (IR) spectra of different FGS models aimed at identifying the IR signatures of different functional groups and defect sites on FGSs. We found that vacancies and edges have significant effects on the IR frequencies of the functional groups on FGSs. In particular, hydroxyl groups close to vacancies have higher stretching and lower bending frequencies in comparison to hydroxyls in defect free regions of FGSs. More interestingly, the OH vibrations of carboxyl groups at edges exhibit unique features in the high frequency IR bands, which originate from the interactions with neighboring groups and the relative orientation of the carboxyl with respect to the FGS plane. Our results are supported by experimental IR measurements on FGS powders.
Nguyen, Ngoc Linh; Borghi, Giovanni; Ferretti, Andrea; Dabo, Ismaila; Marzari, Nicola
2015-04-24
The determination of spectral properties from first principles can provide powerful connections between microscopic theoretical predictions and experimental data, but requires complex electronic-structure formulations that fall outside the domain of applicability of common approaches, such as density-functional theory. We show here that Koopmans-compliant functionals, constructed to enforce piecewise linearity and the correct discontinuity derivative in energy functionals with respect to fractional occupation-i.e., with respect to charged excitations-provide molecular photoemission spectra and momentum maps of Dyson orbitals that are in excellent agreement with experimental ultraviolet photoemission spectroscopy and orbital tomography data. These results highlight the role of Koopmans-compliant functionals as accurate and inexpensive quasiparticle approximations to the spectral potential. PMID:25955063
Roy, Tapta Kanchan; Kopysov, Vladimir; Nagornova, Natalia S; Rizzo, Thomas R; Boyarkin, Oleg V; Gerber, R Benny
2015-05-18
Calculated structures of the two most stable conformers of a protonated decapeptide gramicidin S in the gas phase have been validated by comparing the vibrational spectra, calculated from first- principles and measured in a wide spectral range using infrared (IR)-UV double resonance cold ion spectroscopy. All the 522 vibrational modes of each conformer were calculated quantum mechanically and compared with the experiment without any recourse to an empirical scaling. The study demonstrates that first-principles calculations, when accounting for vibrational anharmonicity, can reproduce high-resolution experimental spectra well enough for validating structures of molecules as large as of 200 atoms. The validated accurate structures of the peptide may serve as templates for in silico drug design and absolute calibration of ion mobility measurements. PMID:25721337
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-bearing radioactive waste. In a second example, we discuss how (17)O NMR can be used to probe the dynamic disorder of H in hydroxyl-humite minerals (nMg2SiO4·Mg(OH)2), and how (19)F NMR can be used to understand F substitution in these systems. The combination of first-principles calculations and multinuclear NMR spectroscopy facilitates the investigation of local structure, disorder, and dynamics in solids. We expect that applications will undoubtedly become more widespread with further advances in computational and experimental methods. Insight into the atomic-scale environment is a crucial first step in understanding the structure-property relationships in solids, and it enables the efficient design of future materials for a range of end uses. PMID:23402741
Roy, Tapta Kanchan; Sharma, Rahul; Gerber, R Benny
2016-01-21
First-principles quantum calculations for anharmonic vibrational spectroscopy of three protected dipeptides are carried out and compared with experimental data. Using hybrid HF/MP2 potentials, the Vibrational Self-Consistent Field with Second-Order Perturbation Correction (VSCF-PT2) algorithm is used to compute the spectra without any ad hoc scaling or fitting. All of the vibrational modes (135 for the largest system) are treated quantum mechanically and anharmonically using full pair-wise coupling potentials to represent the interaction between different modes. In the hybrid potential scheme the MP2 method is used for the harmonic part of the potential and a modified HF method is used for the anharmonic part. The overall agreement between computed spectra and experiment is very good and reveals different signatures for different conformers. This study shows that first-principles spectroscopic calculations of good accuracy are possible for dipeptides hence it opens possibilities for determination of dipeptide conformer structures by comparison of spectroscopic calculations with experiment. PMID:26673682
Hydrogen donors in SnO2 studied by infrared spectroscopy and first-principles calculations
NASA Astrophysics Data System (ADS)
Hlaing Oo, W. M.; Tabatabaei, S.; McCluskey, M. D.; Varley, J. B.; Janotti, A.; van de Walle, C. G.
2010-11-01
Hydrogen is a potentially important source of n -type conductivity in oxide materials. We have investigated hydrogen in tin oxide (SnO2) , a wide-band-gap semiconductor with applications as a transparent conductor and in gas sensors. Infrared (IR) spectroscopy and electrical measurements indicate that hydrogen binds to a host oxygen atom and increases the conductivity. First-principles calculations confirm that interstitial hydrogen acts as a shallow donor (Hi+) . Our calculations also indicate that Hi+ diffuses easily and combines with Sn vacancies into stable (VSn-H)-3 complexes, with the calculated O-H frequencies in agreement with the experimental values. These results suggest that interstitial hydrogen acts as a shallow, mobile donor in a range of oxide materials.
Theoretical characterization of layered silica nanostructures from first-principles prediction
NASA Astrophysics Data System (ADS)
Zhou, Hongcai; Xi, Zexiao; Zhao, Mingwen
2014-10-01
Using first-principles calculations, we study the structural, mechanical and electronic properties of the layered silica nanostructures built on base of silica bilayers consisting of four- and six-membered Si-O ring (4 MR and 6 MR) units. These silica nanostructures have high stability and good flexibility comparable to graphene and can serve as a promising precursor for the fabrication of well-ordered silica nanotubes. The porous structure and wide band gap of the silica nanomaterials may find applications in gas separation, slow-release microcapsules, and catalyst supports.
Inelastic Electron Tunneling Spectroscopy in Molecular Electronic Devices from First-Principles
NASA Astrophysics Data System (ADS)
Ji, Tao
In this thesis, we present the first-principle calculations of inelastic electron tunneling spectroscopy(IETS) in single molecular break junctions. In a two-probe electrode-molecule-electrode setup, density functional theory(DFT) is used for the construction of the Hamiltonian and the Keldysh non-equilibrium Green's function(NEGF) technique will be employed for determining the electron density in non-equilibrium system conditions. Total energy functional, atomic forces and Hessian matrix can be obtained in the DFT-NEGF formalism and self-consistent Born approximation(SCBA) is used to integrate the molecular vibrations (phonons) into the framework once the phonon spectra and eigenvectors are calculated from the dynamic matrix. Geometry optimization schemes will also be discussed as an indispensable part of the formalism as the equilibrium condition is crucial to correctly calculate the phonon properties of the system. To overcome the numerical difficulties, especially the large computational time demand of the electron-phonon coupling problem, we develop a numerical approximation for the electron self-energy due to phonons and the error is controlled within numerical precision. Besides, a direct IETS second order I-V derivative expression is derived to reduce the error of numerical differentiation under reasonable assumptions. These two approximations greatly reduce the computation requirement and make the calculation feasible within current numerical capability. As the application of the DFT-NEGF-SCBA formalism, we calculate the IETS of the gold-octanedithiol(ODT) molecular junction. The I-V curve, conductance and IETS from ab-inito calculations are compared directly to experiments. A microscopic understanding of the electron-phonon coupling mechanism in the molecular tunneling junctions is explained in this example. In addition, comparisons of the hydrogen-dissociative and hydrogen-non-dissociative ODT junctions as well as the different charge transfer behaviors are presented to show the effects of thiol formation in the ODT molecular junction.
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.
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.
Theoretical studies of aluminum and aluminide alloys using CALPHAD and first-principles approach
NASA Astrophysics Data System (ADS)
Jiang, Chao
Heat-treatable aluminum alloys have been widely used in the automobile and aerospace industries as structural materials due to their light weight and high strength. To study the age-hardening process in heat-treatable aluminum alloys, the Gibbs energies of the strengthening metastable phases, e.g. theta ' and theta?, are critical. However, those data are not included in the existing thermodynamic databases for aluminum alloys due to the semi-empirical nature of the CALPHAD approach. In the present study, the thermodynamics of the Al-Cu system, the pivotal age-hardening system, is remodeled using a combined CALPHAD and first-principles approach. The formation enthalpies and vibrational formation entropies of the stable and metastable phases in the Al-Cu system are provided by first-principles calculations. Special Quasirandom Structures (SQS's) are applied to model the substitutionally random fee and bee alloys. SQS's for binary bee alloys are developed and tested in the present study. Finally, a self-consistent thermodynamic description of the Al-Cu system including the two metastable theta? and theta' phases is obtained. During welding of heat-treatable aluminum alloys, a detrimental phenomenon called constitutional liquation, i.e. the local eutectic melting of second-phase particles in a matrix at temperatures above the eutectic temperature but below the solidus of the alloy, may occur in the heat-affected zone (HAZ). In the present study, diffusion code DICTRA coupled with realistic thermodynamic and kinetic databases is used to simulate the constitutional liquation in the model Al-Cu system. The simulated results are in quantitative agreement with experiments. The critical heating rate to avoid constitutional liquation is also determined through computer simulations. Besides the heat-treatable aluminum alloys, intermetallic compounds based on transition metal aluminides, e.g. NiAl and FeAl, are also promising candidates for the next-generation of high-temperature structural materials for aerospace applications due to their high melting temperature and good oxidation resistance. Many important properties of B2 aluminides are governed by the existences of point defects. In the present study, Special Quasirandom Structures (SQS's) are developed to model non-stoichiometric B2 compounds containing large concentrations of constitutional point defects. The SQS's are then applied to study B2 NiAl. The first-principles SQS results provide formation enthalpies, equilibrium lattice parameters and elastic constants of B2 NiAl which agree satisfactorily with the existing experimental data in the literature. It is unambiguously shown that, at T = 0K and zero pressure, Ni vacancies and antisite Ni atoms are the energetically favorable point defects in Al-rich and Ni-rich B2 NiAl, respectively. Remarkably, it is predicted that high defect concentrations can lead to structural instability of B2 NiAl, which explains well the martensitic transformation observed in this compound at high Ni concentrations.
Johnston, Karen E; O'Keefe, Christopher A; Gauvin, Régis M; Trébosc, Julien; Delevoye, Laurent; Amoureux, Jean-Paul; Popoff, Nicolas; Taoufik, Mostafa; Oudatchin, Konstantin; Schurko, Robert W
2013-09-01
A series of transition-metal organometallic complexes with commonly occurring metal-chlorine bonding motifs were characterized using (35)Cl solid-state NMR (SSNMR) spectroscopy, (35)Cl nuclear quadrupole resonance (NQR) spectroscopy, and first-principles density functional theory (DFT) calculations of NMR interaction tensors. Static (35)Cl ultra-wideline NMR spectra were acquired in a piecewise manner at standard (9.4?T) and high (21.1?T) magnetic field strengths using the WURST-QCPMG pulse sequence. The (35)Cl electric field gradient (EFG) and chemical shielding (CS) tensor parameters were readily extracted from analytical simulations of the spectra; in particular, the quadrupolar parameters are shown to be very sensitive to structural differences, and can easily differentiate between chlorine atoms in bridging and terminal bonding environments. (35)Cl?NQR spectra were acquired for many of the complexes, which aided in resolving structurally similar, yet crystallographically distinct and magnetically inequivalent chlorine sites, and with the interpretation and assignment of (35)Cl?SSNMR spectra. (35)Cl?EFG tensors obtained from first-principles DFT calculations are consistently in good agreement with experiment, highlighting the importance of using a combined approach of theoretical and experimental methods for structural characterization. Finally, a preliminary example of a (35)Cl?SSNMR spectrum of a transition-metal species (TiCl4) diluted and supported on non-porous silica is presented. The combination of (35)Cl?SSNMR and (35)Cl?NQR spectroscopy and DFT calculations is shown to be a promising and simple methodology for the characterization of all manner of chlorine-containing transition-metal complexes, in pure, impure bulk and supported forms. PMID:23907813
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 (?-Po) with respect to the trigonal spiral structure exhibited by Se and Te and to the displacive phase transformations into either tetragonal or trigonal phases. The origin of the phase stability of ?-Po is analyzed with the help of densities of states, electronic band structures, and total energies of competing higher-energy structures corresponding to selected stationary points of the total energy. The electronic structures and total energies are calculated both within the generalized gradient approximation and local-density approximation (LDA) to the exchange-correlation energy as well as with and without inclusion of the spin-orbit (SO) coupling. The total energies are displayed in contour plots as functions of selected structural parameters and atomic volume. It turns out that the LDA calculation with SO interaction incorporated provides best agreement with existing experimental data and that the simple cubic structure of ?-Po is stabilized by relativistic effects of core electrons. High elastic anisotropy of ?-Po is explained as a consequence of its simple cubic structure and is compared with elastic properties of other crystal structures. Finally, an uniaxial tensile test for loading along the [001] and [111] directions is simulated; the corresponding theoretical tensile strengths calculated within the LDA+SO approach amount to 4.2 GPa and 4.7 GPa, respectively, which are the lowest values predicted in an element so far. According to Pugh and Frantsevich criteria, ?-Po is predicted to be ductile. Also a positive value of the Cauchy pressure confirms the metallic type of interatomic bonding.
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. PMID:25607949
Beccara, Silvio a; Rivalta, Ivan; Cerullo, Giulio
2014-01-01
The ability of non-linear electronic spectroscopy to track folding/unfolding processes of proteins in solution by monitoring aromatic interactions is investigated by first-principle 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, connecting the initial folded configuration with stacked aromatic side chains and the final unfolded state with distant non-interacting aromatic residues. ?-stacking and excitonic coupling effects are included via ab-initio simulations based on multiconfigurational methods within a hybrid QM/MM scheme. We show that linear absorption spectroscopy in the ultraviolet (UV) 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 (Vis) the excited state absorptions (ESA) that involve charge transfer (CT) states. 2DUV spectroscopy offers the highest sensitivity to the unfolding process, providing the disentanglement of ESA signals belonging to different aromatic chromophores and high correlation between the conformational dynamics and the quartic splitting. PMID:25145908
Nenov, Artur; 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 ?-stacking and excitonic coupling effects are included through ab initio simulations based on multiconfigurational methods within a hybrid quantum mechanics/molecular mechanics scheme. It is shown that linear absorption spectroscopy in the ultraviolet (UV) region is unable to resolve the unstacking dynamics characterized by the three-step process: T-shaped?twisted offset stacking?unstacking. Conversely, pump-probe spectroscopy can be used to resolve aromatic interactions by probing in the visible region, the excited-state absorptions (ESAs) that involve charge-transfer states. 2D?UV spectroscopy offers the highest sensitivity to the unfolding process, by providing the disentanglement of ESA signals belonging to different aromatic chromophores and high correlation between the conformational dynamics and the quartic splitting. PMID:25145908
Nenov, Artur; 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. PMID:26574458
Probing the uniaxial strains in MoS2 using polarized Raman spectroscopy: A first-principles study
NASA Astrophysics Data System (ADS)
Doratotaj, Danna; Simpson, Jeffrey R.; Yan, Jia-An
2016-02-01
Characterization of strain in two-dimensional (2D) crystals is important for understanding their properties and performance. Using first-principles calculations, we study the effects of uniaxial strain on the Raman-active modes in monolayer MoS2. We show that the in-plane E' mode at 384 cm-1 and the out-of-plane A1' mode at 403 cm-1 can serve as fingerprints for the uniaxial strain in this 2D material. Specifically, under a uniaxial strain, the doubly degenerate E' mode splits into two nondegenerate modes: one is the Eâˆ¥' mode in which atoms vibrate in parallel to the strain direction, and the other is the EâŠ¥'mode in which atoms vibrate perpendicular to the strain direction. The frequency of the Eâˆ¥' mode blueshifts for a compressive strain, but redshifts for a tensile strain. In addition, due to the strain-induced anisotropy in the MoS2 lattice, the polarized Raman spectra of the Eâˆ¥' and EâŠ¥' modes exhibit distinct angular dependence for specific laser polarization setups, allowing for a precise determination of the direction of the uniaxial strain with respect to the crystallographic orientation. Furthermore, we find that the polarized Raman intensity of the A1' mode also shows evident dependence on the applied strain, providing additional effective clues for determining the direction of the strain even without knowledge of the crystallographic orientation. Thus, polarized Raman spectroscopy offers an efficient nondestructive way to characterize the uniaxial strains in monolayer MoS2.
NASA Astrophysics Data System (ADS)
Pedesseau, L.; Even, J.; Bondi, A.; Guo, W.; Richard, S.; Folliot, H.; Labbe, C.; Cornet, C.; Dehaese, O.; LeCorre, A.; Durand, O.; Loualiche, S.
2008-08-01
We study the properties of highly strained InAs material calculated from first-principles modelling using ABINIT packages. We first simulate the characteristics of bulk InAs crystals and compare them with both experimental and density functional theory results. Secondly, we focus our attention on the strain effects on InAs crystals with a gradual strain reaching progressively the lattice matched parameters of InP, GaAs and GaP substrates. Section 4 is dedicated to the study of a hypothetical spherical InAs/GaP quantum dot (QD). The effect of hydrostatic deformations for both InAs zinc-blende phase and InAs rocksalt phase is discussed. Section 5 is devoted to the dependence of the lattice parameter aoz versus aox (aoy), Î“ gap energies and band line-ups at the Î“ point for biaxial deformations and interfacial structures. Ab initio results are compared with the empirical calculations which reveal nonlinear behaviour of the conduction band for highly strained InAs material.
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 Â°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 Â± 0.02 Ã…. 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 Â°C and pressures to 750 bar, the dominant species are the di-chloride complex AgCl2- with Ag-Cl distances of 2.40 Â± 0.02 Ã… and Cl-Ag-Cl angle of 160 Â± 10Â°, and the tri-chloride complex AgCl32- of a triangular structure and mean Ag-Cl distances of 2.60 Â± 0.05 Ã…. With increasing temperature, the contribution of the tri-chloride species decreases from Ëœ50% of total dissolved Ag in the most concentrated solution (5.9m Cl) at 200 Â°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 Ã… from the silver atom. The species fractions derived from XAS and FPMD analyses, and total AgCl(s) solubilities, measured in situ in this work from the absorption edge height of XAS spectra, are in accord with thermodynamic predictions using the stability constants of AgCl2- and AgCl32- from Akinfiev and Zotov (2001) and Zotov et al. (1995), respectively, which are based on extensive previous AgCl(s) solubility measurements. These data are thus recommended for chemical equilibrium calculations in mineral-fluid systems above 200 Â°C. In contrast, our data disagree with SUPCRT-based datasets for Ag-Cl species, which predict large fractions of high-order chloride species, AgCl32- and AgCl43- in high-temperature saline fluids. Comparisons of the structural and stability data of Ag-Cl species derived in this study with those of their Au and Cu analogs suggest that molecular-level differences amongst the chloride complexes such as geometry, dipole moment, distances, and resulting outer-sphere interactions with the solvent may account, at least partly, for the observed partitioning of Au, Ag and Cu in vapor-brine and fluid-melt systems. In hydrothermal environments dominated by fluid-rock interactions, the contrasting affinity of these metals for sulfur ligands and the differences both in chemistry and stability of their main solid phases (Ag sulfides, Cu-Fe sulfides, and native Au) largely control the concentration and distribution of these metals in their economic deposits.
A First-Principles Theoretical Study on the Thermoelectric Properties of the Compound Cu5AlSn2S8
NASA Astrophysics Data System (ADS)
Li, Weijian; Zhou, Chenyi; Li, Liangliang
2016-03-01
A new compound of Cu5AlSn2S8, which contained earth-abundant and environment-friendly elements and had a diamond-like crystal structure, was designed, and its electronic structure and thermoelectric transport properties from 300 K to 700 K were investigated by first-principles calculations, Boltzmann transport equations, and a modified Slack's model. The largest power factors of Cu5AlSn2S8 at 700 K were 47.5 Ã— 1010 W m-1 K-2 s-1 and 14.7 Ã— 1010 W m-1 K-2 s-1 for p- and n-type semiconductors, respectively. The lattice thermal conductivity of Cu5AlSn2S8 was calculated with its shear modulus and isothermal bulk modulus, which were also obtained by first-principles calculations. The lattice thermal conductivity was 0.9-2.2 W m-1 K-1 from 300 K to 700 K, relatively low among thermoelectric compounds. This theoretical study showed that Cu5AlSn2S8 could be a potential thermoelectric material.
A First-Principles Theoretical Study on the Thermoelectric Properties of the Compound Cu5AlSn2S8
NASA Astrophysics Data System (ADS)
Li, Weijian; Zhou, Chenyi; Li, Liangliang
2015-10-01
A new compound of Cu5AlSn2S8, which contained earth-abundant and environment-friendly elements and had a diamond-like crystal structure, was designed, and its electronic structure and thermoelectric transport properties from 300 K to 700 K were investigated by first-principles calculations, Boltzmann transport equations, and a modified Slack's model. The largest power factors of Cu5AlSn2S8 at 700 K were 47.5 × 1010 W m-1 K-2 s-1 and 14.7 × 1010 W m-1 K-2 s-1 for p- and n-type semiconductors, respectively. The lattice thermal conductivity of Cu5AlSn2S8 was calculated with its shear modulus and isothermal bulk modulus, which were also obtained by first-principles calculations. The lattice thermal conductivity was 0.9-2.2 W m-1 K-1 from 300 K to 700 K, relatively low among thermoelectric compounds. This theoretical study showed that Cu5AlSn2S8 could be a potential thermoelectric material.
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.
Rieger, Michael; Rogal, Jutta; Reuter, Karsten
2008-01-11
Using the catalytic CO oxidation at RuO2(110) as a showcase, we employ first-principles kinetic Monte Carlo simulations to illustrate the intricate effects on temperature programmed reaction spectroscopy data brought about by the mere correlations between the locations of the active sites at a nanostructured surface. Even in the absence of lateral interactions, this nanostructure alone can cause inhomogeneities that cannot be grasped by prevalent mean-field data analysis procedures, which thus lead to wrong conclusions on the reactivity of the different surface species. PMID:18232791
Sarkar, Tanmay; Kumar, Parveen; Bharadwaj, Mridula Dixit; Waghmare, Umesh
2016-04-14
A double layer Î´-NH4V4O10, due to its high energy storage capacity and excellent rate capability, is a very promising cathode material for Li-ion and Na-ion batteries for large-scale renewable energy storage in transportation and smart grids. While it possesses better stability, and higher ionic and electronic conductivity than the most widely explored V2O5, the mechanisms of its cyclability are yet to be understood. Here, we present a theoretical cyclic voltammetry as a tool based on first-principles calculations, and uncover structural transformations that occur during Li(+)/Na(+) insertion (x) into (Lix/Nax)NH4V4O10. Structural distortions associated with single-phase and multi-phase structural changes during the insertion of Li(+)/Na(+), identified through the analysis of voltage profile and theoretical cyclic voltammetry are in agreement with the reported experimental electrochemical measurements on Î´-NH4V4O10. We obtain an insight into its electronic structure with a lower band gap that is responsible for the high rate capability of (Lix/Nax) Î´-NH4V4O10. The scheme of theoretical cyclic voltammetry presented here will be useful for addressing issues of cyclability and energy rate in other electrode materials. PMID:26996324
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 input files are provided as Supporting Information . PMID:26651871
NASA Astrophysics Data System (ADS)
Choi, Woo Seok; Kim, Dong Geun; Seo, Sung Seok A.; Moon, Soon Jae; Lee, Daesu; Lee, Jung Hyuk; Lee, Ho Sik; Cho, Deok-Yong; Lee, Yun Sang; Murugavel, Pattukkannu; Yu, Jaejun; Noh, Tae W.
2008-01-01
We investigated the electronic structure of multiferroic hexagonal RMnO3 ( R=Gd , Tb, Dy, and Ho) thin films using both optical spectroscopy and first-principles calculations. One of the difficulties in explaining the electronic structures of hexagonal RMnO3 is that they exist in nature with limited rare earth ions (i.e., R=Sc , Y, and Ho-Lu), so a systematic study in terms of the different R ions has been lacking. Recently, our group succeeded in fabricating hexagonal RMnO3 ( R=Gd , Tb, and Dy) using the epitaxial stabilization technique [Adv. Mater. (Weinheim Ger.) 18, 3125 (2006)]. Using artificially stabilized hexagonal RMnO3 , we extended the optical spectroscopic studies on the hexagonal multiferroic manganite system. We observed two optical transitions located near 1.7 and 2.3eV , in addition to the predominant absorption above 5eV . With the help of first-principles calculations, we attributed the low-lying optical absorption peaks to interband transitions from the oxygen states hybridized strongly with different Mn orbital symmetries to the Mn3d3z2-r2 state. As the ionic radius of the rare earth ion increased, we observed a systematic increase of the lowest peak position, which became more evident when compared with previously reported results. We explained this systematic change in terms of a flattening of the MnO5 triangular bipyramid.
Wang Jingyang; Zhou Yanchun; Lin Zhijun; Ohno, Takahisa
2008-03-01
The theoretical elastic stiffness of Y{sub 3}Si{sub 5}N{sub 9}O, the only Y-Si-O-N quaternary crystal that contains a framework of corner-sharing SiN{sub 4} and/or SiON{sub 3} tetrahedron in three dimensions, was investigated using the first-principles total energy calculations. The full set of second order elastic coefficients, polycrystalline bulk and shear moduli, and anisotropic elastic moduli were reported and further compared with those of Y{sub 2}O{sub 3} and {beta}-Si{sub 3}N{sub 4}. The equation of state and compressibility of Y{sub 3}Si{sub 5}N{sub 9}O were investigated at pressures up to 50 GPa. The crystal structure is stable up to 50 GPa and exhibits anisotropic compressibility under hydrostatic pressure. The relatively softer YN{sub 6} and/or YN{sub 5}O polyhedra are more prone to distort or deform than the SiN{sub 4} and/or SiON{sub 3} tetrahedra. Therefore, although the crystal structure of Y{sub 3}Si{sub 5}N{sub 9}O contains a Si-N-O framework similar to that in {beta}-Si{sub 3}N{sub 4}, it displays elastic stiffness between those of Y{sub 2}O{sub 3} and {beta}-Si{sub 3}N{sub 4}.
NASA Astrophysics Data System (ADS)
Honda, Atsushi; Higai, Shin'ichi; Kageyama, Keisuke; Higuchi, Yukio; Shiratsuyu, Kosuke
2015-10-01
We performed first-principles theoretical calculations on microwave dielectric compounds Ba(Zn1/3Ta2/3)O3, to clarify the origin of high Q characteristics in the Zn/Ta ordered structure. It was found that Zn atoms suppress the intrinsic ferroelectric nature of Slater mode vibration in TaO6 octahedra. This results in the improvement of harmonicity in lattice vibration. There are two important mechanisms. One is the compression of TaO6 octahedra by adjacent ZnO6, and then the double-well potential that TaO6 originally has is narrowed to be single-well with higher harmonicity. The other is the weakening of the covalent character of Ta-O bonds by adjacent Zn atoms, which is the origin of the double-well potential. In the fully ordered Ba(Zn1/3Ta2/3)O3, all Ta atoms are adjacent to Zn atoms, where Zn atoms have the maximum effect on improving harmonicity. Thus, highly ordered Ba(Zn1/3Ta2/3)O3 with prolonged heat treatment exhibits a high Q value.
Karre, Rajamallu; Niranjan, Manish K; Dey, Suhash R
2015-05-01
High alloyed Î²-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 Î²-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 Î²-phase. The Î²-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 Î²-Ti-Nb alloy system are found to be lower than that of pure titanium (116 GPa). For Ti-25(at.%)Nb composition the calculated Young's modulus comes out to be ~80 GPa. In ternary Ti-Nb-Zr alloy system, the Young's modulus of Ti-25(at.%)Nb-6.25(at.%)Zr composition is calculated to be ~50 GPa. Furthermore, the directional Young's moduli of these two selected binary (Ti-25(at.%)Nb) and ternary alloy (Ti-25(at.%)Nb-6.25(at.%)Zr) compositions are found to be nearly isotropic in all crystallographic directions. PMID:25746245
Xiao, Kai; Yoon, Mina; Rondinone, Adam Justin; Payzant, E Andrew; Geohegan, David B
2012-01-01
The deterministic growth of oriented crystalline organic nanowires (CONs) from the vapor-solid chemical reaction (VSCR) between small-molecule reactants and metal nanoparticles has been demonstrated in several studies to date, however the growth mechanism has not yet been conclusively understood. Here, the VSCR growth of M-TCNQF4 (where M is Cu- or Ag-) nanowires is investigated both experimentally and theoretically with time-resolved, in-situ x-ray diffraction (XRD) and first-principles atomistic calculations, respectively, to understand how metals (M) direct the assembly of small molecules into CONs, and what determines the selectivity of a metal for an organic vapor reactant in the growth process. Analysis of the real-time growth kinetics data using a modified Avrami model indicates that the formation of CONs from VSCR follows a one-dimensional ion diffusion-controlled tip growth mechanism wherein metal ions diffuse from a metal film through the nanowire to its tip where they react with small molecules to continue growth. The experimental data and theoretical calculations indicate that the selectivity of different metals to induce nanowire growth depends strongly upon effective charge transfer between the organic molecules and the metal. Specifically, the experimental finding that Cu ions can exchange and replace Ag ions in Ag-TCNQF4 to form Cu-TCNQF4 nanowires is explained by the significantly stronger chemical bond between Cu and TCNQF4 molecules than for Ag, due to the strong spin-dependent electronic contribution of Cu. Understanding how to control the VSCR growth process may enable the synthesis of novel organic nanowires with axial or coaxial p/n junctions for organic nanoelectronics and solar energy harvesting.
NASA Astrophysics Data System (ADS)
Lustemberg, Pablo G.; Scherlis, Damián A.
2013-03-01
The adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfaces is examined applying first-principles molecular dynamics simulations. In particular, the vibrational frequencies are obtained beyond the harmonic approximation, through the time correlation functions of the atomic trajectories. In agreement with experiments, at low CO coverages we find an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35 cm-1 on the stoichiometric rutile (110) and anatase (101) faces, respectively. A band falling 8 cm-1 below the frequency corresponding to the perfect face is observed for the reduced rutile (110) surface in the low vacancy concentration limit, where the adsorption is favored on Ti4 + sites. At a higher density of defects, adsorption on Ti3 + sites becomes more stable, accompanied by a downshift in the stretching band. In the case of anatase (101), we analyze the effect of subsurface oxygen vacancies, which have been shown to be predominant in this material. Interestingly, we find that the adsorption of CO on five coordinate Ti atoms placed over subsurface vacancies is favored with respect to other Ti4 + sites (7.25 against 6.95 kcal/mol), exhibiting a vibrational redshift of 20 cm-1. These results provide the basis to quantitatively assess the degree of reduction of rutile and anatase surfaces via IR spectroscopy, and at the same time allow for the assignment of characteristic bands in the CO spectra on TiO2 whose origin has remained ambiguous.
NASA Astrophysics Data System (ADS)
Prendergast, David; Pemmaraju, Sri Chaitanya Das
2015-09-01
With the advent of X-ray free electron lasers and table-top high-harmonic-generation X-ray sources, we can now explore changes in electronic structure on ultrafast time scales -- at or less than 1ps. Transient X-ray spectroscopy of this kind provides a direct probe of relevant electronic levels related to photoinitiated processes and associated interfacial electron transfer as the initial step in solar energy conversion. However, the interpretation of such spectra is typically fraught with difficulty, especially since we rarely have access to spectral standards for nonequilibrium states. To this end, direct first-principles simulations of X-ray absorption spectra can provide the necessary connection between measurements and reliable models of the atomic and electronic structure. We present examples of modeling excited states of materials interfaces relevant to solar harvesting and their corresponding X-ray spectra in either photoemission or absorption modalities. In this way, we can establish particular electron transfer mechanisms to reveal detailed working principles of materials systems in solar applications and provide insight for improved efficiency.
Zipoli, Federico; Car, Roberto; Cohen, Morrel H; Selloni, Annabella
2010-11-01
Bacterial di-iron hydrogenases produce hydrogen efficiently from water. Accordingly, we have studied by first-principles molecular-dynamics simulations (FPMD) electrocatalytic hydrogen production from acidified water by their common active site, the [FeFe]H cluster, extracted from the enzyme and linked directly to the (100) surface of a pyrite electrode. We found that the cluster could not be attached stably to the surface via a thiol link analogous to that which attaches it to the rest of the enzyme, despite the similarity of the (100) pyrite surface to the Fe4S4 cubane to which it is linked in the enzyme. We report here a systematic sequence of modifications of the structure and composition of the cluster devised to maintain the structural stability of the pyrite/cluster complex in water throughout its hydrogen production cycle, an example of the molecular design of a complex system by FPMD. PMID:26617099
Wanbayor, Raina; Deak, Peter; Frauenheim, Thomas; Ruangpornvisuti, Vithaya
2011-03-14
First principles density functional theory calculations were carried out to investigate the adsorption and oxidation of CO on the positively charged (101) surface of anatase, as well as the desorption of CO{sub 2} from it. We find that the energy gain on adsorption covers the activation energy required for the oxidation, while the energy gain on the latter is sufficient for the desorption of CO{sub 2}, leaving an oxygen vacancy behind. Molecular dynamics simulations indicate that the process can be spontaneous at room temperature. The oxidation process described here happens only in the presence of the hole. The possibility of a photocatalytic cycle is discussed assuming electron scavenging by oxygen.
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
Theoretical methods for ultrafast spectroscopy.
Marquardt, Roberto
2013-05-10
Time-resolved spectroscopy in the femtosecond and attosecond time domain is a tool to unravel the dynamics of nuclear and electronic motion in molecular systems. Theoretical insight into the underlying physical processes is ideally gained by solving the time-dependent Schrödinger equation. In this work, methods currently used to solve this equation are reviewed in a compact presentation. These methods involve numerical representations of wavefunctions and operators, the calculation of time evolution operators, the setting up of the Hamiltonian operators and the types of coordinates to be used hereto. The advantages and disadvantages of some methods are discussed. PMID:23606322
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.
Hua, Weijie; Ai, Yue-Jie; Gao, Bin; Li, Hongbao; Ã…gren, Hans; Luo, Yi
2012-07-21
The N1s near-edge X-ray absorption fine structure (NEXAFS) and X-ray emission spectra (XES) of blocked alanine in water solution have been investigated at the first-principles level based on cluster models constructed from classical molecular dynamics simulations. The bulk solvent has been described by both supermolecular and combined supermolecular-continuum models. With the former model we show that NEXAFS spectra convergent with respect to system size require at least the inclusion of the second solvation shell and that averaged spectra over several hundreds of snapshots can well represent the statistical effect of different instantaneous configurations of the solvation shells. With the combined model we demonstrate that calculations of a medium-sized peptide-water supermolecule qualitatively predict the NEXAFS spectrum of the solvated peptide even considering a single geometry. Furthermore, sampling over hundreds of snapshots by the combined model, the explicit inclusion of even a few waters yields an averaged spectrum in good quantitative agreement with the discrete model results. In comparison, the XES spectra show little dependence on the structures of either the solvent shell or the peptide itself. The ramifications of these findings are discussed. PMID:22684434
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.
NASA Astrophysics Data System (ADS)
Ducher, Manoj; Blanchard, Marc; Vantelon, Delphine; Nemausat, Ruidy; Cabaret, Delphine
2015-12-01
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 3d 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.
Raekers, M.; Bartkowski, S.; Prinz, M.; Neumann, M.; Kuepper, K.; Potzger, K.; Zhou, S.; Postnikov, A. V.; Arulraj, A.; Stuesser, N.; Uecker, R.; Yang, W. L.
2009-03-15
The electronic structures of SmScO{sub 3}, GdScO{sub 3}, and DyScO{sub 3} are investigated by means of x-ray photoelectron spectroscopy, x-ray emission spectroscopy (XES), and x-ray absorption spectroscopy (XAS). A strong hybridization between Sc 3d and O 2p is found, and a contribution of the rare-earth 5d states to this hybridization is not excluded. The band gaps of the compounds are determined by combining XES and XAS measurements. For SmScO{sub 3}, GdScO{sub 3}, and DyScO{sub 3} the band gaps were determined to be 5.6, 5.8, and 5.9 eV, respectively. Magnetization versus temperature measurements reveal antiferromagnetic coupling at 2.96 (SmScO{sub 3}), 2.61 (GdScO{sub 3}), and 3.10 K (DyScO{sub 3}). For DyScO{sub 3} a Rietveld refinement of a 2 K neutron-diffraction data set gives the spin arrangement of Dy in the Pbnm structure (Shubnikov group: Pb{sup '}n{sup '}m{sup '})
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
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 ?-radiolysis on the behavior of heptavalent technetium has been investigated in 13 and 18 M H2SO4. Irradiation experiments were performed using ?-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 ?-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. PMID:24422714
DruÅ¼bicki, Kacper; Mikuli, Edward; PaÅ‚ka, Norbert; Zalewski, SÅ‚awomir; Ossowska-ChruÅ›ciel, MirosÅ‚awa 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
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.
First-Principles Theories of Piezoelectric Materials
NASA Astrophysics Data System (ADS)
Cohen, R. E.
Piezoelectrics have long been studied using parameterized models fit to experimental data, starting with the work of Devonshire in 1954 [1]. Much has been learned using such approaches, but they can also miss major phenomena if the materials properties are not well understood, as is exemplified by the realization that low-symmetry monoclinic phases are common around morphotropic phase boundaries, which was missed completed by low-order Devonshire models, and can only appear in higher-order models [2]. In the last 15 years, a new approach has developed using first-principles computations, based on fundamental physics, with no essential experimental input other than the desired chemistry (nuclear charges). First-principles theory laid the framework for a basic understanding of the origins of ferroelectric behavior [3-7] and piezoelectric properties [8-11]. The range of properties accessible to theory continues to expand as does the accuracy of the predictions. We are moving towards the ability to design materials of desired properties computationally. Here, we review some of the fundamental developments of our understanding of piezoelectric material behavior and the ability to predict a wide range of properties using theoretical methods. This is not meant as a review of the literature. Comprehensive reviews of the literature of theoretical studies of ferroelectrics are given by Resta [12] and Rabe and Ghosez [13].
Xiao, Kai; Yoon, Mina; Rondinone, Adam J; Payzant, Edward A; Geohegan, David B
2012-09-01
The deterministic growth of oriented crystalline organic nanowires (CONs) from the vapor-solid chemical reaction (VSCR) between small-molecule reactants and metal nanoparticles has been demonstrated in several studies to date; however, the growth mechanism has not yet been conclusively understood. Here, the VSCR growth of M-TCNQF(4) (where M is Cu- or Ag-) nanowires is investigated both experimentally and theoretically with time-resolved, in situ X-ray diffraction (XRD) and first-principles atomistic calculations, respectively, to understand how metals (M) direct the assembly of small molecules into CONs, and what determines the selectivity of a metal for an organic vapor reactant in the growth process. Analysis of the real-time growth kinetics data using a modified Avrami model indicates that the formation of CONs from VSCR follows a one-dimensional ion diffusion-controlled tip growth mechanism wherein metal ions diffuse from a metal film through the nanowire to its tip where they react with small molecules to continue growth. The experimental data and theoretical calculations indicate that the selectivity of different metals to induce nanowire growth depends strongly upon effective charge transfer between the organic molecules and the metal. Specifically, the experimental finding that Cu ions can exchange and replace Ag ions in Ag-TCNQF(4) to form Cu-TCNQF(4) nanowires is explained by the significantly stronger chemical bond between Cu and TCNQF(4) molecules than for Ag, due to the strong electronic contribution of Cu d-orbitals near the Fermi level. Understanding how to control the VSCR growth process may enable the synthesis of novel organic nanowires with axial or coaxial p/n junctions for organic nanoelectronics and solar energy harvesting. PMID:22506925
First-principles calculations of flexoelectric coefficients
NASA Astrophysics Data System (ADS)
Hong, Jiawang; Vanderbilt, David
2013-03-01
Flexoelectricity, which is the linear response of polarization to a strain gradient, can have a significant effect on the functional properties of dielectric thin films, superlattices and nanostructures. Despite growing experimental interest, there have been relatively few theoretical studies of flexoelectricity, especially in the context of first-principles calculations. In this talk, we present a complete theory of both the electronic (or ``frozen-ion'')[1] and lattice contributions to flexoelectricity, and demonstrate a supercell method for calculating the flexoelectric coefficients using first-principles density-functional methods. Results are presented for cubic materials including CsCl and SrTiO3. In order to obtain all the elements of the flexoelectric tensor, transverse as well as longitudinal, we carry out calculations on supercells extended along different orientations (e.g., [110] as well as [100]), taking special care to carry out conversions between objects calculated under fixed E or fixed D electric boundary conditions in different parts of the procedure. In this way, all the elements of both the electronic and lattice contributions to the flexoelectric tensor are determined.
NASA Astrophysics Data System (ADS)
Popescu, Dana G.; Barrett, Nicholas; Chirila, Cristina; Pasuk, Iuliana; Husanu, Marius A.
2015-12-01
The effects of the bonding mechanism and band alignment in a ferroelectric (FE) BaTiO3/ferromagnetic La0.6Sr0.4MnO3 heterostructure are studied using x-ray photoelectron spectroscopy and first-principles calculations. The band lineup at the interface is determined by a combination of band bending and polarization-induced modification of core-hole screening. A Schottky barrier height for electrons of 1.22 ±0.17 eV is obtained in the case of downwards FE polarization of the top layer. The symmetry of the bonding states is emphasized by integrating the local density of states ±0.2 eV around the Fermi level, and strong dependence on the FE polarization is found: upwards, polarization stabilizes Ti t2 g(x y ) orbitals, while downwards, polarization favors Ti t2 g(y z ) symmetry. It is predicted that the abrupt (La,Sr) | TiO2 interface is magnetoelectrically active, leading to a A-type antiferromagnetic coupling of the first TiO2 interface layer with the underlying manganite layer through a superexchange mechanism.
Mechanical Behaviors of Alloys From First Principles
NASA Astrophysics Data System (ADS)
Hanlumyuang, Yuranan
2011-07-01
Several mesoscale models have been developed to consider a number of mechanical properties and microstructures of Ti-V approximants to Gum Metal and steels from the atomistic scale. In Gum Metal, the relationships between phonon properties and phase stabilities are studied. Our results show that it is possible to design a BCC (beta-phase) alloy that deforms near the ideal strength, while maintaining structural stability with respect to the formation of the o and alpha'' phases. Theoretical diffraction patterns reveal the role of the soft N-point phonon and the BCC-to-HCP transformation path in post-deformation samples. The total energies of the path explain the formation of the giant faults and nano shearbands in Gum Metal. In the study of steels, we focus on the carbon-solute dislocation interactions. The analysis covers the Eshelby's model of point defects and first principles calculations. It is argued that the effects of chemistry and magnetism, omitted in the elasticity model, do not make major contributions to the segregation energy. The predicted solute atmospheres are in good agreement with atom probe measurements.
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 â‰¥107 Hz. Variable-temperature 17O NMR spectra recorded from 22 to 214 Â°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 Â± 0.06 eV was observed for rotation about the Pâ€“O1 axis, with a higher activation energy of 0.50 Â± 0.07 eV being obtained for rotation about the Pâ€“O2, Pâ€“O3d, and Pâ€“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 O1â€“H1 bond. The activation energy of this process is slightly lower than that obtained from the 1H exchange process (0.70 Â± 0.07 eV) (Kim, G.; Blanc, F.; Hu, Y.-Y.; Grey, C. P. J. Phys. Chem. C2013, 117, 6504âˆ’6515) associated with the translational motion of the protons. The relationship between proton jumps and phosphate rotation was analyzed in detail by considering uncorrelated motion, motion of individual PO4 ions and the four connected/H-bonded protons, and concerted motions of adjacent phosphate units, mediated by proton hops. We conclude that, while phosphate rotations aid proton motion, not all phosphate rotations result in proton jumps. PMID:25732257
Kim, Gunwoo; Griffin, John M; Blanc, Frédéric; Haile, Sossina M; Grey, Clare P
2015-03-25
(17)O NMR spectroscopy combined with first-principles calculations was employed to understand the local structure and dynamics of the phosphate ions and protons in the paraelectric phase of the proton conductor CsH2PO4. For the room-temperature structure, the results confirm that one proton (H1) is localized in an asymmetric H-bond (between O1 donor and O2 acceptor oxygen atoms), whereas the H2 proton undergoes rapid exchange between two sites in a hydrogen bond with a symmetric double potential well at a rate ?10(7) Hz. Variable-temperature (17)O NMR spectra recorded from 22 to 214 °C were interpreted by considering different models for the rotation of the phosphate anions. At least two distinct rate constants for rotations about four pseudo C3 axes of the phosphate ion were required in order to achieve good agreement with the experimental data. An activation energy of 0.21 ± 0.06 eV was observed for rotation about the P-O1 axis, with a higher activation energy of 0.50 ± 0.07 eV being obtained for rotation about the P-O2, P-O3(d), and P-O3(a) axes, with the superscripts denoting, respectively, dynamic donor and acceptor oxygen atoms of the H-bond. The higher activation energy of the second process is most likely associated with the cost of breaking an O1-H1 bond. The activation energy of this process is slightly lower than that obtained from the (1)H exchange process (0.70 ± 0.07 eV) (Kim, G.; Blanc, F.; Hu, Y.-Y.; Grey, C. P. J. Phys. Chem. C 2013, 117, 6504-6515) associated with the translational motion of the protons. The relationship between proton jumps and phosphate rotation was analyzed in detail by considering uncorrelated motion, motion of individual PO4 ions and the four connected/H-bonded protons, and concerted motions of adjacent phosphate units, mediated by proton hops. We conclude that, while phosphate rotations aid proton motion, not all phosphate rotations result in proton jumps. PMID:25732257
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 wires and show that interface contacts critically control charge conduction. It was found, for Au/BDT/Au junctions, the H atom in -SH groups energetically prefers to be non-dissociative after the contact formation, which was supported by comparison between computed and measured break-down forces and bonding energies. The H-non-dissociated (HND) junctions give equilibrium conductances from0.054G0 (equilibrium structure) to 0.020G0 (stretched structure) which is within a factor of 2-5 of the measured data. On the other hand, for all H-dissociated contact structures - which were the assumed structures in the literature, the conductance is at least more than an order of magnitude larger that the experimental value. The HND-model significantly narrows down the theory/experiment discrepancy. Finally, a by-product of this work is a comprehensive pseudopotential and atomic orbital basis set database that has been carefully calibrated and can be used by the DFT community at large.
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 (Ëœ 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 relatively modest band masses for both electrons and holes suggesting applications. Optical properties show a infrared-red absorption when doped. This could potentially be useful for combining wavelength filtering and transparent conducting functions. Furthermore, our defect calculations show that Ba 2TeO is intrinsically p-type conducting under Ba-poor condition. However, the spontaneous formation of the donor defects may constrain the p-type transport properties and would need to be addressed to enable applications. Chapter 4 mainly devotes to the thermoelectric properties of the famous phase change material, Ge2Sb2Te5 (GST). GST has been used in data storage for more than a decade because of their fast phase switching between metastable crystalline (cubic) and amorphous phases. It also exhibits interesting thermoelectric properties, and we did a systematic study on the two crystalline phases (hexagonal and cubic) and the amorphous phase. We found a high Seebeck coefficient with a broad doping concentrations for both n-type and p-type, at and below room temperatures (300 K) for both the cubic and amorphous phases. This finding will be of crucial interests in further understand the thermoelectric properties experimentally and find device applications in the ultimate goal. Several magnetic materials that involve lanthanide elements are reported in Chapter 5. First of all, the electronic and magnetic properties of the BaLn2O4 (Ln = La-Lu, Y) family compound are studied. The series has been synthesized for the first time in single crystalline form, using a molten metal flux. They crystallize in the CaV 2O4 structure type with primitive orthorhombic symmetry (space group Pnma, #62). Our calculations show an insulating character with band gaps ranging from 3 eV to 4.5 eV for the three representative compounds, BaLa2O4, BaGd2O4 and BaLu 2O4. Moreover, the superexchange magnetism is also studied. Secondly, a strong correlated system with cerium is investigated. As expected, we find a value of 15 states eV-1 that stems from the Ce 4f orbitals at the Fermi energy which indicates intermetallic heavy fermion behavior. The Fermi surface calculation shows nesting feature which might be useful to further understand the antiferromagnetic magnetism. Thirdly, the DFT calculations of another lanthanide oxide involving transition element, LaMo16O44, are also presented. This material crystallizes with a complicated crystal structure consists of MoO6 magnetic clusters. The band structure calculations indicate a spin-polarized half metal feature that comes from different crystallographic sites of Mo since La occurs as trivalent with empty f shell thus no contribution to the magnetic moment. Last but not least, we studied the electronic properties of another newly found oxytellride that having lanthanide, Ba3Yb 2O5Te. We find a insulating behavior with a direct band-gap value of 1.9 eV using the DFT+U methodology.
Description of charge conjugation from first principles
Lujan-Peschard, C.; Napsuciale, M.
2006-09-25
We construct the charge conjugation operator as a unitary automorphism in the spinor space ((1/2), 0) + (0 (1/2)) from first principles. We calculate its eigenspinors and derive the equation of motion they satisfy. The mapping associated to charge conjugation is constructed from parity eigenstates which are considered as particle and antiparticle.
Rediscovering First Principles through Online Learning.
ERIC Educational Resources Information Center
Kidney, Gary W.; Puckett, Edmond G.
2003-01-01
Describes an evaluation of Web-based instruction at the University of Houston-Clear Lake (Texas) that showed that the design team had been distracted from many first principles of instructional design by the creative chaos on the Web and discusses how self-reflection and role definitions allowed the team to overcome these disappointments andâ€¦
First-principles theory of ultrathin magnetic films
NASA Astrophysics Data System (ADS)
Asada, T.; Bihlmayer, G.; Handschuh, S.; Heinze, S.; Kurz, Ph; Blügel, S.
1999-12-01
We report on a set of systematic first-principles electronic structure investigations of the magnetic spin moments, the magnetic spin configurations, and the magnetic coupling of ultrathin magnetic films on (001)- and (111)-oriented noble-metal substrates and on the Fe(001) substrate. Magnetism is found for 3d-, 4d-, and 5d-transition-metal monolayers on noble-metal substrates. For V, Cr, and Mn on (001) substrates a c(2 × 2) antiferromagnetic superstructure has the lowest energy, and Fe, Co, Ni are ferromagnetic. On (111) substrates, for Cr the energy minimum is found for a 120° non-collinear magnetic configuration in a (icons/Journals/Common/sqrt3" ALT="sqrt3" ALIGN="TOP"/> × icons/Journals/Common/sqrt3" ALT="sqrt3" ALIGN="TOP"/>)R30° unit cell, and for Mn a row-wise antiferromagnetic structure is found. On Fe(001), V and Cr monolayers prefer the layered antiferromagnetic coupling, and Fe, Co, and Ni monolayers favour the ferromagnetic coupling to Fe(001). The magnetic structure of Mn on Fe(001) is a difficult case: at least two competing magnetic states are found within an energy of 7 meV. The Cr/Fe(001) system is discussed in more detail as the surface-alloy formation is investigated, and this system is used as a test case to compare theoretical and experimental scanning tunnelling spectroscopy (STS) results. The possibility of resolving magnetic structures by STS is explored. The results are based on the local spin-density approximation and the generalized gradient approximation to the density functional theory. The calculations are carried out with the full-potential linearized augmented-plane-wave method in film geometry.
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 solidâ€“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 Î£3(112)[1Ì…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 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.
Ferroelectric Phase Transitions from First Principles
NASA Astrophysics Data System (ADS)
Rabe, Karin M.
1997-03-01
For a deeper understanding of structural phase transitions in perovskite-structure oxides, first-principles calculations offer valuable access to microscopic information. With recent advances in algorithms and computational capabilities, structural energetics has been largely met, and high-accuracy density-functional studies for a wide range of perovskite compounds have been presented in the literature. The practical application of these methods to temperature-driven structural transitions involves the construction of an effective Hamiltonian with parameters determined from first-principles calculations. The lattice Wannier function method(K. M. Rabe and U. V. Waghmare, Phys. Rev. B52), 13236 (1995). offers a systematic approach for the construction of first-principles effective Hamiltonians applicable to complex structural transitions involving multiple unstable modes at arbitrary points in the Brillouin zone. The parameters appearing in the LWF effective Hamiltonians for ferroelectric PbTiO3 and antiferroelectric PbZrO3 are obtained from density-functional-theory linear response calculations of phonon frequencies, Z and ?_?, and total-energy calculations using the conjugate-gradients method with optimized pseudopotentials and a plane-wave basis set. The finite-temperature behavior of the model systems is studied using mean field theory and Monte Carlo simulation to yield values of the transition temperature, latent heat and various distribution functions for comparison with experiment. The role of strain coupling in producing the observed transition behavior is investigated. Recent work on the extension to mixed systems will be illustrated by results on Pb_1-xGe_xTe. The use of first-principles effective Hamiltonians to study the temperature dependence of dielectric and piezoelectric response, as well as phonon and domain wall dynamics, will be discussed.
NASA Astrophysics Data System (ADS)
Leng, Xia; Yin, Huabing; Liang, Dongmei; Ma, Yuchen
2015-09-01
Organic semiconductors have promising and broad applications in optoelectronics. Understanding their electronic excited states is important to help us control their spectroscopic properties and performance of devices. There have been a large amount of experimental investigations on spectroscopies of organic semiconductors, but theoretical calculation from first principles on this respect is still limited. Here, we use density functional theory (DFT) and many-body Green's function theory, which includes the GW method and Bethe-Salpeter equation, to study the electronic excited-state properties and spectroscopies of one prototypical organic semiconductor, sexithiophene. The exciton energies of sexithiophene in both the gas and bulk crystalline phases are very sensitive to the exchange-correlation functionals used in DFT for ground-state structure relaxation. We investigated the influence of dynamical screening in the electron-hole interaction on exciton energies, which is found to be very pronounced for triplet excitons and has to be taken into account in first principles calculations. In the sexithiophene single crystal, the energy of the lowest triplet exciton is close to half the energy of the lowest singlet one. While lower-energy singlet and triplet excitons are intramolecular Frenkel excitons, higher-energy excitons are of intermolecular charge-transfer type. The calculated optical absorption spectra and Davydov splitting are in good agreement with experiments.
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. PMID:26395713
The ABINIT software package : theoretical spectroscopy of materials.
NASA Astrophysics Data System (ADS)
Gonze, Xavier
2010-05-01
ABINIT allows one to study, from first-principles, properties of materials (also 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, electronic or optical properties, at different levels of approximation. Used an estimated 1000 scientists worldwide, ABINIT is especially appreciated for its electronic, optical and vibrational spectroscopy capabilities (photoemission, optical, IR/Raman spectroscopy ...). The presentation will cover the description of ABINIT capabilities, and present some applications. ABINIT is freely available (GNU GPL) at [http://www.abinit.org]. For a recent account, see Computer Physics Communications 180 (2009) 2582-2615.
Giant magnetoresistance calculated from first principles
Butler, W.H.; MacLaren, J.M.; Zhang, X.G.
1994-09-01
The Layer Korringa Kohn Rostoker-Coherent Potential Approximation technique was used to calculate the low temperature Giant Magnetoresistance from first principles for Co{vert_bar}Cu and permalloy{vert_bar}Cu superlattices. Our calculations predict large giant magnetoresistance ratios for Co{vert_bar}Cu and extremely large ratios for permalloy{vert_bar}Cu for current perpendicular to the layers. Mechanisms such as spin-orbit coupling which mix spin channels are expected to greatly reduce the GMR effect for permalloy{vert_bar}Cu.
First-principles theory of flexoelectricity
NASA Astrophysics Data System (ADS)
Vanderbilt, David
2013-03-01
Flexoelectricity is the linear response of polarization to a strain gradient. Because strain gradients break inversion symmetry, flexoelectricity occurs in all insulating crystals. The flexoelectric effect is negligible on conventional length scales, but it can become very strong at the nanoscale where large strain gradients can significantly affect the functional properties of dielectric thin films and superlattices. Previous theories have tended to focus either on the lattice or the electronic (i.e., frozen-ion) contribution, and have involved some approximations or limitations. Here we develop a general first-principles theory of the flexoelectric tensor, formulated in such a way that the tensor elements can be computed directly in the context of density-functional calculations. Special attention will be paid to several subtleties, including surface contributions, pseudopotential dependence, the calculation of transverse components, fixed E vs. fixed D boundary conditions, and a degree of non-uniqueness that is present for some strain gradients. We introduce several practical supercell-based methods for calculating the flexoelectric coefficients from first principles, and demonstrate them by computing the coefficients for a variety of insulating materials.(Work done in collaboration with Jiawang Hong. Supported by ONR N00014-12-1-1035.)
Aqueous solvation of methane from first principles.
Rossato, Lorenzo; Rossetto, Francesco; Silvestrelli, Pier Luigi
2012-04-19
Structural, dynamical, bonding, and electronic properties of water molecules around a soluted methane molecule are studied from first principles. The results are compatible with experiments and qualitatively support the conclusions of recent classical molecular dynamics simulations concerning the controversial issue on the presence of "immobilized" water molecules around hydrophobic groups: the hydrophobic solute slightly reduces (by a less than 2 factor) the mobility of many surrounding water molecules rather than immobilizing just the few ones which are closest to methane, similarly to what was obtained by previous first-principles simulations of soluted methanol. Moreover, the rotational slowing down is compatible with the one predicted on the basis of the excluded volume fraction, which leads to a slower hydrogen bond exchange rate. The analysis of simulations performed at different temperatures suggests that the target temperature of the soluted system must be carefully chosen, in order to avoid artificial slowing-down effects. By generating maximally localized Wannier functions, a detailed description of the polarization effects in both solute and solvent molecules is obtained, which better characterizes the solvation process. PMID:22443455
Iron diffusion from first principles calculations
NASA Astrophysics Data System (ADS)
Wann, E.; Ammann, M. W.; Vocadlo, L.; Wood, I. G.; Lord, O. T.; Brodholt, J. P.; Dobson, D. P.
2013-12-01
The cores of Earth and other terrestrial planets are made up largely of iron1 and it is therefore very important to understand iron's physical properties. Chemical diffusion is one such property and is central to many processes, such as crystal growth, and viscosity. Debate still surrounds the explanation for the seismologically observed anisotropy of the inner core2, and hypotheses include convection3, anisotropic growth4 and dendritic growth5, all of which depend on diffusion. In addition to this, the main deformation mechanism at the inner-outer core boundary is believed to be diffusion creep6. It is clear, therefore, that to gain a comprehensive understanding of the core, a thorough understanding of diffusion is necessary. The extremely high pressures and temperatures of the Earth's core make experiments at these conditions a challenge. Low-temperature and low-pressure experimental data must be extrapolated across a very wide gap to reach the relevant conditions, resulting in very poorly constrained values for diffusivity and viscosity. In addition to these dangers of extrapolation, preliminary results show that magnetisation plays a major role in the activation energies for diffusion at low pressures therefore creating a break down in homologous scaling to high pressures. First principles calculations provide a means of investigating diffusivity at core conditions, have already been shown to be in very good agreement with experiments7, and will certainly provide a better estimate for diffusivity than extrapolation. Here, we present first principles simulations of self-diffusion in solid iron for the FCC, BCC and HCP structures at core conditions in addition to low-temperature and low-pressure calculations relevant to experimental data. 1. Birch, F. Density and composition of mantle and core. Journal of Geophysical Research 69, 4377-4388 (1964). 2. Irving, J. C. E. & Deuss, A. Hemispherical structure in inner core velocity anisotropy. Journal of Geophysical Research 116, B04307 (2011). 3. Buffett, B. A. Onset and orientation of convection in the inner core. Geophysical Journal International 179, 711-719 (2009). 4. Bergman, M. Measurements of electric anisotropy due to solidification texturing and the implications for the Earth's inner core. Nature 389, 60-63 (1997). 5. Deguen, R. & Cardin, P. Thermochemical convection in Earth's inner core. Geophysical Journal International 187, 1101-1118 (2011). 6. Reaman, D. M., Daehn, G. S. & Panero, W. R. Predictive mechanism for anisotropy development in the Earth's inner core. Earth and Planetary Science Letters 312, 437-442 (2011). 7. Ammann, M. W., Brodholt, J. P., Wookey, J. & Dobson, D. P. First-principles constraints on diffusion in lower-mantle minerals and a weak D'' layer. Nature 465, 462-5 (2010).
First principles: Systems and their analysis
Woods, T.W.
1993-04-01
This paper is intended to challenge systems professionals to think about systems -- not at the process level but at the foundational level: first principles. System principles at the concept level, and what one understands about them, determine what one practices at the process level -- that is, how one defines ``systems engineering``. When Kant, Kepler, Newton, Einstein, and the others were deriving the natural laws, where was the comparable basic work in the natural order of things: systems? Is our profession one of simply employing some fairly good empirical procedures? Is there a legitimate place for a ``First Law of Systems`` alongside The First Law of Thermodynamics? Who would do this research? Who would fund it? Is now the time? Why should we care?
First principles: Systems and their analysis
Woods, T.W.
1993-04-01
This paper is intended to challenge systems professionals to think about systems -- not at the process level but at the foundational level: first principles. System principles at the concept level, and what one understands about them, determine what one practices at the process level -- that is, how one defines systems engineering''. When Kant, Kepler, Newton, Einstein, and the others were deriving the natural laws, where was the comparable basic work in the natural order of things: systems Is our profession one of simply employing some fairly good empirical procedures Is there a legitimate place for a First Law of Systems'' alongside The First Law of Thermodynamics Who would do this research Who would fund it Is now the time Why should we care
Numerical Inductance Calculations Based on First Principles
Shatz, Lisa F.; Christensen, Craig W.
2014-01-01
A method of calculating inductances based on first principles is presented, which has the advantage over the more popular simulators in that fundamental formulas are explicitly used so that a deeper understanding of the inductance calculation is obtained with no need for explicit discretization of the inductor. It also has the advantage over the traditional method of formulas or table lookups in that it can be used for a wider range of configurations. It relies on the use of fast computers with a sophisticated mathematical computing language such as Mathematica to perform the required integration numerically so that the researcher can focus on the physics of the inductance calculation and not on the numerical integration. PMID:25402467
Phonon-phonon interactions: First principles theory
NASA Astrophysics Data System (ADS)
Gibbons, T. M.; Bebek, M. B.; Kang, By.; Stanley, C. M.; Estreicher, S. K.
2015-08-01
We present the details of a method to perform molecular-dynamics (MD) simulations without thermostat and with very small temperature fluctuations ±?T 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 ?T. 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.
Effective magnetic Hamiltonians from first principles
NASA Astrophysics Data System (ADS)
Drchal, V.; Kudrnovský, J.; Turek, I.
2013-01-01
We construct effective magnetic Hamiltonians by using the first-principles electronic-structure calculations. These Hamiltonians are used to determine the ground state of solids and nanostructures, and, at T > 0, using the methods of statistical mechanics, also their magnetic properties as a function of temperature. The present approach is highly flexible and it makes possible to find magnetic structure of complex systems with the accuracy of ab initio methods. As illustrations we show (i) how the magnetic structure at T = 0 and at T > 0 can be determined for systems with anisotropic exchange and for systems with several sublattices, (ii) why the magnetic moment in fcc-nickel is unstable upon reversal, and (iii) how to calculate the size of magnetic moment in fcc-Ni at the Curie temperature, which is important for explanation of the spin-disorder resistivity in Ni.
Susceptibilities for first principles band structures
NASA Astrophysics Data System (ADS)
Crockford, D. J.; Yeung, W.
1993-04-01
We present a parallel implementation of a new method for calculating the unenhanced susceptibility proposed by us recently. Our implementation uses the first principles LMTO band structure within the tight binding approach as input to calculate the joint density of states. The susceptibility is then obtained by integrating over the product of the joint density of states and a Lindhard function. Our program, which has a simple friendly user interface, runs on the PC with a quadputer board, a Meiko Surface running CSTools powered either by T800 or i860 compute boards and the Intel iPSC/860 hypercube in Daresbury. Our method incorporates the troublesome matrix elements naturally and our results on Pd and Ni show that the decrease in ?( q) as we go away from the Brillouin zone centre is due mainly to the matrix elements rather than to the band energies.
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 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.
Efficient first-principles electronic dynamics.
Liang, Wenkel; Chapman, Craig T; Li, Xiaosong
2011-05-14
An efficient first-principles electronic dynamics method is introduced in this article. The approach we put forth relies on incrementally constructing a time-dependent Fock?Kohn-Sham matrix using active space density screening method that reduces the cost of computing two-electron repulsion integrals. An adaptive stepsize control algorithm is developed to optimize the efficiency of the electronic dynamics while maintaining good energy conservation. A selected set of model dipolar push-pull chromophore molecules are tested and compared with the conventional method of direct formation of the Fock?Kohn-Sham matrix. While both methods considered herein take on identical dynamical simulation pathways for the molecules tested, the active space density screening algorithm becomes much more computationally efficient. The adaptive stepsize control algorithm, when used in conjunction with the dynamically active space method, yields a factor of ?3 speed-up in computational cost as observed in electronic dynamics using the time dependent density functional theory. The total computational cost scales nearly linear with increasing size of the molecular system. PMID:21568492
Efficient first-principles electronic dynamics
NASA Astrophysics Data System (ADS)
Liang, Wenkel; Chapman, Craig T.; Li, Xiaosong
2011-05-01
An efficient first-principles electronic dynamics method is introduced in this article. The approach we put forth relies on incrementally constructing a time-dependent Fock/Kohn-Sham matrix using active space density screening method that reduces the cost of computing two-electron repulsion integrals. An adaptive stepsize control algorithm is developed to optimize the efficiency of the electronic dynamics while maintaining good energy conservation. A selected set of model dipolar push-pull chromophore molecules are tested and compared with the conventional method of direct formation of the Fock/Kohn-Sham matrix. While both methods considered herein take on identical dynamical simulation pathways for the molecules tested, the active space density screening algorithm becomes much more computationally efficient. The adaptive stepsize control algorithm, when used in conjunction with the dynamically active space method, yields a factor of ˜3 speed-up in computational cost as observed in electronic dynamics using the time dependent density functional theory. The total computational cost scales nearly linear with increasing size of the molecular system.
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.
Phonon Engineering in Metals from First Principles
NASA Astrophysics Data System (ADS)
Lanzillo, Nicholas; Thomas, J.; Watson, E. B.; Washington, M.; Nayak, Saroj K.
2013-03-01
The electron-phonon interaction in metallic systems controls the electronic transport properties, including both electrical and thermal resistivity. The effect of compressive strain on the electron-phonon interaction in metals is investigated using first-principles density functional theory, and we propose various ways to ``engineer'' this interaction for various technological applications. In particular, we show that by applying compressive strain on the FCC crystals of Al, Cu, Ag and Au, the net electron-phonon scattering rate decreases and likewise the electrical resistivity decreases with increasing pressure. This trend is corroborated by experimental measurements of the resistance of a 0.5 mm diameter high-purity Al wire pressurized up to 2 GPa in a solid-media pressure apparatus at room temperature. The rate of the decrease in electrical resistivity as a function of pressure as determined by experiment is matched by the rate predicted by theory. Our simulations show that Al nanowires have the same response to strain as the bulk crystal; the net electron-phonon scattering can be reduced through compressive strain. Modifying the electron-phonon interaction in metallic structures shows great promise for future nano-electronic devices.
First principle study of sodium decorated graphyne
NASA Astrophysics Data System (ADS)
Sarkar, Utpal; Bhattacharya, Barnali; Seriani, Nicola
2015-11-01
We present first-principles calculations of the electronic properties of Na-decorated graphyne. This structure of the graphyne family is a direct band gap semiconductor with a band gap of 0.44 eV in absence of sodium, but Na-decorated graphyne compounds are metallic, and can then be employed as carbon-based conductors. Metallization is due to charge donation from sodium to carbon. Pristine graphyne is more stable than Na-decorated graphyne, therefore is seems probable that, if this material should be employed as electrode in Na-ion batteries, it would lead to the formation of metallic sodium rather than well dispersed sodium ions. On the other side, this property might be useful if graphyne is employed in water desalination. Finally, the abrupt change from a semiconducting to a metallic state in presence of a small amount of sodium might be exploited in electronics, e.g. for the production of smooth metal-semiconductor interfaces through spatially selective deposition of sodium.
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.
GPU Based Acceleration of First Principles Calculations
NASA Astrophysics Data System (ADS)
Tomono, Hidekazu; Iitaka, Toshiaki; Tsumuraya, Kazuo
2009-03-01
The saturation of the acceleration using the silicon devices has required the parallel computing using multiple CPU's (central processing units). The parallel computing has been widely used in the field of the high-performance computing. On the other hand, graphics processing units (GPU's) were designed to accelerate graphic applications in 1978. NVIDIA Co. began to provide CUDA for C-language users to manipulate the GPU's in 2007. They applied it to computational fluid dynamics, medical real time simulation and astronomical N-body problem among others. This is the GPGPU (general-purpose computation on GPU's), which is faster in operation than CPU in the fields of linear algebras, FFT, and others. We have experienced that one- dimensional CUFFT ver1.1 (GPU-FFT) is eight times faster than FFTW for single-precision case. We implement the GPU-FFT into our in-house first principles planewave code, in which the hot spot is the FFT routine. We will present the performance of the implementation.
First Principle Modeling of Energetic Storm Particles
NASA Astrophysics Data System (ADS)
Shiota, Daikou; Kataoka, Ryuho; Sugiyama, Tooru; Kusano, Kanya
The origin and cause of solar energetic particles (SEPs) has been one of the most important topics in the field of space weather research. Energetic storm particle (ESP) event is a subset of SEP events, which is characterized as proton flux enhancement of the relatively low energy range (Â¡10 MeV) associated with interplanetary shock arrivals at the Earth. Here we perform the first principle modeling of ESP and quantitatively compare the results with in-situ observations, using hybrid plasma simulation. In this paper we focus on the coronal mass ejection event on 13 Dec 2006. As the input parameters for the simulation, fundamental shock parameters of the plasma beta, Alfven mach, and shock angle are given from a global solar wind simulation of Kataoka et al. (2009). As a result, it is found that proton flux and spectral index of ESP event can be quantitatively reproduced by the interlocked simulation, which is driven by the realistic shock parameters without injection particles. We suggest that ESPs can be originated from thermal solar wind plasma accelerated by the passage of interplanetary shocks.
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.
Electrostatics of superlattices by first principles
NASA Astrophysics Data System (ADS)
Wu, Xifan; Diéguez, Oswaldo; Stengel, Massimiliano; Rabe, Karin; Vanderbilt, David
2007-03-01
A complete theory of epitaxial perovskite superlattices requires an understanding of both epitaxial strain effects and of electrostatic boundary conditions. Here, focusing on the latter issue, we have carried out first-principles calculations of the nonlinear dielectric properties of short-period BaTiO3/SrTiO3 and PbTiO3/SrTiO3 superlattices having the in-plane lattice constant of SrTiO3. In particular, we have calculated the layer polarizations pj as defined using the Wannier-based method of Wu, Di'eguez, Rabe and Vanderbilt for each neutral BaO, SrO, PbO, or TiO2 layer, and modeled pj as a function of displacement field D (which is uniform throughout the superlattice), the chemical identity of the layer itself, and the chemical identity of its near neighbors. We then test our expectation that the dependence on the identity of neighboring layers should decay rapidly with distance. If we apply a cut-off to the range of this interlayer interaction, we arrive at a model description that allows us to predict pj(D) for each layer, and thus the overall P(D) (and trivially, also P vs. electric field and related quantities) for a superlattice of arbitrary layer sequence. X. Wu, O. Di'eguez, K. Rabe and D. Vanderbilt, Phys. Rev. Lett. 97, 107602 (2006).
NASA Astrophysics Data System (ADS)
Rovezzi, Mauro; Schlögelhofer, Wolfgang; Devillers, Thibaut; Szwacki, Nevill Gonzalez; Li, Tian; Adhikari, Rajdeep; Glatzel, Pieter; Bonanni, Alberta
2015-09-01
Synchrotron radiation x-ray absorption and emission spectroscopy techniques, complemented by high-resolution transmission electron microscopy methods and density functional theory calculations, are employed to investigate the effect of Mn in AlxGa1 -xN :Mn samples with an Al content up to 100%. The atomic and electronic structure of Mn is established together with its local environment and valence state. A dilute alloy without precipitation is obtained for AlxGa1 -xN :Mn with Al concentrations up to 82%, and the surfactant role of Mn in the epitaxial process is confirmed.
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
Theoretical Sum Frequency Generation Spectroscopy of Peptides.
Carr, Joshua K; Wang, Lu; Roy, Santanu; Skinner, James L
2015-07-23
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
First-Principles Investigation on Boron Nanostructures
NASA Astrophysics Data System (ADS)
Tang, Hui
2011-12-01
First-principles calculations based on density functional theory are employed to study and predict the properties of boron and Mg boride nanostructures. For boron nanostructures, two-dimensional boron sheets are found to be metallic and made of mixtures of triangles and hexagons which benefit from the balance of two-center bonding and three-center bonding. This unusual bonding in boron sheets results in a self-doping picture where adding atoms to the hexagon centers does not change the number of bonding states but merely increases the electron count. Boron sheets can be either flat or buckled depending on the ratio between hexagons and triangles. Formed by stacking two identical boron sheets, double-layered boron sheets can form interlayer bonds, and the most stable one is semiconducting. Built from single-layered boron sheets, single-walled boron nanotubes have smaller curvature energies than carbon nanotubes and undergo a metal-to-semiconductor transition once the diameter is smaller than Ëœ20 A. Optimal double-walled boron nanotubes with inter-walled bonds formed are metallic and always more stable than single-walled ones. For Mg boride nanostructures, certain Mg boride sheets prefer to curve themselves into nanotubes, which is explained via Mg-Mg interactions governed by the charge state of Mg. In addition, optimal Mg boride sheet structures are explored with a genetic algorithm. Phase diagrams for Mg boride sheet structures are constructed and stable phases under boron-rich environments are identified. Curvature effects on the phase diagram of Mg boride nanotubes are also discussed. As a natural extension to boron sheets, layered boron crystals based on boron sheets are then presented and are shown to be stable under high pressure. Finally, this thesis ends with an investigation of hydrogen-storage properties of pristine and metal doped boron nanostructures.
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 ?1 = 1386.0cm-1,?2 = 621.1 cm-1 and ?3 = 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 ??3 = - 5.4 cm-1 and ? (?1 +?3) = - 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.
Microscopic Dynamics of Supercooled Liquids from First Principles
NASA Astrophysics Data System (ADS)
Janssen, Liesbeth M. C.; Reichman, David R.
2015-11-01
The transition from a liquid to a glass remains one of the most poorly understood phenomena in condensed matter physics, and still no fully microscopic theory exists that can describe the dynamics of supercooled liquids in a quantitative manner over all relevant time scales. Here, we present a theoretical framework that yields near-quantitative accuracy for the time-dependent correlation functions of a glass-forming system over a broad density range. Our approach requires only simple static structural information as input and is based entirely on first principles. Owing to its ab initio nature, the framework offers a unique platform to study the relation between structure and dynamics in glass-forming matter, and paves the way towards a systematically correctable and ultimately fully quantitative theory of microscopic glassy dynamics.
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 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.
A first-principles model for orificed hollow cathode operation
NASA Technical Reports Server (NTRS)
Salhi, A.; Turchi, P. J.
1992-01-01
A theoretical model describing orificed hollow cathode discharge is presented. The approach adopted is based on a purely analytical formulation founded on first principles. The present model predicts the emission surface temperature and plasma properties such as electron temperature, number densities and plasma potential. In general, good agreements between theory and experiment are obtained. Comparison of the results with the available related experimental data shows a maximum difference of 10 percent in emission surface temperature, 20 percent in electron temperature and 35 percent in plasma potential. In case of the variation of the electron number density with the discharge current a maximum discrepancy of 36 percent is obtained. However, in the case of the variation with the cathode internal pressure, the predicted electron number density is higher than the experimental data by a maximum factor of 2.
First principles studies of semiconductor epitaxial growth
NASA Astrophysics Data System (ADS)
Tsai, Bao-Liang
This thesis conducts investigations mainly on the structures, energetics, and recations of semiconductor as well as oxide surfaces using first principles cluster model approach. The first part of the research work addresses the issues in the epitaxial growth of Hgsb{1-x}Cdsb{x}Te (MCT) materials. Hg divalent compounds were studied thoroughly using a variety of quantum chemical methods in order to understand the energetics of Hg precursors for growth. The (001) growth surfaces were then examined in detail using cluster model calculations. Based on these results, a novel metal-organic molecular beam epitaxial (MOMBE) growth strategy with favorable energetics for growing MCT using Hsb2C=CH-CHsb2-Hg-Cequiv C-CHsb3 is proposed. It is hoped that with this new growth strategy, the Hg vacancy and p-doping problems that currently exist in growth can be avoided. The second part of the thesis discusses the molecular beam epitaxial (MBE) growth of cubic GaN on the (001) surface using various N sources. Surface reconstructions and the interactions of gas-phase atomic and molecular nitrogens with the surface were elucidated using cluster models. Using these results an energy phase diagram for the growth of GaN has been constructed. It suggests that excited state molecular Nsb2\\ (sp3Sigmasbsp{u}{+}) is the most favorable of all N species for growth of high quality GaN because it can undergo a dissociative chemisorption process. Ground state atomic N\\ (sp4S) is also good for growth. The doublet excited states N\\ (sp2D and sp2P) might cause surface N abstraction, leading to N vacancies in the material. Finally, a Fe(OH)sb3(Hsb2O)sb3 GVB cluster model of crystalline alpha-Fesb2Osb3 was developed. This simple model can describe the local geometry and bonding of Fe in the bulk oxide. Using quantum mechanical calculations, the orientation of the oleic imidazoline (OI) molecule bonding to the oxide surface has been determined. OI class of molecules are used extensively for corrosion inhibitor in oil field pipeline applications. It is found in this work that OI can make very strong bonding to the Fe of the iron oxide. In aqueous environments they can replace water on the pipe surface to form a protective layer to prevent corrosion.
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…
First Principle Study of Sodium Nanoclusters
NASA Astrophysics Data System (ADS)
Saxena, Prabodh Sahai; Srivastava, Pankaj; Shrivastava, Ashwani Kumar
2011-12-01
The structural and electronic properties of small Nan (n = 2-5) nanoclusters have been investigated by employing an ab-initio self-consistent density functional theory in the local density approximation. The total energy, binding energy, bond length and HOMO-LUMO gap are calculated in large energy interval for various isomeric forms of sodium nanoclusters. The results are compared with the other theoretical calculations.
First principles study of hydroxyapatite surface
NASA Astrophysics Data System (ADS)
Slepko, Alexander; Demkov, Alexander A.
2013-07-01
The biomineral hydroxyapatite (HA) [Ca10(PO4)6(OH)2] is the main mineral constituent of mammal bone. We report a theoretical investigation of the HA surface. We identify the low energy surface orientations and stoichiometry under a variety of chemical environments. The surface most stable in the physiologically relevant OH-rich environment is the OH-terminated (1000) surface. We calculate the work function of HA and relate it to the surface composition. For the lowest energy OH-terminated surface we find the work function of 5.1 eV, in close agreement with the experimentally reported range of 4.7 eV-5.1 eV [V. S. Bystrov, E. Paramonova, Y. Dekhtyar, A. Katashev, A. Karlov, N. Polyaka, A. V. Bystrova, A. Patmalnieks, and A. L. Kholkin, J. Phys.: Condens. Matter 23, 065302 (2011), 10.1088/0953-8984/23/6/065302].
Phonon Thermal Transport in Thermoelectric Materials from First Principles
NASA Astrophysics Data System (ADS)
Broido, David
2013-03-01
Breakthroughs in nanoscience and materials fabrication technology have led to the creation of materials with very low lattice thermal conductivity, a requirement for high thermoelectric efficiency. There is now an unprecedented need for quantitative, predictive theoretical approaches to provide fundamental understanding of lattice thermal transport in thermoelectric materials and insight into the design and development of new materials for enhanced thermoelectric applications. In this talk, I will describe our atomistic first principles approach for calculating lattice thermal conductivity of materials, which combines a complete solution of the Boltzmann transport equation for phonons with harmonic and anharmonic interatomic forces determined from density functional theory. I will present an overview of this theoretical approach along with some of our recent calculated results for a range of test materials such as Si, Ge and III-V compounds. I will also discuss results for thermoelectric alloys such as SixGe1-x and Mg2SixSn1-x and nanoparticle embedded in alloy thermoelectric (NEAT) materials. Finally, I will discuss insights gained from this effort such as the importance of anharmonic coupling of acoustic and optic phonon modes. This research was supported in part by NSF Grants CBET-1066634, CBET-1066404, by DARPA, by the S3TEC, an Energy Frontier Research Center funded by the US DOE, office of Basic Energy Sciences under award N0. DE-FG02-09ER46577, and by an EU IRG Grant.
First principles study of point defects in SnS.
Malone, Brad D; Gali, Adam; Kaxiras, Efthimios
2014-12-21
Photovoltaic cells based on SnS as the absorber layer show promise for efficient solar devices containing non-toxic materials that are abundant enough for large scale production. The efficiency of SnS cells has been increasing steadily, but various loss mechanisms in the device, related to the presence of defects in the material, have so far limited it far below its maximal theoretical value. In this work we perform first principles, density-functional-theory calculations to examine the behavior and nature of both intrinsic and extrinsic defects in the SnS absorber layer. We focus on the elements known to exist in the environment of SnS-based photovoltaic devices during growth. In what concerns intrinsic defects, our calculations support the current understanding of the role of the Sn vacancy (VSn) acceptor defect, namely that it is responsible for the p-type conductivity in SnS. We also present calculations for extrinsic defects and make extensive comparison to experimental expectations. Our detailed treatment of electrostatic correction terms for charged defects provides theoretical predictions on both the high-frequency and low-frequency dielectric tensors of SnS. PMID:25363023
A first-principle calculation of the XANES spectrum of Cu2+ in water
NASA Astrophysics Data System (ADS)
La Penna, G.; Minicozzi, V.; Morante, S.; Rossi, G. C.; Stellato, F.
2015-09-01
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 Cu2+ 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 Cu2+ 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.
Designing Interactive Learning Environments: An Approach from First Principles
ERIC Educational Resources Information Center
Scott, Bernard; Cong, Chunyu
2007-01-01
Purpose: Today's technology supports the design of more and more sophisticated interactive learning environments. This paper aims to argue that such design should develop from first principles. Design/methodology/approach: In the paper by first principles is meant: learning theory and principles of course design. These principles are briefly…
Designing Interactive Learning Environments: An Approach from First Principles
ERIC Educational Resources Information Center
Scott, Bernard; Cong, Chunyu
2007-01-01
Purpose: Today's technology supports the design of more and more sophisticated interactive learning environments. This paper aims to argue that such design should develop from first principles. Design/methodology/approach: In the paper by first principles is meant: learning theory and principles of course design. These principles are brieflyâ€¦
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 study of O defects in CdSe
NASA Astrophysics Data System (ADS)
T-Thienprasert, J.; Limpijumnong, S.; Du, M.-H.; Singh, D. J.
2012-08-01
Recently, the vibrational signatures related to oxygen defects in oxygen-doped CdSe were measured using ultrahigh resolution Fourier transform infrared (FTIR) spectroscopy by Chen et al.(2008) [1]. They observed two absorption bands centered at âˆ¼1991.77 and 2001.3 cm-1, which they attributed to the LVMs of OCd, in the samples grown with the addition of CdO and excess Se. For the samples claimed to be grown with even more excess Se, three high-frequency modes (1094.11, 1107.45, and 1126.33) were observed and assigned to the LVMs of OSe-VCd complex. In this work, we explicitly calculated the vibrational signatures of OCd and OSe-VCd complex defects based on first principles approach. The calculated vibrational frequencies of OCd and OSe-VCd complex are inconsistent with the frequencies observed by Chen et al., indicating that their observed frequencies are from other defects. Potential defects that could explain the experimentally observed modes are suggested.
First 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.
Electronic and vibrational properties of nickel sulfides from first principles
NASA Astrophysics Data System (ADS)
Wang, Jeng-Han; Cheng, Zhe; Brédas, Jean-Luc; Liu, Meilin
2007-12-01
We report the results of first-principles calculations (generalized gradient approximation-Perdew Wang 1991) on the electronic and vibrational properties of several nickel sulfides that are observed on Ni-based anodes in solid oxide fuel cells (SOFCs) upon exposure to H2S contaminated fuels: heazlewoodite Ni3S2, millerite NiS, polydymite Ni3S4, and pyrite NiS2. The optimized lattice parameters of these sulfides are within 1% of the values determined from x-ray diffraction. The electronic structure analysis indicates that all Ni-S bonds are strongly covalent. Furthermore, it is found that the nickel d orbitals shift downward in energy, whereas the sulfur p orbitals shift upward with increasing sulfur content; this is consistent with the decrease in conductivity and catalytic activity of sulfur-contaminated Ni-based electrodes (or degradation in SOFC performance). In addition, we systematically analyze the classifications of the vibrational modes at the ? point from the crystal symmetry and calculate the corresponding vibrational frequencies from the optimized lattice constants. This information is vital to the identification with in situ vibrational spectroscopy of the nickel sulfides formed on Ni-based electrodes under the conditions for SOFC operation. Finally, the effect of thermal expansion on frequency calculations for the Ni3S2 system is also briefly examined.
First Principles Calculations for X-ray Resonant Spectra and Elastic Properties
Yongbin Lee
2006-05-01
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{sub 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-principles theory of multipolar order in actinide dioxides
NASA Astrophysics Data System (ADS)
Magnani, Nicola; Suzuki, Michi-To; Oppeneer, Peter M.
2014-08-01
Magnetic phase transitions that involve multipolar degrees of freedom have been widely studied during the last couple of decades, challenging the common approximation which assumes that the physical properties of a magnetic material could be effectively described by purely dipolar degrees of freedom. Due to the complexity of the problem and to the large number of competing interactions involved, the simple (fcc) crystal structure of the actinide dioxides made them the ideal playground system for such theoretical and experimental studies. In the present paper, we summarize our recent attempts to provide an ab initio description of the ordered phases of UO2, NpO2, and AmO2 by means of state-of-the-art LDA+U first-principles calculations. This systematic analysis of the electronic structures is here naturally connected to the local crystalline fields of the 5f states in the actinide dioxide series. Related to these we find that the mechanisms which lead to the experimentally observed insulating ground states work in distinctly different ways for each compound. xml:lang="fr"
First principles Tafel kinetics of methanol oxidation on Pt(111)
NASA Astrophysics Data System (ADS)
Fang, Ya-Hui; Liu, Zhi-Pan
2015-01-01
Electrocatalytic methanol oxidation is of fundamental importance in electrochemistry and also a key reaction in direct methanol fuel cell. To resolve the kinetics at the atomic level, this work investigates the potential-dependent reaction kinetics of methanol oxidation on Pt(111) using the first principles periodic continuum solvation model based on modified-Poisson-Boltzmann equation (CM-MPB), focusing on the initial dehydrogenation elementary steps. A theoretical model to predict Tafel kinetics (current vs potential) is established by considering that the rate-determining step of methanol oxidation (to CO) is the first Csbnd H bond breaking (CH3OH(aq) ? CH2OH* + H*) according to the computed free energy profile. The first Csbnd H bond breaking reaction needs to overcome a large entropy loss during methanol approaching to the surface and replacing the adsorbed water molecules. While no apparent charge transfer is involved in this elementary step, the charge transfer coefficient of the reaction is calculated to be 0.36, an unconventional value for charge transfer reactions, and the Tafel slope is deduced to be 166 mV. The results show that the metal/adsorbate interaction and the solvation environment play important roles on influencing the Tafel kinetics. The knowledge learned from the potential-dependent kinetics of methanol oxidation can be applied in general for understanding the electrocatalytic reactions of organic molecules at the solid-liquid interface.
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.
First-Principles Calculations for Insulators at Constant Polarization
NASA Astrophysics Data System (ADS)
Diéguez, Oswaldo; Vanderbilt, David
2006-02-01
We develop an exact formalism for performing first-principles calculations for insulators at fixed electric polarization. As shown by Sai, Rabe, and Vanderbilt (SRV) [Phys. Rev. BPRBMDO0163-1829 66, 104108 (2002)10.1103/PhysRevB.66.104108], who designed an approximate method to tackle the same problem, such an approach allows one to map out the energy landscape as a function of polarization, providing a powerful tool for the theoretical investigation of polar materials. We apply our method to a system in which the ionic contribution to the polarization dominates (a broken-inversion-symmetry perovskite), one in which this is not the case (a III-V semiconductor), and one in which an additional degree of freedom plays an important role (a ferroelectric phase of KNO3). We find that while the SRV method gives rather accurate results in the first case, the present approach provides important improvements to the physical description in the latter cases.
First principles characterization of silicate sites in clay surfaces.
Alvim, Raphael S; Miranda, Caetano R
2015-02-21
Aluminosilicate clays like Montmorillonite (MMT) and Muscovite Mica (MT) have siloxane cavities on the basal plane. The hydroxyl groups localized in these cavities and van der Waals (vdW) forces contribute significantly to adsorption processes. However, the basal sites are found to be difficult to characterize experimentally. Here, (001) surfaces of MMT and MT clays were investigated using first-principles calculations to understand how these silicate surface sites are influenced by hydroxyl groups and the effective role of inner layer vdW interactions. Based on density-functional theory (DFT) within the generalized gradient approximation (GGA), different types of exchange-correlation functionals were tested to check the effect of vdW dispersion correction. Noncontact atomic force microscopy (nc-AFM), X-ray absorption spectroscopy (XAS) in the near-edge region and solid-state nuclear magnetic resonance (SS-NMR) spectroscopy were simulated. In both clays, the oxygen surface sites are directly affected by the intralayer interaction through hydroxyl groups. Our results indicated that the chemical environment of the hydroxyl groups is distinct in the MMT and MT structures. The vdW correction was essential for a better description of the surface oxygen sites and correctly describes the similarity between both clays. Particularly, the bulk apical oxygen sites in the MT structure are less influenced by vdW interaction. Compared to MMT, the silicon surface sites of MT are more sensitive to the intralayer changes in Si-Oapical-Al and with less effect of the hydroxyl groups. These results provide a clear understanding of influence of the siloxane cavity on the oxygen and silicon surface sites in aluminosilicates. PMID:25592132
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.
NMR characterization of hydrocarbon adsorption on calcite surfaces: a first principles study.
Bevilaqua, Rochele C A; Rigo, Vagner A; VerÃssimo-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 (101Â¯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
NMR characterization of hydrocarbon adsorption on calcite surfaces: A first principles study
Bevilaqua, Rochele C. A.; Miranda, Caetano R.; Rigo, Vagner A.; VerÃssimo-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 Â¯}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.
Theoretical Study of the Vibrational Spectroscopy of the Ethyl Radical
NASA Astrophysics Data System (ADS)
Tabor, Daniel P.; Sibert, Edwin. L. Sibert, Iii
2013-06-01
The rich spectroscopy of the ethyl radical has attracted the attention of several experimental and theoretical investigations. The purpose of these studies was to elucidate the signatures of hyperconjugation, torsion, inversion, and Fermi coupling in the molecular spectra. Due to the number of degrees of freedom in the system, previous theoretical studies have implemented reduced-dimensional models. Our ultimate goal is a full-dimensional theoretical treatment of the vibrations using both Van Vleck and variational approaches. The methods will be combined with the potential that we have calculated using the CCSD(T) method on the cc-pVTZ basis set. In this talk we will discuss our initial work, which builds up from these reduced-dimensional models. Our calculations use coordinates that exploit the system's G_{12} PI symmetry in a simple fashion. By systematically adding more degrees of freedom to our model, we can determine the effects of specific couplings on the spectroscopy. T. Häber, A. C. Blair, D. J. Nesbitt and M. D. Schuder J. Chem. Phys. {124}, 054316, (2006). G .E. Douberly, unpublished. R. S. Bhatta, A. Gao and D. S. Perry J. Mol. Struct.: THEOCHEM {941}, 22, (2010).
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.
Transport and first-principles study of novel thermoelectric materials
NASA Astrophysics Data System (ADS)
Chi, Hang
Thermoelectric materials can recover waste industrial heat and convert it to electricity as well as provide efficient local cooling of electronic devices. The efficiency of such environmentally responsible and exceptionally reliable solid state energy conversion is determined by the dimensionless figure-of-merit ZT = alpha2 sigmaT/kappa, where alpha is the Seebeck coefficient, sigma is the electrical conductivity, kappa is the thermal conductivity, and T is the absolute temperature. The goal of the thesis is to (i) illustrate the physics to achieve high ZT of advanced thermoelectric materials and (ii) explore fundamental structure and transport properties in novel condensed matter systems, via an approach combining comprehensive experimental techniques and state-of-the-art first-principles simulation methods. Thermo-galvanomagnetic transport coefficients are derived from Onsager's reciprocal relations and evaluated via solving Boltzmann transport equation using Fermi-Dirac statistics, under the relaxation time approximation. Such understanding provides insights on enhancing ZT through two physically intuitive and very effective routes: (i) improving power factor PF = alpha2sigma; and (ii) reducing thermal conductivity kappa, as demonstrated in the cases of Mg2Si1-xSnx solid solution and Ge/Te double substituted skutterudites CoSb3(1-x)Ge1.5x Te1.5x, respectively. Motivated by recent theoretical predictions of enhanced thermoelectric performance in highly mismatched alloys, ZnTe:N molecular beam epitaxy (MBE) films deposited on GaAs (100) substrates are carefully examined, which leads to a surprising discovery of significant phonon-drag thermopower (reaching 1-2 mV/K-1) at ~13 K. Further systematic study in Bi2Te3 MBE thin films grown on sapphire (0001) and/or BaF2 (111) substrates, reveal that the peak of phonon drag can be tuned by the choice of substrates with different Debye temperatures. Moreover, the detailed transport and structure studies of Bi2-xTl xTe3 single crystals demonstrate that thallium doping leads to a bulk insulating state for such a topological insulator, which opens an avenue for further investigations of transport phenomena related to surface states. Finally, using the combined theoretical and experimental approaches, a new layered transition metal dichalcogenide type of ground state of Cu 2Se is proposed, which exhibits extraordinary weak anti-localization type of magnetoresistance at liquid helium temperatures.
Static dielectric permittivity of ice from first principles.
Bonnet, NicÃ©phore; Marzari, Nicola
2014-12-12
The static permittivity of ice is computed from first principles as a function of the electric field, together with the generalized Kirkwood factor. The molecular dipole in ice is unambiguously obtained by an original method combining a slab approach and Berry phase calculations, and the fluctuations of the polarization are sampled by Monte Carlo runs using first-principles model Hamiltonians for different proton configurations. Common approximations in the exchange-correlation functionals overestimate the dielectric permittivity and enhance ferroelectric configurations and the Kirkwood factor, whereas dielectric saturation effects compare well with experiment. PMID:25541777
First-principles lattice dynamics, thermodynamics, and elasticity of Cr2O3
NASA Astrophysics Data System (ADS)
Wang, Yi; Fang, Huazhi; Zacherl, Chelsey L.; Mei, Zhigang; Shang, Shunli; Chen, Long-Qing; Jablonski, Paul D.; Liu, Zi-Kui
2012-09-01
We present the calculation of the lattice dynamics of chromia (Cr2O3), a typical Mott-Hubbard insulator, employing the first-principles density functional theory plus U approach. We first report the phonon dispersions at the theoretical equilibrium volume. Then the phonon density-of-states is calculated as a function of volume. Finally, the atomic volume, heat capacity, linear thermal expansion coefficient, bulk modulus, Grüneisen constant, and elastic constants are calculated as functions of temperature.
Hybrid first-principles/neural networks model for column flotation
Gupta, S.; Liu, P.H.; Svoronos, S.A.; Sharma, R.; Abdel-Khalek, N.A.; Cheng, Y.; El-Shall, H.
1999-03-01
A new model for phosphate column flotation is presented which for the first time relates the effects of operating variables such as frother concentration on column performance. This is a hybrid model that combines a first-principles model with artificial neural networks. The first-principles model is obtained from material balances on both phosphate particles and gangue (undesired material containing mostly silica). First-order rates of net attachment are assumed for both. Artificial neural networks relate the attachment rate constants to the operating variables. Experiments were conducted in a 6-in.-dia. (152-mm-dia.) laboratory column to provide data for neural network training and model validation. The model successfully predicts the effects of frother concentration, particle size, air flow rate and bubble diameter on grade and recovery.
First principles molecular dynamics of molten NaCl
NASA Astrophysics Data System (ADS)
Galamba, N.; Costa Cabral, B. J.
2007-03-01
First principles Hellmann-Feynman molecular dynamics (HFMD) results for molten NaCl at a single state point are reported. The effect of induction forces on the structure and dynamics of the system is studied by comparison of the partial radial distribution functions and the velocity and force autocorrelation functions with those calculated from classical MD based on rigid-ion and shell-model potentials. The first principles results reproduce the main structural features of the molten salt observed experimentally, whereas they are incorrectly described by both rigid-ion and shell-model potentials. Moreover, HFMD Green-Kubo self-diffusion coefficients are in closer agreement with experimental data than those predicted by classical MD. A comprehensive discussion of MD results for molten NaCl based on different ab initio parametrized polarizable interionic potentials is also given.
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 modeling of hard and soft matter
NASA Astrophysics Data System (ADS)
Car, Roberto
2013-03-01
Electronic and atomistic processes are key to bio-inspired functional materials and nanocatalysts for energy applications. This talk will review recent simulation studies and discuss the challenges that first-principles quantum mechanical approaches face when addressing these issues. Supported by DOE-DE-FG02-06ER-46344, DOE-DE-SC0008626, DOE-DE-SC0005180, and NSF-CHE-0956500.
First-principles total-energy calculation of gallium nitride
NASA Astrophysics Data System (ADS)
Min, B. J.; Chan, C. T.; Ho, K. M.
1992-01-01
A first-principles total-energy calculation is performed on gallium nitride (GaN). The equilibrium lattice parameters, the bulk modulus, and the cohesive energy of GaN in the wurtzite structure is calculated and compared with experimental values. In our calculation, the ground state of GaN is a zinc-blende structure, and the difference between these two phases is around 1.4 mRy.
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 hotmoreÂ Â» 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.Â«Â less
Evolutionary approach for determining first-principles hamiltonians
NASA Astrophysics Data System (ADS)
Hart, Gus L. W.; Blum, Volker; Walorski, Michael J.; Zunger, Alex
2005-05-01
Modern condensed-matter theory from first principles is highly successful when applied to materials of given structure-type or restricted unit-cell size. But this approach is limited where large cells or searches over millions of structure types become necessary. To treat these with first-principles accuracy, one 'coarse-grains' the many-particle Schrödinger equation into 'model hamiltonians' whose variables are configurational order parameters (atomic positions, spin and so on), connected by a few 'interaction parameters' obtained from a microscopic theory. But to construct a truly quantitative model hamiltonian, one must know just which types of interaction parameters to use, from possibly 106-108 alternative selections. Here we show how genetic algorithms, mimicking biological evolution ('survival of the fittest'), can be used to distil reliable model hamiltonian parameters from a database of first-principles calculations. We demonstrate this for a classic dilemma in solid-state physics, structural inorganic chemistry and metallurgy: how to predict the stable crystal structure of a compound given only its composition. The selection of leading parameters based on a genetic algorithm is general and easily applied to construct any other type of complex model hamiltonian from direct quantum-mechanical results.
Understanding and Predicting Thiolated Gold Nanoclusters from First Principles
Jiang, Deen
2010-01-01
This is an exciting time for studying thiolated gold nanoclusters. Single crystal structures of Au{sub 102}(SR){sub 44} and Au{sub 25}(SR){sub 18}{sup -} (-SR being an organothiolate group) bring both surprises and excitement in this field. First principles density functional theory (DFT) simulations turn out to be an important tool to understand and predict thiolated gold nanoclusters. In this review, I summarize the progresses made by us and others in applying first principles DFT to thiolated gold nanoclusters, as inspired by the recent experiments. First, I will give some experimental background on synthesis of thiolated gold nanoclusters, followed by a description of the recent experimental breakthroughs. Then I will introduce the superatom complex concept as a way to understand the electronic structure of thiolated gold nanoclusters or smaller nanoparticles. Next, I will describe in detail how first principles DFT is used to understand the Au-thiolate interface, predict structures for Au{sub 38}(SR){sub 24}, screen good dopants for the Au{sub 25}(SR){sub 18}{sup -} cluster, design the smallest magic thiolated gold cluster, and demonstrate the need for the trimer protecting motif. I will conclude with a grand challenge: the real time monitoring of nucleation of thiolated gold nanoclusters.
Atomic-scale structures in complex solids by Z-contrast STEM and first-principles theory
Pennycook, S.J.; Pantelides, S.T.; Maiti, A. |; Chisholm, M.F.; Yan, Y.
1998-02-01
Modern high-resolution scanning transmission electron microscopes are capable of forming electron probes below 2{angstrom}, sufficiently small to allow an atomic resolution Z-contrast image to be formed from many materials. Such images, formed with a high angle annular detector, are incoherent in nature, and can be described as a convolution of the probe intensity profile with an object function peaked sharply at the atomic sites. Unlike phase contrast microscopy, there is no phase problem associated with an incoherent image, and direct inversion to the projected atomic structure is possible. A quantitative method for structure retrieval is maximum entropy analysis, although under Scherzer incoherent conditions the probe tails are small, and peaks in the image intensity correspond closely to atomic positions. In such cases, an intuitive structure determination may be possible. Electron energy loss spectroscopy (EELS) may also be performed simultaneously, using the Z-contrast image to position the probe over selected atomic columns, providing complementary information on composition and electronic structure. On the theoretical side, present-day computers are now capable of electronic structure calculations that can be used to determine the preferred atomic arrangements in complex systems. Both first-principles and semiempirical approaches have been developed with complementary capabilities. Here the authors present several examples where a synergistic approach combining Z-contrast STEM, spatially resolved EELS and theoretical results have led to the elucidation of complex atomic structures. The ability to determine atomic structures experimentally to high accuracy provides both a perfect starting point for theoretical calculations and an ideal test of theoretical predictions. Theoretical studies can explore the new dimensions of time and energy. This combined approach leads to a detailed and comprehensive picture of complex atomistic mechanisms.
NASA Astrophysics Data System (ADS)
Wan, Quan; Galli, Giulia
2015-12-01
We present a first-principles framework to compute sum-frequency generation (SFG) vibrational spectra of semiconductors and insulators. The method is based on density functional theory and the use of maximally localized Wannier functions to compute the response to electric fields, and it includes the effect of electric field gradients at surfaces. In addition, it includes quadrupole contributions to SFG spectra, thus enabling the verification of the dipole approximation, whose validity determines the surface specificity of SFG spectroscopy. We compute the SFG spectra of ice Ih basal surfaces and identify which spectra components are affected by bulk contributions. Our results are in good agreement with experiments at low temperature.
First-principles 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.
First-principles constitutive equation for suspension rheology
NASA Astrophysics Data System (ADS)
Brader, J. M.; Cates, M. E.; Fuchs, M.
2012-08-01
We provide a detailed derivation of a recently developed first-principles approach to calculating averages in systems of interacting, spherical Brownian particles under time-dependent flow. Although we restrict ourselves to flows which are both homogeneous and incompressible, the time dependence and geometry (e.g., shear and extension) are arbitrary. The approximations formulated within mode-coupling theory are particularly suited to dense colloidal suspensions and capture the slow relaxation arising from particle interactions and the resulting glass transition to an amorphous solid. The delicate interplay between slow structural relaxation and time-dependent external flow in colloidal suspensions thus may be studied within a fully tensorial theory.
First principles calculations of doped MnBi compounds
NASA Astrophysics Data System (ADS)
Ababtin, Sultana Abdullah
We investigate the effect of the substitution of Ni, Ti and Co in MnBi using first principles calculations based on density functional theory (DFT) within the generalized gradient approximation (GGA). We also performed total energy calculations to compare different structures to determine the ground state structures and investigate their magnetic properties. Our calculation shows that the substitution of Ni, Co and Ti lowers the total magnetization of MnBi. We also found that the stable structure of Ni and Ti substitute is to replace Mn atoms in their regular site while the substitute Co is most stable when Co occupies the interstitial site of MnBi unit cell.
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 investigation of Nitrosyl formation in zirconia
Yu, Z.G.; Zhang, J.; Singh, David J; Wu, Ping
2012-01-01
We report first-principles calculations aimed at understanding the properties of nitrogen in ZrO{sub 2}. We find that interstitial N occurs covalently bonded to O in the form of NO units, in contrast to previous expectations of a N substitutional for O. This reveals a different chemistry for N in ZrO{sub 2} and perhaps other highly stable oxide species. This leads to a natural oxygen vacancy formation mechanism in ZrO{sub 2} in the presence of nitrogen.
First principles pseudopotential calculations on aluminum and aluminum alloys
Davenport, J.W.; Chetty, N.; Marr, R.B.; Narasimhan, S.; Pasciak, J.E.; Peierls, R.F.; Weinert, M.
1993-12-31
Recent advances in computational techniques have led to the possibility of performing first principles calculations of the energetics of alloy formation on systems involving several hundred atoms. This includes impurity concentrations in the 1% range as well as realistic models of disordered materials (including liquids), vacancies, and grain boundaries. The new techniques involve the use of soft, fully nonlocal pseudopotentials, iterative diagonalization, and parallel computing algorithms. This approach has been pioneered by Car and Parrinello. Here the authors give a review of recent results using parallel and serial algorithms on metallic systems including liquid aluminum and liquid sodium, and also new results on vacancies in aluminum and on aluminum-magnesium alloys.
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.
Diffusion in thorium carbide: A first-principles study
NASA Astrophysics Data System (ADS)
PÃ©rez 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.
Pentaprismane and hypostrophene from first-principles, with plane waves
NASA Astrophysics Data System (ADS)
Jenkins, S. J.; King, D. A.
2000-02-01
We present results of first-principles plane wave density functional studies of the polycyclic hydrocarbons pentaprismane and hypostrophene, which demonstrate, for the first time, that a plane wave basis can be successfully deployed with such strained molecules. Calculated structural parameters are found to be in good agreement with previous quantum chemical calculations and X-ray diffraction data (where available). A series of calculations for related C 10H 10 isomers, in which the five-fold rings of pentaprismane and hypostrophene are cleaved progressively further apart, are also reported. The way is now open for the calculation of the properties of these complex molecules on surfaces.
First principles calculations of shock compressed fluid helium.
Militzer, B
2006-10-27
The properties of hot dense helium at megabar pressures are studied with two first principles computer simulation techniques: path integral Monte Carlo simulation and density functional molecular dynamics. The simulations predict that the compressibility of helium is substantially increased by electronic excitations that are present in the hot fluid at thermodynamic equilibrium. A maximum compression ratio of 5.24(4)-fold the initial density was predicted for 360 GPa and 150,000 K. This result distinguishes helium from deuterium, for which simulations predicted a maximum compression ratio of 4.3(1). Hugoniot curves for statically precompressed samples are also discussed. PMID:17155480
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.
Ingber, M. S.; Mondy, L. A.; Graham, A.; Brenner, H.
2001-03-31
The objective of this research is to combine recent advances in high performance computing (HPC), theoretical mechanics, and parallel nonlinear algorithms to make fundamental advances in the ability to predict transport phenomena in concentrated, multiphase, dispersed systems from first principles. The. ability to accurately model multiphase flow is central to the development of many energy-related technologies such as transport of muds, cements, proppants, and produced solids in petroleum production; transport of coal slurry feedstocks and design of fluidized bed reactors in synfuel production; and the manufacture of semiconductors, turbine blades, and advanced composite materials for energy conservation.
First-Principles Molecular Structure Search with a Genetic Algorithm.
Supady, Adriana; Blum, Volker; Baldauf, Carsten
2015-11-23
The identification of low-energy conformers for a given molecule is a fundamental problem in computational chemistry and cheminformatics. We assess here a conformer search that employs a genetic algorithm for sampling the low-energy segment of the conformation space of molecules. The algorithm is designed to work with first-principles methods, facilitated by the incorporation of local optimization and blacklisting conformers to prevent repeated evaluations of very similar solutions. The aim of the search is not only to find the global minimum but to predict all conformers within an energy window above the global minimum. The performance of the search strategy is (i) evaluated for a reference data set extracted from a database with amino acid dipeptide conformers obtained by an extensive combined force field and first-principles search and (ii) compared to the performance of a systematic search and a random conformer generator for the example of a drug-like ligand with 43 atoms, 8 rotatable bonds, and 1 cis/trans bond. PMID:26484612
Step decoration studied with first-principles statistical mechanics
NASA Astrophysics Data System (ADS)
Zhang, Yongsheng; Reuter, Karsten
2008-03-01
With respect to oxidation catalysis or oxide formation, surface defects like steps, kinks, or vacancies are widely believed to play a decisive role, e.g. in form of active sites or as nucleation centers. Despite this suggested importance, first-principles investigations qualifying this role for gas-phase conditions that are representative of these applications are scarce. This is mostly due to the limitations of electronic-structure calculations in tackling the large system sizes and huge configuration spaces involved. We overcome these limitations with a first-principles statistical mechanics approach coupling density-functional theory (DFT) calculations with grand-canonical Monte Carlo simulations, and apply it to obtain the phase diagram of on-surface O adsorption at a (111) step on a Pd(100) surface. The link between the electronic and mesoscopic techniques is achieved by a lattice-gas Hamiltonian expansion, in which we parameterize the lateral interactions affected by the step from DFT calculations at a Pd(117) vicinal surface, and all remaining lateral interactions from calculations at Pd(100). For a wide range of O gas-phase conditions we find the (111) step to be decorated by a characteristic zig-zag structure. Intriguingly, this structure prevails even up to the elevated temperatures characteristic for catalytic combustion reactions, where only small amounts of disordered oxygen remain at the Pd(100) surface.
Static Dielectric Properties of Carbon Nanotubes from First Principles
Kozinsky, Boris; Marzari, Nicola N.
2006-04-24
The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. We characterize the response of isolated single-wall (SWNT) and multiwall (MWNT) carbon nanotubes and nanotube bundles to static electric fields using first-principles calculations and densityfunctional theory. The longitudinal polarizability of SWNTs scales as the inverse square of the band gap, while in MWNTs and bundles it is given by the sum of the polarizabilities of the constituent tubes. The transverse polarizability of SWNTs is insensitive to band gaps and chiralities and is proportional to the square of the effective radius; in MWNTs, the outer layers dominate the response. The transverse response is intermediate between metallic and insulating, and a simple electrostatic model based on a scale-invariance relation captures accurately the first-principles results. The dielectric response of nonchiral SWNTs in both directions remains linear up to very high values of applied field.
First-Principles Informed Thermodynamics of CRUD Deposition
NASA Astrophysics Data System (ADS)
O'Brien, Christopher John
The recent emphasis in the United States on developing abundant domestic sources of energy, together with an increasing awareness of the environmental hazards of fossil fuels, has led to a fresh look at the challenges of nuclear energy within the science and engineering community. One of these challenges is controlling the precipitation of porous oxide deposits onto the nuclear fuel rod cladding from the primary coolant during operation of pressurized light-water reactors (PWRs). These deposits, called CRUD (an acronym for Chalk River Unidentified Deposits), are a major concern to reactor operation because they reduce fuel lifetime and efficiency by reducing heat transfer to the coolant, promote corrosion, and depress neutron flux. This dissertation provides fundamental insights into the process by which CRUD is formed in PWRs by providing a framework linking the results of first-principles calculations to experimental data. The technique developed to facilitate the investigation is referred to as Density Functional Theory (DFT) referenced semi-empirical thermodynamics; It links 0K first-principles calculations with high temperature thermodynamics by redefining the reference chemical potentials of the constituent elements. The technique permits aqueous chemistry to be incorporated into thermodynamic calculations and allows for the prediction of temperature and pressure dependent free energies of materials that are experimentally inaccessible or have not yet been measured. The ability to extend accurate first-principles calculations to high temperatures and aqueous environments allows the stability of crystal surfaces, calculated with DFT techniques, to be predicted at conditions representative of an operating PWR. Accurate values of surface energies are used in fulfilling the principal goal of this dissertation, which is to investigate the aqueous thermodynamics of formation of nickel oxide (NiO) and nickel ferrite (NiFe 2O4) crystallites as representative CRUD components. Specifically, this dissertation investigates the thermodynamics of the homogeneous nucleation of crystallites and proposes a mechanism for their agglomeration. This work also contributes to a more complete understanding of CRUD by generating improved thermodynamic data to enable modeling of the incorporation of boron into CRUD through the formation of Bonaccordite (Ni2FeBO5) which has formed in PWRs operating at high-power conditions.
First principles calculations of thermal phonon transport in nanostructures
NASA Astrophysics Data System (ADS)
Jayasekera, Thushari; Calzolari, A.; Chen, Y. F.; Kim, K. W.; Buongiorno Nardelli, M.
2010-03-01
As the size of the electronic devices gets smaller, the power density associated with the devices becomes a very significant aspect that requires a great attention. The devices need to be designed in such a way that the heat removal from the system is efficient, thus, it is very important to understand the heat transport at the nanoscale for better thermal management. In this talk, we will discuss recent advances in the development of efficient techniques to compute the phonon contributions to thermal conductance from first principles. We will show examples of prototypical applications, ranging from atomic chains to graphene nanoribbon junctions and constrictions This work was supported, in part, by the NERC/NIST SWAN-NRI and the DARPA/HRL CERA programs.
First-principles calculations of Pt surface phonon dispersion curves
NASA Astrophysics Data System (ADS)
Hong, Sampyo; Rahman, Talat S.; Heid, Rolf; Bohnen, Klaus Peter
2003-03-01
We have calculated dispersion curves for surface phonons of Pt(100), Pt(110) and Pt(111), using first-principles, total energy calculations based on a mixed-basis set and norm-conserving pseudopotentials. Linear response theory and the harmonic approximation of lattice dynamics are also invoked. For bulk Pt our calculations show the experimentally observed anomaly along (110) direction, and the calculated relaxations on the three surfaces are also in agreement with previous results. The dispersion of the Rayleigh wave and the resonance modes on Pt(111) are in good agreement with data from He scattering measurements. Our results of phonon dispersion for Pt(100) and Pt(110) are for their unreconstructed surfaces. The richness in the phonon disersion curves for the three surfaces will be compared and conclusions presented about the changes in the surface force constants from the bulk values. The propensity of two of the surfaces to reconstruct will also be discussed.
Ziegler Natta heterogeneous catalysis by first principles computer experiments
NASA Astrophysics Data System (ADS)
Boero, M.; Parrinello, M.; Terakura, K.
1999-09-01
In this work we present a first attempt to study the polymerization process of ethylene in a realistic Ziegler-Natta heterogeneous system by means of first principles molecular dynamics. In particular, we simulate, in a very unbiased way, both the deposition of the catalyst TiCl 4 on the (110) active surface of a solid MgCl 2 support and the polymer chain formation. By using a constrained molecular dynamics approach, we work out the energetics and the reaction pathway of the polymerization process as it occurs in a laboratory or an industrial plant. The good agreement of the results of our simulations with the available experimental data indicates that these kinds of simulations can be used as a skilful approach to study the details of the reaction mechanism which are not accessible to experimental probes. This offers a tool to improve the production and/or to design reactants and products for practical use.
First principles modeling of panchromatic dyes for solar cells applications.
NASA Astrophysics Data System (ADS)
di Felice, Rosa; Calzolari, Arrigo; Dong, Rui; Buongiorno Nardelli, Marco
2013-03-01
The state-of-the-art dye in Grätzel solar cells, N719, exhibits a total solar-to-electric conversion efficiency of 11.2%. However, it severely lacks absorption in the red and the near infrared regions of the electromagnetic spectrum, which represent more than 70% of the solar radiation spectrum. Using calculations from first principles in the time-dependent domain, we have studied the electronic and optical response of a novel class of panchromatic sensitizers that can harvest solar energy efficiently across the visible and near infrared regions, which have been recently synthesized [A. El-Shafei, M. Hussain, A. Atiq, A. Islam, and L. Han, J. Mater. Chem. 22, 24048 (2012)]. Our calculations show that, by tuning the properties of antenna groups, one can achieve a substantial improvement of the optical properties.
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 studies of electron tunneling in proteins
Hayashi, Tomoyuki; Stuchebrukhov, Alexei A.
2014-01-01
A first principles study of electronic tunneling along the chain of seven Fe/S clusters in respiratory complex I, a key enzyme in the respiratory electron transport chain, is described. The broken-symmetry states of the Fe/S metal clusters calculated at both DFT and semi-empirical ZINDO levels were utilized to examine both the extremely weak electronic couplings between Fe/S clusters and the tunneling pathways, which provide a detailed atomistic-level description of the charge transfer process in the protein. One-electron tunneling approximation was found to hold within a reasonable accuracy, with only a moderate induced polarization of the core electrons. The method is demonstrated to be able to calculate accurately the coupling matrix elements as small as 10?4 cm?1. A distinct signature of the wave properties of electrons is observed as quantum interferences of multiple tunneling pathways. PMID:25383312
Ferroelectric Transitions at Ferroelectric Domain Walls Found from First Principles
NASA Astrophysics Data System (ADS)
Wojde?, Jacek C.; Íñiguez, Jorge
2014-06-01
We present a first-principles study of model domain walls (DWs) in prototypic ferroelectric PbTiO3. At high temperature the DW structure is somewhat trivial, with atoms occupying high-symmetry positions. However, upon cooling the DW undergoes a symmetry-breaking transition characterized by a giant dielectric anomaly and the onset of a large and switchable polarization. Our results thus corroborate previous arguments for the occurrence of ferroic orders at structural DWs, providing a detailed atomistic picture of a temperature-driven DW-confined transformation. Beyond its relevance to the field of ferroelectrics, our results highlight the interest of these DWs in the broader areas of low-dimensional physics and phase transitions in strongly fluctuating systems.
Terahertz dynamics of ferroelectric vortices from first principles
NASA Astrophysics Data System (ADS)
Gui, Zhigang; Bellaiche, L.
2014-02-01
A first-principles-based effective Hamiltonian is used to reveal dynamics of vortices in ferroelectrics. In addition to the "usual" dielectric modes that are generated by the fluctuation of the electrical polarization, additional toroidic modes, resulting from the electric toroidal moment fluctuations, are also found in the THz regime. Such latter modes can have their own dynamics, with a resonant frequency that softens via a square-root law when the temperature approaches the critical temperature at which electric vortices form. These dynamics are also found to be governed by the fluctuation of the self-organized azimuthal component of individual electric dipoles. Toroidic modes thus behave as pendulums, in contrast to springs that represent polarization dynamics.
Hydroxylated graphyne and graphdiyne: First-principles study
NASA Astrophysics Data System (ADS)
Zhang, Peiran; Ma, ShuangYing; Sun, L. Z.
2016-01-01
First-principles calculations are performed to study the favorable geometries and corresponding electronic properties of hydroxyl functional group (-OH) decorated monolayer graphyne and graphdiyne (MGY and MGDY). The energetically favorable high coverage -OH decoration configuration shows ordered chiral-like structure on adjacent big triangle carbon rings and hexagon due to the stabilization of the hydrogen bonding between these groups. In contrast to the hydrogenated graphyne and graphdiyne, it is an unstable geometry when all carbon bonds are saturated to single bond. It is found that -OH can introduce magnetism in the systems but the magnetic properties can only survive at low -OH concentration. -OH pair forming hydrogen bond between two -OH groups are usually nonmagnetic due to the antiferromagnetic coupling between them.
Helium diffusion in olivine based on first principles calculations
NASA Astrophysics Data System (ADS)
Wang, Kai; Brodholt, John; Lu, Xiancai
2015-05-01
As a key trace element involved in mantle evolution, the transport properties of helium in the mantle are important for understanding the thermal and chemical evolution of the Earth. However, the mobility of helium in the mantle is still unclear due to the scarcity of measured diffusion data from minerals under mantle conditions. In this study, we used first principles calculations based on density functional theory to calculate the absolute diffusion coefficients of the helium in olivine. Using the climbing images nudged elastic band method, we defined the diffusion pathways, the activation energies (Ea), and the prefactors. Our results demonstrate that the diffusion of helium has moderate anisotropy. The directionally dependent diffusion of helium in olivine can be written in Arrhenius form as follows.
First-principles calculation of mobility in silicon
NASA Astrophysics Data System (ADS)
Wu, Yuning; Zhang, X.-G.; Pantelides, Sokrates T.
2014-03-01
We introduce a new first-principles method to calculate Coulomb-scattering-limited electron mobility in silicon. The lifetime of a Bloch state due to scattering can be interpreted as arising from an additional imaginary part of electron self-energy. By introducing an artificial imaginary potential, the electron self-energy can be extracted from the complex band structure of a periodic system while eliminating the interference effect due to multiple scattering between impurities. This allows an implementation using density functional theory within the Quantum-Espresso package. The calculated electron mobility agrees with the experimental data. A portion of the research is conducted at the CNMS sponsored at ORNL by the Office of Basic Energy Sciences, U.S. Department of Energy.
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 âˆ¼2â€‰â€‰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
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.
Properties of molten sodium under pressure from first principles theory.
NASA Astrophysics Data System (ADS)
Raty, Jean-Yves; Schwegler, Eric; Bonev, Stanimir
2006-03-01
Recent measurements of the melting curve of sodium [1] have found a sharp decline in the melting temperatures from 1000 to 300 K in the pressure range from 30 to 120 GPa. In this study, we investigate the stability and structural properties of solid and liquid sodium at high pressure and temperature using first principles molecular dynamics. The experimental melting curve is reproduced from 0 to 120 GPa. The local structure of the liquid is found to be strongly correlated to the multiple finite temperature crystalline phases of sodium. Based on a quantitative analysis of the structural and electronic properties of the solid and liquid phases, we propose an explanation for the unusual melting curve and a new perspective on the phase diagram of sodium. [1] Gregoryantz et al., Phys. Rev. Lett. 94, 185502 (2005).
Hydrogen storage in LiH: A first principle study
Banger, Suman Nayak, Vikas Verma, U. P.
2014-04-24
First principles calculations have been performed on the Lithium hydride (LiH) using the full potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory. We have extended our calculations for LiH+2H and LiH+6H in NaCl structure. The structural stability of three compounds have been studied. It is found that LiH with 6 added Hydrogen atoms is most stable. The obtained results for LiH are in good agreement with reported experimental data. Electronic structures of three compounds are also studied. Out of three the energy band gap in LiH is âˆ¼3.0 eV and LiH+2H and LiH+6H are metallic.
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.
Auger recombination in sodium-iodide scintillators from first principles
McAllister, Andrew; Ã…berg, Daniel; Schleife, AndrÃ©; Kioupakis, Emmanouil
2015-04-06
Scintillator radiation detectors suffer from low energy resolution that has been attributed to non-linear light yield response to the energy of the incident gamma rays. Auger recombination is a key non-radiative recombination channel that scales with the third power of the excitation density and may play a role in the non-proportionality problem of scintillators. In this work, we study direct and phonon-assisted Auger recombination in NaI using first-principles calculations. Our results show that phonon-assisted Auger recombination, mediated primarily by short-range phonon scattering, dominates at room temperature. We discuss our findings in light of the much larger values obtained by numerical fits to z-scan experiments.
First-principles simulation of molecular dissociation-recombination equilibrium
Kylaenpaeae, Ilkka; Rantala, Tapio T.
2011-09-14
For the first time, the equilibrium composition of chemical dissociation-recombination reaction is simulated from first-principles, only. Furthermore, beyond the conventional ab initio Born-Oppenheimer quantum chemistry the effects from the thermal and quantum equilibrium dynamics of nuclei are consistently included, as well as, the nonadiabatic coupling between the electrons and the nuclei. This has been accomplished by the path integral Monte Carlo simulations for full NVT quantum statistics of the H{sub 3}{sup +} ion. The molecular total energy, partition function, free energy, entropy, and heat capacity are evaluated in a large temperature range: from below room temperature to temperatures relevant for planetary atmospheric physics. Temperature and density dependent reaction balance of the molecular ion and its fragments above 4000 K is presented, and also the density dependence of thermal ionization above 10 000 K is demonstrated.
Lattice instabilities of cubic NiTi from first principles
NASA Astrophysics Data System (ADS)
Huang, Xiangyang; Bungaro, Claudia; Godlevsky, Vitaliy; Rabe, Karin M.
2002-01-01
The phonon dispersion relation of NiTi in the simple cubic B2 structure is computed using first-principles density-functional perturbation theory with pseudopotentials and a plane-wave basis set. The structure is shown not to be a local energy minimum, so that its observation at high temperatures must be due to entropic stabilization by large fluctuations around the average atomic positions. Lattice instabilities are observed to occur across nearly the entire Brillouin zone, excluding three interpenetrating tubes of stability along the (001) directions and small spheres of stability centered at R. The strongest instability is that of the doubly degenerate M5' mode. The distorted structure produced by freezing a particular choice of unstable M5' eigenvector into the reference cubic structure yields, with an appropriate choice of overall amplitude, an excellent approximation to the observed ground-state B19' structure.
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 Î¼{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.
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.
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 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, 181â€“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, 152â€“158 (2000)], indicating the absence of the ODS attractive force.
Ferroelectric transitions at ferroelectric domain walls found from first principles.
WojdeÅ‚, Jacek C; ÃÃ±iguez, Jorge
2014-06-20
We present a first-principles study of model domain walls (DWs) in prototypic ferroelectric PbTiO(3). At high temperature the DW structure is somewhat trivial, with atoms occupying high-symmetry positions. However, upon cooling the DW undergoes a symmetry-breaking transition characterized by a giant dielectric anomaly and the onset of a large and switchable polarization. Our results thus corroborate previous arguments for the occurrence of ferroic orders at structural DWs, providing a detailed atomistic picture of a temperature-driven DW-confined transformation. Beyond its relevance to the field of ferroelectrics, our results highlight the interest of these DWs in the broader areas of low-dimensional physics and phase transitions in strongly fluctuating systems. PMID:24996110
Fundamental limits on transparency: first-principles calculations of absorption
NASA Astrophysics Data System (ADS)
Peelaers, Hartwin
2013-03-01
Transparent conducting oxides (TCOs) are a technologically important class of materials with applications ranging from solar cells, displays, smart windows, and touch screens to light-emitting diodes. TCOs combine high conductivity, provided by a high concentration of electrons in the conduction band, with transparency in the visible region of the spectrum. The requirement of transparency is usually tied to the band gap being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers can also lead to optical absorption by excitation of electrons to higher conduction-band states. A fundamental understanding of the factors that limit transparency in TCOs is essential for further progress in materials and applications. The Drude theory is widely used, but it is phenomenological in nature and tends to work poorly at shorter wavelengths, where band-structure effects are important. First-principles calculations have been performed, but were limited to direct transitions; as we show in the present work, indirect transitions assisted by phonons or defects actually dominate. Our calculations are the first to address indirect free-carrier absorption in a TCO completely from first principles. We present results for SnO2, but the methodology is general and is also being applied to ZnO and In2O3. The calculations provide not just quantitative results but also deeper insights in the mechanisms that govern absorption processes in different wavelength regimes, which is essential for engineering improved materials to be used in more efficient devices. For SnO2, we find that absorption is modest in the visible, and much stronger in the ultraviolet and infrared. Work performed in collaboration with E. Kioupakis and C.G. Van de Walle, and supported by DOE, NSF, and BAEF.
First-principles statistical mechanics for heterogeneous catalysis
NASA Astrophysics Data System (ADS)
Reuter, Karsten
2006-03-01
We present a first-principles approach to heterogeneous catalysis that quantitatively describes the activity over a wide range of realistic environmental situations of varying temperatures and pressures. Within a first-principles statistical mechanics setup [1], density-functional theory is first used together with transition state theory to accurately obtain the energetics of all relevant processes. Subsequently the statistical mechanics problem is solved by kinetic Monte Carlo simulations. This two-step approach enables us to gain microscopic insight into the system, following its full dynamics from picoseconds up to seconds and explicitly considering the detailed statistical interplay of all elementary processes, i.e., by fully accounting for the correlations, fluctuations and spatial distributions of the chemicals at the catalyst surface. In the application to CO oxidation at a RuO2(110) model catalyst, we compute the composition and structure of the catalyst surface in reactive environments ranging from ultra- high vacuum to technologically relevant conditions with pressures of the order of atmospheres and elevated temperatures [2]. For all these conditions the obtained conversion rates are in unprecedented quantitative agreement with existing experimental data. The catalytic activity is narrowly peaked in environments, where the surface kinetics builds a disordered and dynamic adsorbate composition at the surface. In the full concert of the large number of processes occurring in this active state, the chemical reaction with the most favorable energy barrier contributes only little to the overall CO2 production. [1] K. Reuter, D. Frenkel, and M. Scheffler, Phys. Rev. Lett. 93, 116105 (2004). [2] K. Reuter and M. Scheffler, Phys. Rev. B, submitted.
Pascal, Tod A.; Prendergast, David; Boesenberg, Ulrike; Kostecki, Robert; Richardson, Thomas J.; Weng, Tsu-Chien; Sokaras, Dimosthenis; Nordlund, Dennis; McDermott, Eamon; Moewes, Alexander; Cabana, Jordi; Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60605
2014-01-21
We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li{sub 2}SO{sub 4}, Li{sub 2}O, Li{sub 3}N, and Li{sub 2}CO{sub 3} using high resolution X-ray Raman spectroscopy (XRS) measurements and first-principles density functional theory calculations within the eXcited electron and Core Hole approach. Based on thermodynamic sampling via ab initio molecular dynamics simulations, we find calculated XAS in much better agreement with experiment than those computed using the rigid crystal structure alone. We show that local instantaneous distortion of the atomic lattice perturbs the symmetry of the Li 1s core-excited-state electronic structure, broadening spectral line-shapes and, in some cases, producing additional spectral features. The excellent agreement with high-resolution XRS measurements validates the accuracy of our first-principles approach to simulating XAS, and provides both accurate benchmarks for model compounds and a predictive theoretical capability for identification and characterization of multi-component systems, such as lithium-ion batteries, under working conditions.
Pascal, Tod A; Boesenberg, Ulrike; Kostecki, Robert; Richardson, Thomas J; Weng, Tsu-Chien; Sokaras, Dimosthenis; Nordlund, Dennis; McDermott, Eamon; Moewes, Alexander; Cabana, Jordi; Prendergast, David
2014-01-21
We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li2SO4, Li2O, Li3N, and Li2CO3 using high resolution X-ray Raman spectroscopy (XRS) measurements and first-principles density functional theory calculations within the eXcited electron and Core Hole approach. Based on thermodynamic sampling via ab initio molecular dynamics simulations, we find calculated XAS in much better agreement with experiment than those computed using the rigid crystal structure alone. We show that local instantaneous distortion of the atomic lattice perturbs the symmetry of the Li 1s core-excited-state electronic structure, broadening spectral line-shapes and, in some cases, producing additional spectral features. The excellent agreement with high-resolution XRS measurements validates the accuracy of our first-principles approach to simulating XAS, and provides both accurate benchmarks for model compounds and a predictive theoretical capability for identification and characterization of multi-component systems, such as lithium-ion batteries, under working conditions. PMID:25669363
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 Siâ€“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 507â€“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 177â€“212]. The most striking changes are of inter-tetrahedral Oâ€“O distances and Siâ€“Oâ€“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 Siâ€“Oâ€“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 (dÎ½i/dP) of the 12 Raman frequencies are obtained at 0 GPaâ€“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 study of the spectroscopy of Al2(+)
NASA Technical Reports Server (NTRS)
Rosi, Marzio; Bauschlicher, Charles W., Jr.; Langhoff, Stephen R.
1991-01-01
The electronic states of Al2(+) below about 40,000/cm are studied using a CASSCF/MRCI approach in a large Gaussian basis set. The computed spectroscopic constants, excitation energies, Einstein coefficients, and radiative lifetimes should give insight into the spectroscopy of this ion.
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 Fe 2 As 2 system. Both further examination of the underlying physics and development of new approximate functionals are needed. Supported by DOE/BES and NSF/DMR, computed at NERSC and UF/HPC.
First Principles Models for Ferroelectricity and Piezoelectricity in Solid Solutions
NASA Astrophysics Data System (ADS)
Cockayne, Eric
1998-03-01
The largest piezoelectric coefficients are observed not in pure compounds, but in solid solutions such as Pb(Zr_1-xTi_x)O3 and Pb(A_1/3Nb_2/3)O_3--PbTiO_3, (A = Zn, Mg)(S.-E. Park and T. R. Strout, J. Appl. Phys.) 82, 1804 (1997).. In this talk, we describe our first-principles approach to modeling ferroelectricity and piezoelectricity in solid solutions(E. Cockayne and K. M. Rabe, Phys. Rev. B) 56, 7947 (1997)., and apply it to Pb_1-xGe_xTe. By exploiting the microscopic information available from first principles calculations, we can identify factors that enhance piezoelectricity. For Pb_1-xGe_xTe, detailed ab initio calculations on various ordered supercells show significant differences in the low-temperature piezoelectric response of different ordered configurations, even at fixed composition. We have identified the following trends: (1) The highest piezoelectricity is often associated with the greatest deviation of local structure from average structure. Local relaxation occurs due to size mismatch between the substituting ions. Locally, this relaxation can have an effect similar to applying a large positive or negative local pressure to the corresponding pure system, driving it into a regime of large response. (2) As the strength of instability of the high symmetry prototype structure decreases, the piezoelectric response increases. Fitting of the ab initio results to double-well potentials explains the latter trend, showing that in the limit of marginal instability piezoelectricity diverges. Our results suggest the possibility of tuning ferroelectric and piezoelectric properties through controlling configurations. Typically, the piezoelectric response of a ferroelectric is sharply peaked near the paraelectric-ferroelectric transition temperature. In solid solutions, it becomes possible to obtain a large response over a range of temperatures, which is desirable for applications. We present an effective Hamiltonian approach to modeling the temperature dependence of the polarization and piezoelectricity in Pb_1-xGe_xTe.
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]. One contribution discusses the challenges of translating the results at the microscale into the macroscopic response of the material in a multiscale approach [Makov et al], and the issue closes with a discussion of neglected gaps in the first-principles modelling of defects, important problems that are commonly overlooked and perhaps deserving greater attention [Stoneham]. All papers were peer-reviewed following the standard procedure established by the Editorial Board of Modelling and Simulation in Materials Science and Engineering. Peter A Schultz Sandia National Laboratories, USA Guest Editor
First-principles study on surface stability of tantalum carbides
NASA Astrophysics Data System (ADS)
Yan, Wen-Li; Sygnatowicz, Michael; Lu, Guang-Hong; Liu, Feng; Shetty, Dinesh K.
2016-02-01
Using first-principles method, surface energies of crystal planes of different tantalum carbide phases have been calculated. Quantum size effects are shown to possibly play a considerable role in determining accurate surface energies of these metallic films, which have been neglected in previous works. The Î³-TaC phase has a more stable (0 0 1) surface than the close-packed (1 1 1) surface. In the Î±-Ta2C phase, (0 0 1) surface with only Ta termination is more stable than that of mixed Ta-C termination because the metallic bonds between Ta atoms are weaker than the Ta-C covalent bonds. The same is true for the Î¶-Ta4C3 phase. The introduction of structural vacancies in the Î¶-Ta4C3 -x phase creates more direct Ta metallic bonds, making the Ta-terminated surfaces even more stable. This is consistent with the experimental observations of cleavage of the basal planes, lamellae bridging of cracks, and the high fracture toughness of Î¶-Ta4C3 -x.
Anisotropic intrinsic lattice thermal conductivity of phosphorene from first principles.
Qin, Guangzhao; Yan, Qing-Bo; Qin, Zhenzhen; Yue, Sheng-Ying; Hu, Ming; Su, Gang
2015-02-21
Phosphorene, the single layer counterpart of black phosphorus, is a novel two-dimensional semiconductor with high carrier mobility and a large fundamental direct band gap, which has attracted tremendous interest recently. Its potential applications in nano-electronics and thermoelectrics call for fundamental study of the phonon transport. Here, we calculate the intrinsic lattice thermal conductivity of phosphorene by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations. The thermal conductivity of phosphorene at 300 K is 30.15 W m(-1) K(-1) (zigzag) and 13.65 W m(-1) K(-1) (armchair), showing an obvious anisotropy along different directions. The calculated thermal conductivity fits perfectly to the inverse relationship with temperature when the temperature is higher than Debye temperature (?D = 278.66 K). In comparison to graphene, the minor contribution around 5% of the ZA mode is responsible for the low thermal conductivity of phosphorene. In addition, the representative mean free path (MFP), a critical size for phonon transport, is also obtained. PMID:25594447
First-principles investigation of Ag-doped gold nanoclusters.
Zhang, Xiao-Dong; Guo, Mei-Li; Wu, Di; Liu, Pei-Xun; Sun, Yuan-Ming; Zhang, Liang-An; She, Yi; Liu, Qing-Fen; Fan, Fei-Yue
2011-01-01
Gold nanoclusters have the tunable optical absorption property, and are promising for cancer cell imaging, photothermal therapy and radiotherapy. First-principle is a very powerful tool for design of novel materials. In the present work, structural properties, band gap engineering and tunable optical properties of Ag-doped gold clusters have been calculated using density functional theory. The electronic structure of a stable Au(20) cluster can be modulated by incorporating Ag, and the HOMO-LUMO gap of Au(20-) (n)Ag(n) clusters is modulated due to the incorporation of Ag electronic states in the HOMO and LUMO. Furthermore, the results of the imaginary part of the dielectric function indicate that the optical transition of gold clusters is concentration-dependent and the optical transition between HOMO and LUMO shifts to the low energy range as the Ag atom increases. These calculated results are helpful for the design of gold cluster-based biomaterials, and will be of interest in the fields of radiation medicine, biophysics and nanoscience. PMID:21686162
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 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.
Electronic band engineering in epitaxial graphene: First principles calculations
NASA Astrophysics Data System (ADS)
Sirikumara, Hansika Iroshini
In this research work, we have investigated the band engineering of epitaxial graphene using first principles calculations. Epitaxial graphene on SiC (0001) surface is modified by using different methods such as intercalation, doping, passivation and oxidation. The calculations are done using Density functional theory which is implemented in quantum espresso package. In the presence of H intercalation, epitaxial graphene is shown to have p type behavior with monolayer graphene. However this behavior is different for multilayer epitaxial graphene systems, and it depended on the concentration of the H atoms. When epitaxial graphene is intercalated with Ge atoms, the Ge atoms make clusters and these clusters are responsible for the electronic properties of the epitaxial graphene systems. As a result of oxidation of epitaxial SiC surface, the graphene layer is mostly stable on the surface for both silicates and oxynitrides structures. For silicate/SiC configurations, the epitaxial graphene is shown to be less n type. For oxynitrides/ SiC configurations, epitaxial graphene is shown to be neutral. In the presence of oxygen intercalation with silicate/SiC, epitaxial graphene is shown to have p type behavior. These systematic studies of epitaxial graphene will opens up great potential for electronic applications. Additionally the resultant models can be used to guide further studies.
First principles calculations for Gd doped GaN
NASA Astrophysics Data System (ADS)
Mitra, Chandrima; Lambrecht, Walter
2008-03-01
Gd doped GaN has been reported by Dhar et al. to have magnetic moments of order a few 1000 ?B per Gd in the very dilute limit of 10^15 Gd/cm^3 and to show above room temperature ferromagnetism. Here we present first principle electronic structure calculations to study the spin splitting of the conduction band with varying concentration of Gd in GaN. Our calculations show that the spin splitting varies linearly with the concentration of Gd which suggests an almost zero splitting if one were to extrapolate to the 1 ppm dilute concentration of Gd. Thus the large magnetic moments cannot be explained simply by assuming donor electrons (for example from oxygen) will fill the spin-split conduction band. The spin polarization of the Ga and N atoms around Gd atom in a supercell of 1.5% Gd were found to be small and to become negligible beyond second nearest neighbors. In these, studies, we either added oxygen or Si as co-dopants or a background charge to fill the spin-split conduction band. This indicates that the proposed model of Dhar of polarization of the host is not supported by our calculations. The magnetic exchange interaction parameter, for nearest neighbour Gd atoms have also been calculated by mapping the energy differences between the ferromagnetic and antiferromagnetic arrangement onto the Heisenberg's model. Effects of strain, supercell size and shape, and other dopants on the exchange interactions were investigated.
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.
Chemistry and Deformation: First Principles Studies of Local Plasticity
NASA Astrophysics Data System (ADS)
Woodward, Christopher
2011-03-01
In order to understand the how chemistry influences deformation, an adequate description of the strain field near the center of dislocations (i.e. the core) is required. Continuum level descriptions of deformation ignore this short-range coupling between dislocations and the local atomic lattice. Interactions at this scale are non-linear and can strongly influence plastic deformation in fcc and bcc metals. Here, density functional theory is used in conjunction with a flexible boundary condition method to calculate the equilibrium dislocation core structure in a variety of bcc and fcc metals. The problem is divided into two parts: a solution for the nonlinear dislocation-core region and a solution for the long-range elastic response. Solving these individual problems is straightforward and by iteratively coupling the two solutions we can efficiently solve for the strain field in all space. Chemical effects, in the form of local solute-dislocation interactions, can also be calculated using this method. Derived solute-dislocation interactions are used to inform new models of solution hardening (and softening) in bcc Mo-X (X=Re, Pt) and fcc Al-X (X=Mg, Cr, Si, Cu) alloys. Currently, solute dislocation interactions are being assessed in bcc Fe-H alloys using this first principles technique.
Thermalisation of a quantum system from first principles
NASA Astrophysics Data System (ADS)
Ithier, Gregoire; Benaych-Georges, Florent
2015-03-01
How does a quantum system reach thermodynamical equilibrium? Answering such a question from first principles is, perhaps surprisingly, still an open issue (Popescu Nat. Phys. 2006, Goldstein PRL 2006, Genway PRL 2013). We present here a new model comprising an arbitrary quantum system interacting with a large arbitrary quantum environment, both initially prepared in a quantum pure state. We then demonstrate that thermalisation is an emergent property of the unitary evolution under a Schrödinger equation of this large composite system. The key conceptual tool of our method is the phenomenon of ``measure concentration'' appearing with functions defined on large dimension Hilbert spaces, a phenomenon which cancels out any effect of the microscopic structure of interaction Hamiltonians. Using our model, we first characterize the transient evolution or decoherence of the system and show its universal character. We then focus on the stationary regime and recover the canonical state well known from statistical thermodynamics. This finding leads us to propose an alternative and more general definition of the canonical partition function, that includes, among other things, the possibility of describing partial thermalisation.
Ions in solutions: Determining their polarizabilities from first-principles
NASA Astrophysics Data System (ADS)
Molina, John J.; Lectez, Sébastien; Tazi, Sami; Salanne, Mathieu; Dufrêche, Jean-François; Roques, Jérôme; Simoni, Eric; Madden, Paul A.; Turq, Pierre
2011-01-01
Dipole polarizabilities of a series of ions in aqueous solutions are computed from first-principles. The procedure is based on the study of the linear response of the maximally localized Wannier functions to an applied external field, within density functional theory. For most monoatomic cations (Li ^+, Na ^+, K ^+, Rb ^+, Mg ^{2+}, Ca ^{2+} and Sr ^{2+}) the computed polarizabilities are the same as in the gas phase. For Cs ^+ and a series of anions (F ^-, Cl ^-, Br ^- and I ^-), environmental effects are observed, which reduce the polarizabilities in aqueous solutions with respect to their gas phase values. The polarizabilities of H ^+_(aq), OH ^-_(aq) have also been determined along an ab initio molecular dynamics simulation. We observe that the polarizability of a molecule instantaneously switches upon proton transfer events. Finally, we also computed the polarizability tensor in the case of a strongly anisotropic molecular ion, UO _2^{2+}. The results of these calculations will be useful in building interaction potentials that include polarization effects.
Selenium adsorption on Mo(110): A first-principles investigation
NASA Astrophysics Data System (ADS)
Roma, Guido; Chiodo, Letizia
2013-06-01
Selenium adsorption on molybdenum surfaces is a relevant process in the production of thin-film solar cells, in particular as far as the formation of the layered compound MoSe2 is concerned. In this paper we investigate the energetics of Se adsorption on the (110) surface of molybdenum using first-principles calculations in the two limiting cases of low and high coverage, and we establish a comparison with the more extensively investigated case of sulfur adsorption at submonolayer coverage. The studied system provides the opportunity for testing the most crucial approximations, namely, the choice of the exchange-correlation functional and the pseudopotential generation. We find that semicore states of molybdenum have an influence on calculated surface energies and, to a lesser extent, on adsorption energies. We compare some more or less popular semilocal exchange-correlation functionals, including one recently proposed as an improvement for quasi-two-dimensional systems. The results show that the preferred adsorption site changes with coverage and suggest a strong variation of the adsorption energy with coverage.
Electrostatic engineering of strained ferroelectric perovskites from first principles
NASA Astrophysics Data System (ADS)
Cazorla, Claudio; Stengel, Massimiliano
2015-12-01
Design of novel artificial materials based on ferroelectric perovskites relies on the basic principles of electrostatic coupling and in-plane lattice matching. These rules state that the out-of-plane component of the electric displacement field and the in-plane components of the strain are preserved across a layered superlattice, provided that certain growth conditions are respected. Intense research is currently directed at optimizing materials functionalities based on these guidelines, often with remarkable success. Such principles, however, are of limited practical use unless one disposes of reliable data on how a given material behaves under arbitrary electrical and mechanical boundary conditions. Here we demonstrate, by focusing on the prototypical ferroelectrics PbTiO3 and BiFeO3 as test cases, how such information can be calculated from first principles in a systematic and efficient way. In particular, we construct a series of two-dimensional maps that describe the behavior of either compound (e.g., concerning the ferroelectric polarization and antiferrodistortive instabilities) at any conceivable choice of the in-plane lattice parameter, a , and out-of-plane electric displacement, D . In addition to being of immediate practical applicability to superlattice design, our results bring new insight into the complex interplay of competing degrees of freedom in perovskite materials and reveal some notable instances where the behavior of these materials depart from what naively is expected.
Solubility of nonelectrolytes: a first-principles computational approach.
Jackson, Nicholas E; Chen, Lin X; Ratner, Mark A
2014-05-15
Using a combination of classical molecular dynamics and symmetry adapted intermolecular perturbation theory, we develop a high-accuracy computational method for examining the solubility energetics of nonelectrolytes. This approach is used to accurately compute the cohesive energy density and Hildebrand solubility parameters of 26 molecular liquids. The energy decomposition of symmetry adapted perturbation theory is then utilized to develop multicomponent Hansen-like solubility parameters. These parameters are shown to reproduce the solvent categorizations (nonpolar, polar aprotic, or polar protic) of all molecular liquids studied while lending quantitative rigor to these qualitative categorizations via the introduction of simple, easily computable parameters. Notably, we find that by monitoring the first-order exchange energy contribution to the total interaction energy, one can rigorously determine the hydrogen bonding character of a molecular liquid. Finally, this method is applied to compute explicitly the Flory interaction parameter and the free energy of mixing for two different small molecule mixtures, reproducing the known miscibilities. This methodology represents an important step toward the prediction of molecular solubility from first principles. PMID:24773531
Coarse graining approach to First principles modeling of structural materials
Odbadrakh, Khorgolkhuu; Nicholson, Don M; Rusanu, Aurelian; Samolyuk, German D; Wang, Yang; Stoller, Roger E; Zhang, X.-G.; Stocks, George Malcolm
2013-01-01
Classical Molecular Dynamic (MD) simulations characterizing extended defects typically require millions of atoms. First principles calculations employed to understand these defect systems at an electronic level cannot, and should not deal with such large numbers of atoms. We present an e cient coarse graining (CG) approach to calculate local electronic properties of large MD-generated structures from the rst principles. We used the Locally Self-consistent Multiple Scattering (LSMS) method for two types of iron defect structures 1) screw-dislocation dipoles and 2) radiation cascades. The multiple scattering equations are solved at fewer sites using the CG. The atomic positions were determined by MD with an embedded atom force eld. The local moments in the neighborhood of the defect cores are calculated with rst-principles based on full local structure information, while atoms in the rest of the system are modeled by representative atoms with approximated properties. This CG approach reduces computational costs signi cantly and makes large-scale structures amenable to rst principles study. Work is sponsored by the USDoE, O ce of Basic Energy Sciences, Center for Defect Physics, an Energy Frontier Research Center. This research used resources of the Oak Ridge Leadership Computing Facility at the ORNL, which is supported by the O ce of Science of the USDoE under Contract No. DE-AC05-00OR22725.
First-Principle Characterization for Singlet Fission Couplings.
Yang, Chou-Hsun; Hsu, Chao-Ping
2015-05-21
The electronic coupling for singlet fission, an important parameter for determining the rate, has been found to be too small unless charge-transfer (CT) components were introduced in the diabatic states, mostly through perturbation or a model Hamiltonian. In the present work, the fragment spin difference (FSD) scheme was generalized to calculate the singlet fission coupling. The largest coupling strength obtained was 14.8 meV for two pentacenes in a crystal structure, or 33.7 meV for a transition-state structure, which yielded a singlet fission lifetime of 239 or 37 fs, generally consistent with experimental results (80 fs). Test results with other polyacene molecules are similar. We found that the charge on one fragment in the S1 diabatic state correlates well with FSD coupling, indicating the importance of the CT component. The FSD approach is a useful first-principle method for singlet fission coupling, without the need to include the CT component explicitly. PMID:26263271
First-principles modelling of magnesium titanium hydrides.
Er, Süleyman; van Setten, Michiel J; de Wijs, Gilles A; Brocks, Geert
2010-02-24
Mixing Mg with Ti leads to a hydride Mg(x)Ti((1 - x))H(2) with markedly improved (de)hydrogenation properties for x ? 0.8, as compared to MgH(2). Optically thin films of Mg(x)Ti((1 - x))H(2) have a black appearance, which is remarkable for a hydride material. In this paper we study the structure and stability of Mg(x)Ti((1 - x))H(2), x = 0-1 by first-principles calculations at the level of density functional theory. We give evidence for a fluorite to rutile phase transition at a critical composition x(c) = 0.8-0.9, which correlates with the experimentally observed sharp decrease in (de)hydrogenation rates at this composition. The densities of states of Mg(x)Ti((1 - x))H(2) have a peak at the Fermi level, composed of Ti d states. Disorder in the positions of the Ti atoms easily destroys the metallic plasma, however, which suppresses the optical reflection. Interband transitions result in a featureless optical absorption over a large energy range, causing the black appearance of Mg(x)Ti((1 - x))H(2). PMID:21386386
First-principles thermodynamic modeling of lanthanum chromate perovskites
NASA Astrophysics Data System (ADS)
Dalach, P.; Ellis, D. E.; van de Walle, A.
2012-01-01
Tendencies toward local atomic ordering in (A,A')(B,B')O3-? mixed composition perovskites are modeled to explore their influence on thermodynamic, transport, and electronic properties. In particular, dopants and defects within lanthanum chromate perovskites are studied under various simulated redox environments. (La1-x,Srx)(Cr1-y,Fey)O3-? (LSCF) and (La1-x,Srx)(Cr1-y,Ruy)O3-? (LSCR) are modeled using a cluster expansion statistical thermodynamics method built upon a density functional theory database of structural energies. The cluster expansions are utilized in lattice Monte Carlo simulations to compute the ordering of Sr and Fe(Ru) dopant and oxygen vacancies (Vac). Reduction processes are modeled via the introduction of oxygen vacancies, effectively forcing excess electronic charge onto remaining atoms. LSCR shows increasingly extended Ru-Vac associates and short-range Ru-Ru and Ru-Vac interactions upon reduction; LSCF shows long-range Fe-Fe and Fe-Vac interaction ordering, inhibiting mobility. First principles density functional calculations suggest that Ru-Vac associates significantly decrease the activation energy of Ru-Cr swaps in reduced LSCR. These results are discussed in view of experimentally observed extrusion of metallic Ru from LSCR nanoparticles under reducing conditions at elevated temperature.
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 300â€‰K, the thermal conductivity of monolayer silicene is found to be 9.4â€‰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.
A new model of spongy icing from first principles
NASA Astrophysics Data System (ADS)
Blackmore, R. Z.; Makkonen, L.; Lozowski, E. P.
2002-11-01
A new icing model has been developed to predict the sponginess (liquid fraction) and growth rate of freshwater ice accretions growing under a surface film of unfrozen water. This model is developed from first principles and does not require experimental sponginess data to tune the model parameters. The model identifies icing conditions that include no accretion, dry accretion, glaze accretion, spongy nonshedding, and spongy shedding regimes. It is a steady state model for a stationary vertical cylinder intercepting horizontally directed spray. The model predicts both the accretion mass growth flux and the accretion sponginess. The model results suggest that spongy shedding and spongy nonshedding regimes are common under the high liquid flux conditions typical of freshwater ship icing. Moreover, the unfrozen liquid incorporated into the spongy ice matrix can substantially increase the ice accretion load over that which would be predicted purely thermodynamically. Despite differences in the experimental setup, the model's performance compares well with two independent freshwater experimental data sets for icing on horizontal rotating cylinders. The model performs well in its prediction of both accretion sponginess and growth rate. The model predicts sponginess with a variation in liquid mass fraction of about 0.2-0.5, over the range of air temperature of 0°C to -30°C, in agreement with observations.
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.
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 (0.24â€“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.
NASA Astrophysics Data System (ADS)
Zhugayevych, Andriy; Tretiak, Sergei; Bazan, Guillermo
2013-03-01
We discuss the predictive power and accuracy of first principles modeling of small-molecule crystalline donors for organic solar cells. First of all, in order to understand where the theory can help us in improving the performance of photovoltaic devices, we clarify what factors constituting power conversion efficiency needed to be improved. We argue these are short circuit current and fill factor, rather than bandgap and open circuit voltage. This implies that the optimization of intramolecular properties (e.g. HOMO/LUMO), which is best suitable for theoretical search, will not give the anticipated gain in efficiency. The intermolecular properties are amenable to first principles modeling on a single-crystallite scale and we discuss some challenges in this avenue. As an example of how theory can provide design rules for architecturing small-molecule crystals we analyze the dependence of charge carrier mobility on the intermolecular geometry of a pi-stack. In the other case study we show that changes in device performance due to small changes in chemical composition can be well tracked by the theory. Finally, we analyze the performance of commonly used density functionals for typical molecular systems used in organic electronics (oligomers, polymers, dimers, crystals).
Achieving accuracy in first-principles calculations for EOS: basis completeness at high temperatures
NASA Astrophysics Data System (ADS)
Wills, John; Mattsson, Ann
2013-06-01
First-principles electronic structure calculations can provide EOS data in regimes of pressure and temperature where accurate experimental data is difficult or impossible to obtain. This lack, however, also precludes validation of calculations in those regimes. Factors that influence the accuracy of first-principles data include (1) theoretical approximations and (2) computational approximations used in implementing and solving the underlying equations. In the first category are the approximate exchange/correlation functionals and approximate wave equations approximating the Dirac equation; in the second are basis completeness, series convergence, and truncation errors. We are using two rather different electronic structure methods (VASP and RSPt) to make definitive the requirements for accuracy of the second type, common to both. In this talk, we discuss requirements for converged calculation at high temperature and moderated pressure. At convergence we show that both methods give identical results. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
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 electron density before it could reach the boundaries and reflect. The CAPs were zero in the region of the emitting structure and in regions were measurements were made. Still, we wanted to make sure that complex potentials were an accurate and efficient solution to the boundary problem. To evaluate this, we once again turned to benchmarks from transport calculations. The results showed CAPs to be an extremely accurate and efficient computational tool, and were incorporated into all of this dissertation's calculations. Our calculations show that adsorbate atoms significantly increase the field emission current of carbon nanotubes. Adsorbate atoms also cause strong differences and nonlinearity in the Fowler-Nordheim plot. The source of the enhanced current is electronic states introduced by the adsorbate atom. We have investigated the emission of electrons from nanostructures induced by short intense laser pulses in the presence of a weaker uniform static field. Based on the results of our simulations, two important qualitative features of this process have been determined: (1) a significant enhancement of the emission when a laser pulse is applied, and (2) the field emission current has a peak of some duration and the position of this peak correlates with the time of the pulse arrival. These two features suggest the possibility of using short laser pulses for making few-electron emitters of nanoscale size [212]. Such emitters could have many desirable properties, especially very high spatial and time resolutions. The field emission from nanotubes of various composition has been studied. The calculations predict that the GaN, SiC, and Si nanotubes are particularly good field emitters. The highest-current nanotube, Si, is predicted to produce a current an order of magnitude higher than BN or C nanotubes. The calculations predict that carbon nanotubes with various adsorbates can be used as spin-polarized current sources. These are the first first-principles calculations to show spin-polarized field emission for carbon nanotubes with iron adsorbates. Also, there i
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 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.
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 that require a data fidelity term based on the matching of DRRs and projections.
First-principles study of codoping in lanthanum bromide
NASA Astrophysics Data System (ADS)
Erhart, Paul; Sadigh, Babak; Schleife, André; Åberg, Daniel
2015-04-01
Codoping of Ce-doped LaBr3 with Ba, Ca, or Sr improves the energy resolution that can be achieved by radiation detectors based on these materials. Here, we present a mechanism that rationalizes this enhancement on the basis of first-principles electronic structure calculations and point defect thermodynamics. It is shown that incorporation of Sr creates neutral VBr-SrLa complexes that can temporarily trap electrons. As a result, Auger quenching of free carriers is reduced, allowing for a more linear, albeit slower, scintillation light yield response. Experimental Stokes shifts can be related to different CeLa-SrLa-VBr triple complex configurations. Codoping with other alkaline as well as alkaline-earth metals is considered as well. Alkaline elements are found to have extremely small solubilities on the order of 0.1 ppm and below at 1000 K. Among the alkaline-earth metals the lighter dopant atoms prefer interstitial-like positions and create strong scattering centers, which has a detrimental impact on carrier mobilities. Only the heavier alkaline-earth elements (Ca, Sr, Ba) combine matching ionic radii with sufficiently high solubilities. This provides a rationale for the experimental finding that improved scintillator performance is exclusively achieved using Sr, Ca, or Ba. The present mechanism demonstrates that codoping of wide-gap materials can provide an efficient means for managing charge carrier populations under out-of-equilibrium conditions. In the present case dopants are introduced that manipulate not only the concentrations but also the electronic properties of intrinsic defects without introducing additional gap levels. This leads to the availability of shallow electron traps that can temporarily localize charge carriers, effectively deactivating carrier-carrier recombination channels. The principles of this mechanism are therefore not specific to the material considered here but can be adapted for controlling charge carrier populations and recombination in other wide-gap materials.
Vibrational spectra of vitreous germania from first-principles
NASA Astrophysics Data System (ADS)
Giacomazzi, Luigi; Umari, P.; Pasquarello, Alfredo
2006-10-01
We report on a first-principles investigation of the structural and vibrational properties of vitreous germania (v-GeO2) . Our work focuses on a periodic model structure of 168 atoms, but three smaller models are also studied for comparison. We first carry out a detailed structural analysis both in real and reciprocal spaces. Our study comprises the partial pair correlation functions, the angular distributions, the total neutron correlation function, the neutron and x -ray total structure factors, and the Faber-Ziman and Bhatia-Thornthon partial structure factors. We find overall good agreement with available experimental data. We then obtain the vibrational frequencies and eigenmodes. We analyze the vibrational density of states in terms of Ge and O motions, and further in terms of rocking, bending, and stretching contributions. The inelastic neutron spectrum is found to differ only marginally from the vibrational density of states. Using a methodology based on the application of finite electric fields, we derive dynamical Born charge tensors and Raman coupling tensors. For the infrared spectra, we calculate the real and imaginary parts of the dielectric function, including the high-frequency and static dielectric constants. The Raman spectra are shown to be sensitive to the medium-range structure and support an average Ge-O-Ge angle of 135°. We identify the shoulder X2 as a signature of breathing O vibrations in three-membered rings. Four-membered rings are found to contribute to the main Raman peak. We advance an interpretation for the shoulder X1 in terms of delocalized bond-bending modes. We derive bond polarizability parameters from the calculated Raman coupling tensors and demonstrate their level of reliability in reproducing the spectra. The calculated vibrational spectra all show good agreement with the respective experimental spectra.
First-principles investigation of hydrous post-perovskite
NASA Astrophysics Data System (ADS)
Townsend, Joshua P.; Tsuchiya, Jun; Bina, Craig R.; Jacobsen, Steven D.
2015-07-01
A stable, hydrogen-defect structure of post-perovskite (hy-ppv, Mg1-xSiH2xO3) has been determined by first-principles calculations of the vibrational and elastic properties up to 150 GPa. Among three potential hy-ppv structures analyzed, one was found to be stable at pressures relevant to the lower-mantle Dâ€³ region. Hydrogen has a pronounced effect on the elastic properties of post-perovskite due to magnesium defects associated with hydration, including a reduction of the zero-pressure bulk (K0) and shear (G0) moduli by 5% and 8%, respectively, for a structure containing âˆ¼1 wt.% H2O. However, with increasing pressure the moduli of hy-ppv increase significantly relative to ppv, resulting in a structure that is only 1% slower in bulk compressional velocity and 2.5% slower in shear-wave velocity than ppv at 120 GPa. In contrast, the reduction of certain anisotropic elastic constants (Cij) in hy-ppv increases with pressure (notably, C55, C66, and C23), indicating that hydration generally increases elastic anisotropy in hy-ppv at Dâ€³ pressures. Calculated infrared absorption spectra show two O-H stretching bands at âˆ¼3500 cm-1 that shift with pressure to lower wavenumber by about 2 cm-1/GPa. At 120 GPa the hydrogen bonds in hy-ppv are still asymmetric. The stability of a hy-ppv structure containing 1-2 wt.% H2O at Dâ€³ pressures implies that post-perovskite may be a host for recycled or primordial hydrogen near the Earth's core-mantle boundary.
Wan, Quan; Galli, Giulia
2015-12-11
We present a first-principles framework to compute sum-frequency generation (SFG) vibrational spectra of semiconductors and insulators. The method is based on density functional theory and the use of maximally localized Wannier functions to compute the response to electric fields, and it includes the effect of electric field gradients at surfaces. In addition, it includes quadrupole contributions to SFG spectra, thus enabling the verification of the dipole approximation, whose validity determines the surface specificity of SFG spectroscopy. We compute the SFG spectra of ice I_{h} basal surfaces and identify which spectra components are affected by bulk contributions. Our results are in good agreement with experiments at low temperature. PMID:26705645
Investigating Orientational Defects in Energetic Material RDX Using First-Principles Calculations.
Pal, Anirban; Meunier, Vincent; Picu, Catalin R
2016-03-24
Orientational defects are molecular-scale point defects consisting of misaligned sterically trapped molecules. Such defects have been predicted in Î±-RDX using empirical force fields. These calculations indicate that their concentration should be higher than that of vacancies. In this study we confirm the stability of a family of four orientational defects in Î±-RDX using first-principles calculations and evaluate their formation energies and annealing barrier heights. The charge density distribution in the defective molecules is evaluated and it is shown that all four orientational defects exhibit some level of charge reduction at the midpoint of the N-N bond, which has been previously related to the sensitivity to initiation of the material. We also evaluate the vibrational spectrum of the crystal containing orientational defects and observe band splitting relative to the perfect crystal case. This may assist the experimental identification of such defects by Raman spectroscopy. PMID:26943238
First-principles calculations of Raman spectra in Li-doped Si nanocrystals
NASA Astrophysics Data System (ADS)
Scott Bobbitt, N.; Chelikowsky, James R.
2016-02-01
We examine the vibrational and Raman spectra for Li doped Si nanocrystals using real-space pseudopotentials constructed within density functional theory. We calculate differences in the Raman spectra using the Placzek approximation. The insertion of Li atoms into Si nanocrystals disrupts the Si crystal structure forming a region of Li-Si alloy in which the regular crystal structure is significantly disrupted. The Raman spectrum for this alloy exhibits a Li induced peak at 440-480 cm-1. We find an accompanying reduction in the size of the dominant bulk-like Si peak at 520 cm-1. Both of these results are consistent with experiment. Our analysis of the calculated spectrum confirms the utility of using Raman spectroscopy, coupled with first principle computations, to predict the structural and electronic properties of Li doped Si nanocrystals.
Mechanisms of molecular doping of graphene: A first-principles study
NASA Astrophysics Data System (ADS)
Saha, Srijan Kumar; Chandrakanth, Reddy Ch.; Krishnamurthy, H. R.; Waghmare, U. V.
2009-10-01
Doping graphene with electron donating or accepting molecules is an interesting approach to introduce carriers into it, analogous to electrochemical doping accomplished in graphene when used in a field-effect transistor. Here, we use first-principles density-functional theory to determine changes in the electronic-structure and vibrational properties of graphene that arise from the adsorption of aromatic molecules such as aniline and nitrobenzene. Identifying the roles of various mechanisms of chemical interaction between graphene and a molecule, we bring out the contrast between electrochemical and molecular doping of graphene. Our estimates of various contributions to shifts in the Raman-active modes of graphene with molecular doping are fundamental to the possible use of Raman spectroscopy in (a) characterization of the nature and concentration of carriers in graphene with molecular doping, and (b) graphene-based chemical sensors.
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.
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, ? , were calculated with first-principles methods for ferroelectric transition metal prototypical ABO3 perovskites [SrTiO3, BaTiO 3, PbTiO3 and PbZrO3] and A(B,B')O3 perovskite alloys Pb(Zr1/2Ti1/2)O3 (PZT) and Pb(Mg1/3Nb2/3)O3 (PMN). The principal findings are 1) a large anisotropy between deshielded sigma xx(O) ? sigmayy(O) and shielded sigma zz(O) components; 2) a nearly linear dependence on nearest-distance transition-metal/oxygen bond length, rs, was found for both isotropic deltaiso(O) and axial deltaax(O) chemical shifts ( d?=? reference- ? ), across all the systems studied, with deltaiso(O) varying by ? 400 ppm; 3) the demonstration that the anisotropy and linear variation arise from large paramagnetic contributions to sigmaxx(O) and sigmayy(O), due to virtual transitions between O(2p) and unoccupied B(nd) states. Using these results, an argument against Ti clustering in PZT, as conjectured from recent 17O NMR magic-angle-spinning measurements, is made. The linear dependence of the chemical shifts on rs provides a scale for determining transition-metal/oxygen bond lengths from experimental 17O NMR spectra. As such, it can be used to assess the degree of local tetragonality in perovskite solid solutions for piezoelectric applications. Results for transition metal atoms show less structural sensitivity, compared to 17O NMR, in homovalent B-site materials, but could be more useful in heterovalent B-site perovskite alloys. This work shows that both 17O and B-site NMR spectroscopy, coupled with first principles calculations, can be an especially useful probe of local structure in complex perovskite alloys.
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] Taku Tsuchiya, Renata M. Wentzcovitch, Cesar R.S. da Silva, and Stefano de Gironcoli, Phys. Rev. Lett. 96, 198501 (2006). [4] Han Hsu, Peter Blaha, Matteo Cococcioni, and Renata M. Wentzcovitch, Phys. Rev. Lett. 106, 118501 (2011).
Subsystem-based theoretical spectroscopy of biomolecules and biomolecular assemblies.
Neugebauer, Johannes
2009-12-21
The absorption properties of chromophores in biomolecular systems are subject to several fine-tuning mechanisms. Specific interactions with the surrounding protein environment often lead to significant changes in the excitation energies, but bulk dielectric effects can also play an important role. Moreover, strong excitonic interactions can occur in systems with several chromophores at close distances. For interpretation purposes, it is often desirable to distinguish different types of environmental effects, such as geometrical, electrostatic, polarization, and response (or differential polarization) effects. Methods that can be applied for theoretical analyses of such effects are reviewed herein, ranging from continuum and point-charge models to explicit quantum chemical subsystem methods for environmental effects. Connections to physical model theories are also outlined. Prototypical applications to optical spectra and excited states of fluorescent proteins, biomolecular photoreceptors, and photosynthetic protein complexes are discussed. PMID:19911405
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}.
The stability of free-standing germanane in oxygen: First-principles investigation
NASA Astrophysics Data System (ADS)
Liu, G.; Liu, S. B.; Xu, B.; Ouyang, C. Y.; Li, L. X.; Song, H. Y.
2015-04-01
The O2 dissociation and O2-dissociation-induced O atoms adsorption on free-standing germanane are studied by using first-principles calculations in this letter. In comparison to the extremely active silicene or silicane with energy barrier of 0.26 eV in oxygen, germanane is more stable than silicene or silicane from the kinetic point of view. Moreover, the most favorable adsorption of O atom on germanene is different to that on silicene or silicane, resulting in a strong OH-group on the germanane surface. Furthermore, the migration and desorption of O atoms are very difficult under room temperature in the O-adsorbed germanane, which is in favor of forming germoxene. The results provide convincing theoretical evidence to show that free-standing gemanane is relatively stable in oxygen,which is different to silicene essentially.
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 calculations on thermodynamic properties of BaTiO3 rhombohedral phase.
Bandura, Andrei V; Evarestov, Robert A
2012-07-01
The calculations based on the linear combination of atomic orbitals have been performed for the low-temperature phase of BaTiO(3) crystal. Structural and electronic properties, as well as phonon frequencies were obtained using hybrid PBE0 exchange-correlation functional. The calculated frequencies and total energies at different volumes have been used to determine the equation of state and thermal contribution to the Helmholtz free energy within the quasiharmonic approximation. For the first time, the bulk modulus, volume thermal expansion coefficient, heat capacity, and GrÃ¼neisen parameters in BaTiO(3) rhombohedral phase have been estimated at zero pressure and temperatures form 0 to 200 K, based on the results of first-principles calculations. Empirical equation has been proposed to reproduce the temperature dependence of the calculated quantities. The agreement between the theoretical and experimental thermodynamic properties was found to be satisfactory. PMID:22514059
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 prediction of the deformation modes in austenitic Fe-Cr-Ni alloys
NASA Astrophysics Data System (ADS)
Li, Wei; Lu, Song; Kim, Dongyoo; Kokko, Kalevi; Hertzman, Staffan; Kwon, Se Kyun; Vitos, Levente
2016-02-01
First-principles alloy theory is used to establish the Î³-surface of Fe-Cr-Ni alloys as function of chemical composition and temperature. The theoretical stacking fault energy (SFE) versus chemistry and temperature trends agree well with experiments. Combining our results with the recent plasticity theory based on the Î³-surface, the stacking fault formation is predicted to be the leading deformation mechanism for alloys with effective stacking fault energy below Ëœ18 mJ m-2. Alloys with SFE above this critical value show both twinning and full slip at room temperature. Interestingly, twinning remains a possible deformation mode in addition to full slip even at elevated temperatures, in line with observations.
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.
Discharge Reaction Mechanisms in Na/FeS2 Batteries: First-Principles Calculations
NASA Astrophysics Data System (ADS)
Momida, Hiroyoshi; Kitajou, Ayuko; Okada, Shigeto; Yamashita, Tomoki; Oguchi, Tamio
2015-12-01
We have studied microscopic discharge reaction mechanisms in Na/FeS2 batteries by first-principles calculations. The calculated Na-Fe-S phase diagram shows that the discharge reactions can proceed by converting 4Na and FeS2 into 2Na2S and Fe as a fully discharged state. As an intermediate discharge reaction, we find that NaxFeS2 (x ˜ 1.5) intermediate products can be generated in the cathode, giving two major plateaus in voltage-capacity curves. The calculated voltage-capacity characteristics and X-ray absorption spectra at S and Fe K-edges of Na-discharged FeS2 cathode materials are compared with experimental results, showing that theoretically determined reaction formulas can account for the experimental discharge reactions.
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.
A first-principles study on the helium doped grain boundary in metal Al
NASA Astrophysics Data System (ADS)
Chen, J.; Long, Y.
2012-10-01
The He doped Al ?3 grain boundary (GB) is investigated by using a first principles method. The segregation energy and elastic coefficient of He doped Al ?3 GB are calculated. The theoretical tensile tests on the He + ?3 GB are implemented and the calculated stress-strain curve shows that He doping will reduce the strength of the material and induces a double stress maxima effect in the fracture process. The bondlength and charge distribution analysis indicate that the reducing strength of the He-doped GB model can be attributed to the weak interaction of the Al-He bond and the weakening of the Al-Al bond nearby the He atom.
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.
NASA Astrophysics Data System (ADS)
Kang, Wei; Zhao, Shijun; Zhang, Shen; Zhang, Ping; Chen, Q. F.; He, Xian-Tu
2016-02-01
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.
Jiang, Xue; Zhao, Jijun; Jiang, Xin
2011-10-01
The atomic and electronic structures, heat of formation, Young's modulus, and ideal strength of hydrogenated diamond nanowires (DNWs) with different cross-sections (from 0.06 to 2.80 nm(2)) and crystallographic orientations ((100), (110), (111), and (112)) have been investigated by means of first-principles simulations. For thinner DNWs (cross-sectional area less than 0.6 nm(2)), preferential growth orientation along (111) is observed. The Young's modulus and ideal strength of these DNWs decrease with decreasing cross-section and show anisotropic effects. Moreover, the band gap of DNWs is sensitive to the size, crystallographic orientation and tensile strain, implying the possibility of a tunable gap. The effective mass at the edges of the conduction band and valence band are also obtained. These theoretical results are helpful for designing novel optoelectronic devices and electromechanical sensors using diamond nanowires. PMID:21911933
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 Kâ€“1200 K. Kulwinder Kaur thanks Council of Scientific & Industrial Research (CSIR), India for providing fellowship.
Order-disorder phase boundary in Ice VII-VIII investigated by first principles
NASA Astrophysics Data System (ADS)
Wentzcovitch, Renata; Umemoto, Koichiro; de Gironcoli, Stefano; Baroni, Stefano
2010-03-01
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. Our calculation accounts well for several important aspects: equation of state of ice VII, negative Clapeyron slope of the phase boundary, and the isotope effect. We will discuss also some factors to be improved, including XC functionals. Research was supported by NSF grants EAR 0810272, EAR 0635990, ATM 0428774 (VLab), EAR 0757903.
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
Obtaining Mixed-Basis Ising-Like Expansions of Binary Alloys from First Principles
NASA Astrophysics Data System (ADS)
Hart, Gus L. W.; Sanati, Mahdi; Wang, Ligen; Zunger, Alex
2002-03-01
Many electronic and structural properties of A_1-xBx alloys can be predicted theoretically if one can find (and quickly compute) the ``configurational energy function''--that is, the energy for any given configuration of A and B atoms on the crystal lattice. Cluster expansion methods provide one such approach. We describe our mixed-basis cluster expansion (MBCE) based on first-principles total energy calculations for only a few ordered A_mBn compounds. Our MBCE can robustly predict a variety of material properties including ground states, phase diagrams, precipitate formation, etc. Specifically, we illustrate how systematic choice of interaction parameters, numerical parameters, and choice of input structures can significantly increase the accuracy and the predictive capability of the expansion. We illustrate how the fit of LDA data can be done essentially automatically. Examples include Cu-Au, Ni-Pt, and Sc_1-xBox_xS.
First-principles approach to investigate toroidal property of magnetoelectric multiferroic GaFeO3
NASA Astrophysics Data System (ADS)
Nie, Yung-mau
2016-01-01
A first-principles approach incorporating the concept of toroidal moments as a measure of the spin vortex is proposed and applied to simulate the toroidization of magnetoelectric multiferroic GaFeO3. The nature of space-inversion and time-reversal violations of ferrotoroidics is reproduced in the simulated magnetic structure of GaFeO3. For undoped GaFeO3, a toroidal moment of -22.38 ?B Å per unit cell was obtained, which is the best theoretical estimate till date. Guided by the spin vortex free-energy minimization perturbed by an externally applied field, it was discovered that the minority spin markedly biases the whole toroidization. In summary, this approach not only calculates the toroidal moment but provides a way to understand the toroidal nature of magnetoelectric multiferroics.
First-principles study on phase transition and ferroelectricity in lithium niobate and tantalate
NASA Astrophysics Data System (ADS)
Toyoura, Kazuaki; Ohta, Masataka; Nakamura, Atsutomo; Matsunaga, Katsuyuki
2015-08-01
The phase transitions and ferroelectricity of LiNbO3 and LiTaO3 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 dxz-px/dyz-py orbitals (Ï€ orbitals), from the electronic point of view.
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 (?/?) and power-factor (S2?/?) 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.
Sulfur dioxide adsorbed on graphene and heteroatom-doped graphene: a first-principles study
NASA Astrophysics Data System (ADS)
Shao, Li; Chen, Guangde; Ye, Honggang; Wu, Yelong; Qiao, Zhijuan; Zhu, Youzhang; Niu, Haibo
2013-02-01
The adsorption of sulfur dioxide (SO2) on intrinsic graphene and heteroatom-doped (B, N, Al, Si, Cr, Mn, Ag, Au, and Pt) graphene samples was theoretically studied using first-principles approach based on density functional theory to exploit their potential applications as SO2 gas sensors. The structural and electronic properties of the graphene-molecule adsorption adducts are strongly dependent on the dopants. SO2 molecule is adsorbed weakly on intrinsic graphene, and B-, N-doped graphene; in general, strong chemisorption is observed on Al-, Si-, Cr-, Mn-, Ag-, Au-, and Pt-doped graphene. The adsorption mechanisms are discussed from charge transfers and density of states. This work reveals that the sensitivity of graphene-based chemical gas sensors for SO2 can be drastically improved by introducing appropriate dopant, and Cr, as well as Mn, may be the best choices among all the dopants.
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 1957â€…cm{sup âˆ’1} for CNT(6,6) while it is 596 to 658â€…cm{sup âˆ’1} for SiNT.
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 Î¼{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 calculation of dielectric response in molecule-based materials.
Heitzer, Henry M; Marks, Tobin J; Ratner, Mark A
2013-07-01
The dielectric properties of materials are of fundamental significance to many chemical processes and the functioning of numerous solid-state device technologies. While experimental methods for measuring bulk dielectric constants are well-established, far less is known, either experimentally or theoretically, about the origin of dielectric response at the molecular/multimolecular scale. In this contribution we report the implementation of an accurate first-principles approach to calculating the dielectric response of molecular systems. We assess the accuracy of the method by reproducing the experimental dielectric constants of several bulk Ï€-electron materials and demonstrating the ability of the method to capture dielectric properties as a function of frequency and molecular orientation in representative arrays of substituted aromatic derivatives. The role of molecular alignment and packing density on dielectric response is also examined, showing that the local dielectric behavior of molecular assemblies can diverge significantly from that of the bulk material. PMID:23734640
First Principles Modeling of Bimolecular Reactions with Diffusion
NASA Astrophysics Data System (ADS)
Hansen, S. K.; Scher, H.; Berkowitz, B.
2013-12-01
We consider three approaches to modeling A + B ? C irreversible reactions in natural media: 1) a discretized diffusion-reaction equation (DRE), 2) a particle tracking (PT) scheme in which reaction occurs if and only if an A and B particle pair are within a fixed distance, r (the "reaction radius"), and 3) a PT scheme using an alternative to the fixed reaction radius: a collocation probability distribution derived directly from first principles. Each approach has advantages. In some cases a discretized DRE may be the most computationally efficient method. For PT simulations, robust codes exist based on use of a fixed reaction radius. And finally, collocation probabilities may be derived directly from the Fick's Law constant, D, which is a well-established property for most species. In each approach, a single parameter governs the 'promiscuity' of the reaction (i.e. the thermodynamic favorability of reaction, predicated on the particles being locally well mixed). For the DRE, fixed-reaction-radius PT, and collocation-based PT, these parameters are, respectively: a second-order decay rate, r, and D. We established a number of new results enhancing these approaches and relating them to each other (and to nature). In particular, a thought experiment concerning a simple system in which the predictions of each approach can be computed analytically was used to derive formulas establishing a universal one-to-one correspondence among each of the governing parameters. We thus showed the conditions for equivalence of the three approaches, and grounded both the DRE approach and the fixed-radius PT approach in the Fick's Law D. We further showed that the existing collocation-based PT theory is based on a probability distribution that is only correct for infinitesimally small times, but which can be modified to be accurate for larger times by means of continuous time random walk analysis and first-passage probability distributions. Finally, we employed a novel mathematical approach to adapt this into a workable collocation-based particle tracking technique.
Simulations of silicate glasses under pressure with first principles approach
NASA Astrophysics Data System (ADS)
Ghosh, D. B.; Karki, B. B.
2011-12-01
Silicate glasses have long been studied because they are considered as analog of geologically relevant silicate melts. Experimental studies have suggested that silicate glasses undergo significant structural changes including irreversibility under compression. Here, we perform first-principles molecular dynamics simulations to explore the pressure-induced structural changes in two silicate glasses, MgSiO3 and CaAl2Si2O8, at 300 K over relevant mantle pressures. Our simulations show that at ambient conditions Si atoms remain mainly in tetrahedral arrangement (> 95 % four-fold coordination) with a mean coordination of slightly greater than 4. Al-O coordination environment consists of both tetrahedral and pentahedral species in large amounts. In contrary to a complete tetrahedral network in pure SiO2 glass, the linkage is partial (via bridging oxygens) in these Mg- and Ca-bearing systems. Upon compression, five-coordinated Si's appear initially in increasing abundance and they gradually turn into six-fold state and even 6+ coordinated states appear at very high pressure. In case of Ca and Mg, where ambient pressure coordination is a mixture of four-fold to seven-fold species, pressure induces gradual increase in coordination (seven- to nine-fold). In a similar fashion, O-Si and O-Al coordinations increase at the expense of free and non-bridging oxygens on compression. Appearance of oxygen tri-clusters (O atoms coordinated with 3 Si/Al atoms) at the compressed volumes is consistent with the experimental findings. Our prediction of various odd coordination species is supported by experimental studies of silicate glasses. We find that glass densification is rather rapid initially and becomes more gradual at pressures above 20 GPa. This behavior can be associated with the way the glass structure changes in terms of bond distances, bond angles and coordination environments as pressure increases. On decompression, the system fails to revert back to its initial structural state, consistent with the experimental observations, and this irreversibility can be attributed to the memory effects, i.e., to continuing existence of high coordination species (pentahedra and octahedra) to larger volumes.
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.; CÃ´tÃ©, 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 vibrational lifetimes, etc. Solution method: Software application based on Density-Functional Theory and Many-Body Perturbation Theory, pseudopotentials, with planewaves, Projector-Augmented Waves (PAW) or wavelets as basis functions. Running time: From less than one second for the simplest tests, to several weeks. The vast majority of the >600 provided tests run in less than 30 seconds. References:[1] http://inac.cea.fr/LSim/BigDFT. [2] http://etsf.eu/index.php?page=standardization. [3] http://www.tddft.org/programs/octopus/wiki/index.php/Libxc. [4] http://www.unidata.ucar.edu/software/netcdf. [5] http://en.wikipedia.org/wiki/MessagePassingInterface. [6] http://www.wannier.org.
First-principles calculations of conductivity in transparent semiconducting oxides
NASA Astrophysics Data System (ADS)
Varley, Joel Basile
2011-12-01
Transparent conducting oxides (TCOs) are exceptional materials that possess the unique combination of nearly metallic conductivity and optical transparency over the visible portion of the spectrum. With such features, TCOs have become critical components of many present and emerging technologies. Today these materials are already ubiquitous, appearing in windows, flat-panel displays, portable electronics, solar cells, solid-state light-emitters, and transistors. Thanks to the ever-growing list of applications that rely on TCOs, the recent surge of interest in these materials has focused on understanding the fundamental properties and doping opportunities in a variety of traditional as well as promising new TCOs. Using state-of-the-art first-principles calculations, we address several important issues in a number of technologically relevant TCOs. First, the origins of unintentional conductivity. Many TCOs exhibit high levels of n-type conductivity, even when not intentionally doped. For SnO 2, In2O3 and Ga2O3, we demonstrate that this is not due to oxygen vacancies, as is commonly assumed, but must be attributed to unintentional incorporation of impurities, with hydrogen being a prime candidate. Second, the push for higher doping levels. We suggest several donor impurities as candidate dopants with high solubility. We also investigate limitations on doping due to the formation or incorporation of compensating centers. Among intrinsic defects, cation vacancies are the most likely candidates; we also study impurities that act as acceptors. In the case of SnO2, group-V impurities are intriguing since they can act either as donors on the Sn site or acceptors on the O site. Third, the prospects for p-type doping. Here we find that none of the investigated acceptors will lead to effective hole doping. We demonstrate that the reason for this behavior is the tendency for strong localization of holes in the oxygen-derived valence bands, and relate this to the issue of polaron formation. Finally, we apply our acquired expertise to the issue of reducing the absorption edge of another wide-band-gap semiconducting oxide, TiO2, which is widely used for photocatalysis. Our conclusions resolve long-standing questions on the properties of N-doped titania, suggesting another application of acceptor doping in TCO materials.
de Oliveira, Vanessa E; Almeida, Eduardo W C; Castro, Harlem V; Edwards, Howell G M; Dos Santos, Hélio F; de Oliveira, Luiz Fernando C
2011-08-01
In the present study, the inclusion processes of ?-carotene, astaxanthin, lycopene, and norbixin (NOR) into the ?-cyclodextrin (?-CD) cavity were investigated by means of Raman spectroscopy and quantum mechanics calculations. The Raman ?(1) band assigned to C?C stretching was sensitive to the host-guest interaction and in general undergoes a blue shift (3-13 cm(-1)) after inclusion takes place, which is the consequence of the localization of single and double bonds. This is supported by the molecular modeling prediction, which inclusion complexes show the ?(1) band blue shifted by 1-8 cm(-1). The calculated complexation energies was small for most of derivatives and was found to be -11.1 kcal mol(-1) for inclusion of AST and +0.27 kcal mol(-1) for NOR. The stability order was qualitatively correlated to topological parameters accounting for the opening angle of the chain. This means that after inclusion the guest molecules assume a slightly more extended conformation, which enhances the host-guest contact, improving the interaction energy. The results discussed here clearly demonstrate the matrix effect on the carotenes' spectroscopic profile and should contribute to fully characterize the raw samples. PMID:21728366
Antirelaxation coatings in coherent spectroscopy: Theoretical investigation and experimental test
NASA Astrophysics Data System (ADS)
Nasyrov, K.; Gozzini, S.; Lucchesini, A.; Marinelli, C.; Gateva, S.; Cartaleva, S.; Marmugi, L.
2015-10-01
We describe a theoretical model, based on a density matrix and the Liouville equation, for the investigation of magneto-optical resonances in alkali-metal atomic vapor, in particular in the case of the electromagnetically induced transparency (EIT) in the presence of antirelaxation coatings. The influence of the coating is parametrized with an empirical coefficient describing its efficiency; the calculations are extended to a broad range of coating quality, contrary to previous works, and to uncoated cells. The model takes into account also different configurations for the EIT formation and different efficiency of optical pumping, as determined by the coating characteristics and the atomic energy structure. The model is validated by investigating the EIT with degenerate Zeeman levels in 39K D1 and Cs D2 lines, which exhibit respectively an almost negligible and a relevant impact of hyperfine optical pumping. The results are compared to experimental data, exhibiting good agreement; in particular, for the 39K D1 line, recent findings are shown here in the case of degenerate and nondegenerate EIT with amplitude-modulated light. Our results demonstrate an effective approach for the investigation of antirelaxation coatings and their contribution in the formation of magneto-optical resonances in alkali-metal atoms, in different regimes and with largely different efficiencies. This sheds new light on well-known but not yet entirely clarified phenomena and their behavior as a function of experimental parameters.
Vibrational spectroscopy and theoretical studies on 2,4-dinitrophenylhydrazine
NASA Astrophysics Data System (ADS)
Chi?, V.; Filip, S.; Micl?u?, V.; Pîrn?u, A.; T?n?selia, C.; Alm??an, V.; Vasilescu, M.
2005-06-01
In this work, we will report a combined experimental and theoretical study on molecular and vibrational structure of 2,4-dinitrophenylhydrazine. FT-IR, FT-IR/ATR and Raman spectra of normal and deuterated DNPH have been recorded and analyzed in order to get new insights into molecular structure and properties of this molecule, with particular emphasize on its intra- and intermolecular hydrogen bonds (HB's). For computational purposes we used density functional theory (DFT) methods, with B3LYP and BLYP exchange-correlation functionals, in conjunction with 6-31G(d) basis set. All experimental vibrational bands have been discussed and assigned to normal modes on the basis of DFT calculations and isotopic shifts and by comparison to other dinitro- substituted compounds [V. Chi?, Chem. Phys., 300 (2004) 1]. To aid in mode assignments, we based on the direct comparison between experimental and calculated spectra by considering both the frequency sequence and the intensity pattern of the experimental and computed vibrational bands. It is also shown that semiempirical AM1 method predicts geometrical parameters and vibrational frequencies related to the HB in a pleasant agreement with experiment, being surprisingly accurate from this perspective.
First-principles models of equilibrium tellurium isotope fractionation
NASA Astrophysics Data System (ADS)
Haghnegahdar, M. A.; Schauble, E. A.; Fornadel, A. P.; Spry, P. G.
2013-12-01
In this study, equilibrium mass-dependent isotopic fractionation among representative Te-bearing species is estimated with first-principles thermodynamic calculations. Tellurium is a group 16 element (along with O, S, and Se) with eight stable isotopes ranging in mass from 120Te to 130Te, and six commonly-occurring oxidation states: -II, -I, 0, +II, +IV, and +VI. In its reduced form, Te(-II), tellurium has a unique crystal-chemical role as a bond partner for gold and silver in epithermal and orogenic gold deposits, which likely form when oxidized Te species (e.g., H2TeO3, TeO32-) or perhaps polytellurides (e.g., Te22-) interact with precious metals in hydrothermal solution. Te(IV) is the most common oxidation state at the Earth's surface, including surface outcrops of telluride ore deposits, where tellurite and tellurate minerals form by oxidation. In the ocean, dissolved tellurium tends to be scavenged by particulate matter. Te(VI) is more abundant than Te(IV) in the ocean water (1), even though it is thought to be less stable thermodynamically. This variety of valence states in natural systems and range of isotopic masses suggest that tellurium could exhibit geochemically useful isotope abundance variations. Tellurium isotope fractionations were determined for representative molecules and crystals of varying complexity and chemistry. Gas-phase calculations are combined with supermolecular cluster models of aqueous and solid species. These in turn are compared with plane-wave density functional theory calculations with periodic boundary conditions. In general, heavyTe/lightTe is predicted to be higher for more oxidized species, and lower for reduced species, with 130Te/125Te fractionations as large as 4‰ at 100?C between coexisting Te(IV) and Te(-II) or Te(0) compounds. This is a much larger fractionation than has been observed in naturally occurring redox pairs (i.e., Te (0) vs. Te(IV) species) so far, suggesting that disequilibrium processes may control oxidation and reduction. Se- and S-isotope redox systems also show kinetically-controlled isotopic disequilibrium, and may serve as useful analogues. Among Te(-II) species, fractionations are smaller (< 1‰ at 100?C). (Au,Ag)Te2 minerals (calaverite, krennerite) and (Au,Ag)2Te minerals (petzite, hessite) are expected to have similar isotopic compositions to vapor-phase H2Te, and there appears to be little discernable effect from Ag,Au solid solution. Altaite (PbTe) will have somewhat lower 130Te/125Te. Calculated fractionation factors for gas-phase species like H2Te and TeF6, based on hybrid density functional theory (B3LYP), agree well with earlier estimates (2). For telluride-bearing crystals, cluster models are in qualitative agreement with periodic boundary condition calculations, especially when larger basis sets (e.g., aug-cc-PVDZ-PP) are used; however, cluster models tend to predict higher 130Te/125Te for reasons that are not yet clear. References: 1. Hein et al. (2003) GCA 67:6. 2. Smithers et al. (1968) Can. J. Chem. 46:4.
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 revealed that, for small layer thicknesses, the material responded rather homogeneously. However, for large layer thicknesses, each compositionally distinct block responded quite independently, and responded like its equivalent bulk system. This led to some overall systems possessing phases that do not exist in BST bulks (such as monoclinic phases), and to the apparent merging of two phase transitions in unstrained systems. When compositionally graded BST systems were modeled under a compressive epitaxial strain, only the z-component of the polarization (that is the component along the growth direction) was found to increase from zero below a single critical temperature. As in unstrained systems, some strained systems were found to have monodomain and striped domain states, with the monodomain representing the ground state and the striped domains representing a local minimum. Development of a BST thin-film code was also undertaken. Initial simulations of BST thin-films were performed using this code, on both disordered and graded systems. These results verified that the new BST thin-film code was functional, as well as revealed interesting phenomena related to compositional grading in two-dimensional materials.
First principles calculations of interlayer exchange coupling in bcc Fe/Cu/Fe structures
Kowalewski, M.; Heninrich, B.; Schulthess, T.C.; Butler, W.H.
1998-01-01
The authors report on theoretical calculations of interlayer exchange coupling between two Fe layers separated by a modified Cu spacer. These calculations were motivated by experimental investigations of similar structures by the SFU group. The multilayer structures of interest have the general form: Fe/Cu(k)/Fe and Fe/Cu(m)/X(1)/Cu(n)/Fe where X indicates one AL (atomic layer) of foreign atoms X (Cr, Ag, or Fe) and k, m, n represent the number of atomic layers of Cu. The purpose of the experimental and theoretical work was to determine the effect of modifying the pure Cu spacer by replacing the central Cu atomic layer with the atomic layer of foreign atoms X. The first principles calculation were performed using the Layer Korringa-Kohn-Rostoker (LKKR) method. The theoretical thickness dependence of the exchange coupling between two semi-infinite Fe layers was calculated for pure Cu spacer thicknesses in the range of 0 < k < 16. The effect of the foreign atoms X on the exchange coupling was investigated using the structure with 9 AL Cu spacer as a reference sample. The calculated changes in the exchange coupling are in qualitative agreement with experiment.
Shock Hugoniot of osmium up to 800 GPa from first principles calculations.
Joshi, K D; Gupta, Satish C; Banerjee, S
2009-10-14
First principles total energy calculations on hcp, ? (a three atom simple hexagonal), ? (bcc) and fcc phases of osmium have been performed as a function of hydrostatic compression employing the FP-LAPW method. The comparison of total energies of these phases up to a maximum compression V/V(0) = 0.58 (pressure?700 GPa) shows that the hcp structure remains stable up to this compression. The 300 K isotherm is determined after adding finite temperature thermal contributions to the total energy calculated as a function of volume at 0 K. From the theoretically determined isotherm, we have derived the shock Hugoniot of this metal and determined the shock parameters C(0) and s to be 4.48 km s(-1) and 1.32, respectively. Employing the theoretically calculated Gruneisen parameter in the differential form of the Lindemann melting rule, we have determined the variation of melting point of the osmium with pressure. The theoretically derived melting curve and the temperature rise along the Hugoniot predict the shock melting of osmium at ?447 GPa with a corresponding temperature of ?9203 K. PMID:21693986
First-principles theory, coarse-grained models, and simulations of ferroelectrics.
Waghmare, Umesh V
2014-11-18
CONSPECTUS: A ferroelectric crystal exhibits macroscopic electric dipole or polarization arising from spontaneous ordering of its atomic-scale dipoles that breaks inversion symmetry. Changes in applied pressure or electric field generate changes in electric polarization in a ferroelectric, defining its piezoelectric and dielectric properties, respectively, which make it useful as an electromechanical sensor and actuator in a number of applications. In addition, a characteristic of a ferroelectric is the presence of domains or states with different symmetry equivalent orientations of spontaneous polarization that are switchable with large enough applied electric field, a nonlinear property that makes it useful for applications in nonvolatile memory devices. Central to these properties of a ferroelectric are the phase transitions it undergoes as a function of temperature that involve lowering of the symmetry of its high temperature centrosymmetric paraelectric phase. Ferroelectricity arises from a delicate balance between short and long-range interatomic interactions, and hence the resulting properties are quite sensitive to chemistry, strains, and electric charges associated with its interface with substrate and electrodes. First-principles density functional theoretical (DFT) calculations have been very effective in capturing this and predicting material and environment specific properties of ferroelectrics, leading to fundamental insights into origins of ferroelectricity in oxides and chalcogenides uncovering a precise picture of electronic hybridization, topology, and mechanisms. However, use of DFT in molecular dynamics for detailed prediction of ferroelectric phase transitions and associated temperature dependent properties has been limited due to large length and time scales of the processes involved. To this end, it is quite appealing to start with input from DFT calculations and construct material-specific models that are realistic yet simple for use in large-scale simulations while capturing the relevant microscopic interactions quantitatively. In this Account, we first summarize the insights obtained into chemical mechanisms of ferroelectricity using first-principles DFT calculations. We then discuss the principles of construction of first-principles model Hamiltonians for ferroelectric phase transitions in perovskite oxides, which involve coarse-graining in time domain by integrating out high frequency phonons. Molecular dynamics simulations of the resulting model are shown to give quantitative predictions of material-specific ferroelectric transition behavior in bulk as well as nanoscale ferroelectric structures. A free energy landscape obtained through coarse-graining in real-space provides deeper understanding of ferroelectric transitions, domains, and states with inhomogeneous order and points out the key role of microscopic coupling between phonons and strain. We conclude with a discussion of the multiscale modeling strategy elucidated here and its application to other materials such as shape memory alloys. PMID:25361389
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+:Î±-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.
NASA Astrophysics Data System (ADS)
Muthu, D. V. S.; Teredesai, Pallavi; Saha, S.; Suchitra, Waghmare, U. V.; Sood, A. K.; Rao, C. N. R.
2015-06-01
We report high-pressure Raman-scattering studies on single-crystal Re O3 up to 26.9 GPa at room temperature, complemented by first-principles density functional calculations to assign the modes and to develop understanding of the subtle features of the low-pressure phase transition. The pressure (P ) dependence of phonon frequencies (? ) reveals three phase transitions at 0.6, 3, and 12.5 GPa with characteristic splitting and changes in the slope of ? (P ). Our first-principles theoretical analysis confirms the role of the rotational modes of Re O6 ,M3, to the lowest pressure structural transition, and shows that the transition from the P m 3 m to the I m 3 structure is a weak first-order transition, originating from the strong anharmonic coupling of the M3 modes with the acoustic modes (strain).
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.
Strain sensitivity and superconducting properties of Nb3Sn from first principles calculations.
De Marzi, G; Morici, L; Muzzi, L; della Corte, A; Nardelli, M Buongiorno
2013-04-01
Using calculations from first principles based on density-functional theory we have studied the strain sensitivity of the A15 superconductor Nb3Sn. The Nb3Sn lattice cell was deformed in the same way as observed experimentally on multifilamentary, technological wires subject to loads applied along their axes. The phonon dispersion curves and electronic band structures along different high-symmetry directions in the Brillouin zone were calculated, at different levels of applied strain, ?, on both the compressive and the tensile side. Starting from the calculated averaged phonon frequencies and electron-phonon coupling, the superconducting characteristic critical temperature of the material, T(c), has been calculated by means of the Allen-Dynes modification of the McMillan formula. As a result, the characteristic bell-shaped T(c) versus ? curve, with a maximum at zero intrinsic strain, and with a slight asymmetry between the tensile and compressive sides, has been obtained. These first-principle calculations thus show that the strain sensitivity of Nb3Sn has a microscopic and intrinsic origin, originating from shifts in the Nb3Sn critical surface. In addition, our computations show that variations of the superconducting properties of this compound are correlated to stress-induced changes in both the phononic and electronic properties. Finally, the strain function describing the strain sensitivity of Nb3Sn has been extracted from the computed T(c)(?) curve, and compared to experimental data from multifilamentary, composite wires. Both curves show the expected bell-shaped behavior, but the strain sensitivity of the wire is enhanced with respect to the theoretical predictions for bulk, perfectly binary and stoichiometric Nb3Sn. An understanding of the origin of this difference might open potential pathways towards improvement of the strain tolerance in such systems. PMID:23478497
Zhang, Jian; Yang, Ping; Sun, Zhenrong; Wang, Xue B.
2014-09-18
Molecular species with electron affinities (EAs) larger than that of the chlorine atom (3.6131 eV) are superhalogens. The corresponding negative ions, namely, superhalogen anions, are intrinsically very stable with high electron binding energies (EBEs), and widely exist as building blocks of bulk materials and ionic liquids. The most common superhalogen anions proposed and confirmed to date are either ionic salts or compact inorganic species. Herein we report a new class of superhalogen species, a series of tetracoordinated organoboron anions [BL4]â€“ (L = phenyl (1), 4-fluorophenyl (2), 1-imidazolyl (3), L4 = H(pyrazolyl)3 (4)) with bulky organic ligands covalently bound to the central B atom. Negative ion photoelectron spectroscopy (NIPES) reveals all of these anions possessing EBEs higher than that of Cl- with the adiabatic / vertical detachment energy (ADE / VDE) of 4.44/4.8 (1), 4.78/5.2 (2), 5.08/5.4 (3), and 4.59/4.9 eV (4), respectively. First-principles calculations confirmed high EBEs of [BL4]â€“ and predicted that these anions are thermodynamically stable against fragmentation. The unraveled superhalogen nature of these species provides a molecular basis to explain the wide-range applications of tetraphenylborate (TPB) (1) and trispyrazolylborate (Tp) (4) in many areas spanning from industrial waste treatment to soft material synthesis and organometallic chemistry
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.
First principles investigation of L-alanine in terahertz region.
Zheng, Zhuan-Ping; Fan, Wen-Hui
2012-06-01
Terahertz absorption spectrum (0.5-4.0 THz) of L-alanine in the solid phase was measured by terahertz time-domain spectroscopy at room temperature. Simulations utilizing gaseous-state and solid-state theory were performed to determine the origins of the observed vibrational features. Our calculations showed that the measured features in solid-state materials could be well understood by considering the crystal packing interactions in a solid-state density functional theory calculation. Furthermore, intermolecular vibrations of L-alanine are found to be the dominating contributions to these measured spectral features in the range of 0.5-4.0 THz, except that located at 3.11 THz. PMID:23729906
First-Principles Study of Casimir Repulsion in Metamaterials
Yannopapas, Vassilios; Vitanov, Nikolay V.
2009-09-18
We examine theoretically the Casimir effect between a metallic plate and several types of magnetic metamaterials in pursuit of Casimir repulsion, by employing a rigorous multiple-scattering theory for the Casimir effect. We first examine metamaterials in the form of two-dimensional lattices of inherently nonmagnetic spheres such as spheres made from materials possessing phonon-polariton and exciton-polariton resonances. Although such systems are magnetically active in infrared and optical regimes, the force between finite slabs of these materials and metallic slabs is plainly attractive since the effective electric permittivity is larger than the magnetic permeability for the studied spectrum. When lattices of magnetic spheres made from superparamagnetic composites are employed, we achieve not only Casimir repulsion but almost total suppression of the Casimir effect itself in the micrometer scale.
First principles study of halogens adsorption on intermetallic surfaces
NASA Astrophysics Data System (ADS)
Zhu, Quanxi; Wang, Shao-qing
2016-02-01
Halides are often present at electrochemical environment, they can directly influence the electrode potential or zero charge potential through the induced work-function change. In this work, we focused in particular on the halogen-induced work function change as a function of the coverage of fluorine, chlorine, bromine and iodine on Al2Au and Al2Pt (110) surfaces. Results show that the real relation between work function change and dipole moment change for halogens adsorption on intermetallic surfaces is just a common linear relationship rather than a directly proportion. Besides, the different slopes between fitted lines and the theoretical slope employed in pure metal surfaces demonstrating that the halogens adsorption on intermetallic surfaces are more complicated. We also present a weight parameter Î² to describe different factors effect on work function shift and finally qualify which factor dominates the shift direction.
Pressure Dependent Electronic Properties of Organic Semiconductors from First Principles
NASA Astrophysics Data System (ADS)
Knuth, Franz; Carbogno, Christian; Blum, Volker; Scheffler, Matthias
2015-03-01
The electronic properties of organic semiconductors typically exhibit a significant dependence on the strain, stress, and pressure. In this contribution, we present the theoretical background, assessment of approximations, and results of electronic and transport properties in the framework of density-functional theory. Our implementation considers the analytical strain derivatives (stress tensor) including the contributions that stem from (a) van-der-Waals interactions and (b) the Fock-exchange in hybrid functionals. We validate our approach by investigating the geometric and electronic changes that occur in polyacetylene and anthracene under hydrostatic pressure. We show that the fraction of exact exchange included in the calculations is critical - and non-trivial to choose - for a correct description of these systems. Furthermore, we point out trends for the electrical conductivity under pressure and identify the dominant charge carriers and transport directions.
First-principles study of Casimir repulsion in metamaterials.
Yannopapas, Vassilios; Vitanov, Nikolay V
2009-09-18
We examine theoretically the Casimir effect between a metallic plate and several types of magnetic metamaterials in pursuit of Casimir repulsion, by employing a rigorous multiple-scattering theory for the Casimir effect. We first examine metamaterials in the form of two-dimensional lattices of inherently nonmagnetic spheres such as spheres made from materials possessing phonon-polariton and exciton-polariton resonances. Although such systems are magnetically active in infrared and optical regimes, the force between finite slabs of these materials and metallic slabs is plainly attractive since the effective electric permittivity is larger than the magnetic permeability for the studied spectrum. When lattices of magnetic spheres made from superparamagnetic composites are employed, we achieve not only Casimir repulsion but almost total suppression of the Casimir effect itself in the micrometer scale. PMID:19792414
Lattice dynamics and ferroelectric instability of PZT from first principles
NASA Astrophysics Data System (ADS)
Bungaro, Claudia; Rabe, Karin M.
2001-03-01
The lattice dynamics of the prototypical cubic perovskite structure has long been considered central to the understanding of the ferroelectric and piezoelectric behavior of perovskite oxides. In solid solutions such as PZT, compositional disorder greatly complicates the theoretical study of the lattice dynamics. The virtual crystal approximation (VCA) is an easily implemented approach to the calculation of interatomic force constants (IFC) in solid solutions; however, as shown by comparison with full ab-initio calculations for ordered alloy configurations, it provides a poor description of the lattice dynamics of PZT. An alternative ab-initio based approach, proposed in [1], is to transfer the IFC's computed for the endpoint compounds to the solid solution. We will present results of the application of this approach to PZT and discuss the implications for effective Hamiltonian simulations of finite-temperature behavior. 1. Ph. Ghosez, E. Cockayne, U. V. Waghmare and K. M. Rabe, Phys. Rev. B60, 836 (1999).
First principles effective electronic couplings for hole transfer in natural and size-expanded DNA
Migliore, Agostino; Corni, Stefano; Varsano, Daniele; Klein, Michael L.; Di Felice, Rosa
2009-01-01
Hole transfer processes between base pairs in natural DNA and size-expanded DNA (xDNA) are studied and compared, by means of an accurate first principles evaluation of the effective electronic couplings (also known as transfer integrals), in order to assess the effect of the base augmentation on the efficiency of charge transport through double-stranded DNA. According to our results, the size expansion increases the average electronic coupling, and thus the CT rate, with potential implications in molecular biology and in the implementation of molecular nanoelectronics. Our analysis shows that the effect of the nucleobase expansion on the charge-transfer (CT) rate is sensitive to the sequence of base pairs. Furthermore, we find that conformational variability is an important factor for the modulation of the CT rate. From a theoretical point of view, this work offers a contribution to the CT chemistry in ?-stacked arrays. Indeed, we compare our methodology against other standard computational frameworks that have been adopted to tackle the problem of CT in DNA, and unravel basic principles that should be accounted for in selecting an appropriate theoretical level. PMID:19537767
A first principles method to simulate electron mobilities in 2D materials
NASA Astrophysics Data System (ADS)
Restrepo, Oscar D.; Krymowski, Kevin E.; Goldberger, Joshua; Windl, Wolfgang
2014-10-01
We examine the predictive capabilities of first-principles theoretical methods to calculate the phonon- and impurity-limited electron mobilities for a number of technologically relevant two-dimensional materials in comparison to experiment. The studied systems include perfect graphene, graphane, germanane and MoS2, as well as graphene with vacancies, and hydrogen, gold, and platinum adsorbates. We find good agreement with experiments for the mobilities of graphene (? = 2 × 105 cm2 V-1s-1) and graphane (? = 166 cm2 V-1s-1) at room temperature. For monolayer MoS2 we obtain ? = 225 cm2 V-1s-1. This value is higher than what is observed experimentally (0.5-200 cm2 V-1s-1) but is on the same order of magnitude as other recent theoretical results. For bulk MoS2 we obtain ? = 48 cm2 V-1s-1. We obtain a very high mobility of 18 200 cm2 V-1s-1 for single-layer germanane. The calculated reduction in mobility from the different impurities compares well to measurements where experimental data are available, demonstrating that the proposed method has good predictive capabilities and can be very useful for validation and materials design.
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.
Lattice dynamics and thermal conductivity of calcium fluoride via first-principles investigation
NASA Astrophysics Data System (ADS)
Qi, Yuan-Yuan; Zhang, Tian; Cheng, Yan; Chen, Xiang-Rong; Wei, Dong-Qing; Cai, Ling-Cang
2016-03-01
The lattice thermal conductivity of CaF2 is accurately computed from a first-principles theoretical approach based on an iterative solution of the Boltzmann transport equation. The second- and third-order interatomic force constants are generated from a real-space finite-difference supercell approach. Then, the force constants for both the second- and third-order potential interactions are used to calculate the lattice thermal conductivity and related physical quantities of CaF2 at temperatures ranging from 30 K to 1500 K. The obtained lattice thermal conductivity 8.6 W/(m.K) for CaF2 at room temperature agrees better with the experimental value than other theoretical data, demonstrating the promise of this parameter-free approach in providing precise descriptions of the lattice thermal conductivity of materials. The obtained dielectric parameters and phonon spectrum of CaF2 accord well with available data. Meanwhile, the temperature dependence curves of the lattice thermal conductivity, heat capacity, and phonon mean free path are presented.
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.
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.)
Theoretical investigation of nuclear quadrupole interactions in DNA at first-principles level
NASA Astrophysics Data System (ADS)
Mahato, Dip N.; Dubey, Archana; Pink, R. H.; Scheicher, R. H.; Badu, S. R.; Nagamine, K.; Torikai, E.; Saha, H. P.; Chow, Lee; Huang, M. B.; Das, T. P.
We have studied the nuclear quadrupole interactions (NQI) of the 14N, 17O and 2H nuclei in the nucleobases cytosine, adenine, guanine and thymine in the free state as well as when they are bonded to the sugar ring in DNA, simulated through a CH3 group attached to the nucleobases. The nucleobase uracil, which replaces thymine in RNA, has also been studied. Our results show that there are substantial indirect effects of the bonding with the sugar group in the nucleic acids on the NQI parameters e 2 qQ/h and ?. It is hoped that measurements of these NQI parameters in DNA will be available in the future to compare with our predictions. Our results provide the conclusion that for any property dependent on the electronic structures of the nucleic acids, the effects of the bonding between the nucleobases and the nucleic acid backbones have to be included.
Theoretical investigation of nuclear quadrupole interactions in DNA at first-principles level
NASA Astrophysics Data System (ADS)
Mahato, Dip N.; Dubey, Archana; Pink, R. H.; Scheicher, R. H.; Badu, S. R.; Nagamine, K.; Torikai, E.; Saha, H. P.; Chow, Lee; Huang, M. B.; Das, T. P.
2008-01-01
We have studied the nuclear quadrupole interactions (NQI) of the 14N, 17O and 2H nuclei in the nucleobases cytosine, adenine, guanine and thymine in the free state as well as when they are bonded to the sugar ring in DNA, simulated through a CH3 group attached to the nucleobases. The nucleobase uracil, which replaces thymine in RNA, has also been studied. Our results show that there are substantial indirect effects of the bonding with the sugar group in the nucleic acids on the NQI parameters e 2 qQ/h and ?. It is hoped that measurements of these NQI parameters in DNA will be available in the future to compare with our predictions. Our results provide the conclusion that for any property dependent on the electronic structures of the nucleic acids, the effects of the bonding between the nucleobases and the nucleic acid backbones have to be included.
Silicane and germanane: tight-binding and first-principles studies
NASA Astrophysics Data System (ADS)
Zólyomi, V.; Wallbank, J. R.; Fal'ko, V. I.
2014-06-01
We present a first-principles and tight-binding model study of silicane and germanane, the hydrogenated derivatives of two-dimensional silicene and germanene. We find that the materials are stable in freestanding form, analyse the orbital composition, and derive a tight-binding model using first-principles calculations to fit the parameters.
First Principles Equations of State of LLM-105
NASA Astrophysics Data System (ADS)
Manaa, Riad
2015-06-01
Equations of states (EOS) of unreacted energetic materials extending to high-pressure and temperatures regimes are provide fundamental information about the associated thermodynamic properties of these materials at extreme conditions. Using dispersion-corrected density functional theoretical calculations, we performed large-scale constant-volume, constant-pressure and temperature molecular dynamics simulations on crystal 2,6-diamino-3, 5-dinitropyrazine-1-oxide (LLM-105) for pressures ranging from ambient to 35 GPa, and temperatures ranging from 300 K to 1400 K. These calculations allowed us to construct an unreacted P-V-T EOS and obtain bulk modulus for each P-V isotherm. We also obtained the thermal expansion coefficient of LLM-105 in the temperature range of this study. Finally, we conducted a quantum-based molecular dynamics study at the C-J point to characterize the decomposition products of reacting LLM-105 at complete reactivity condition. This work performed under the auspices of the U.S. Department of Energy Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Thermal transport properties of complex oxides from first principles
NASA Astrophysics Data System (ADS)
Bhatti, Aqyan; Jain, Ankit; McGaughey, Alan; Benedek, Nicole
2015-03-01
Thermal transport properties of materials are key parameters in the design of many engineering devices. For this reason, it is highly desirable to be able to control or tailor the thermal properties of materials for specific applications. Complex oxides are attractive in this regard, due to their low and potentially highly tunable thermal conductivity. However, the theoretical description of the thermal transport properties of oxides presents a number of challenges compared to conventional semiconductors. For example, oxides tend to have complex crystal structures and the atoms interact through long-range electrostatic forces. In this talk, we use the example of PbTiO3 to discuss some of the challenges and opportunities associated with thermal transport predictions in complex oxides. For example, many oxides contain very low-lying optical branches, which may provide important acoustic-optical scattering channels. In addition, it is often possible to tune the frequencies of such optical modes with epitaxial strain. We also link the observed negative thermal expansion behavior of PbTiO3 to two zone-boundary modes with large, negative Grüneisen parameters and comment on the consequences of this finding for the thermal transport properties of this material.
A first principles study of the acetylene-water interaction
Tzeli, Demeter; Mavridis, Aristides; Xantheas, Sotiris S.
2000-04-08
We present an extensive study of the stationary points on the acetylene-water (AW) ground-state potential energy surface (PES) aimed in establishing accurate energetics for the two different bonding scenarios that are considered. Those include arrangements in which water acts either as a proton acceptor from one of the acetylene hydrogen atoms or a proton donor to the triple bond. We used a hierarchy of theoretical methods to account for electron correlation [MP2 (second-order Moller-Plesset), MP4 (fourth-order Moller-Plesset), and CCSD(T) (coupled-cluster single double triple)] coupled with a series of increasing size augmented correlation consistent basis sets (aug-cc-pVnZ, n=2,3,4). We furthermore examined the effect of corrections due to basis set superposition error (BSSE). We found that those have a large effect in altering the qualitative features of the PES of the complex. They are responsible for producing a structure of higher (C{sub 2v}) symmetry for the global minimum. Zero-point energy (ZPE) corrections were found to increase the stability of the C{sub 2v} arrangement. For the global (water acceptor) minimum of C{sub 2v} symmetry our best estimates are {delta}E{sub e}=-2.87 kcal/mol ({delta}E{sub 0}=-2.04 kcal/mol) and a van der Waals distance of R{sub e}=2.190 Aa. The water donor arrangement lies 0.3 kcal/mol (0.5 kcal/mol including ZPE corrections) above the global minimum. The barrier for its isomerization to the global minimum is E{sub e}=0.18 kcal/mol; however, inclusion of BSSE- and ZPE-corrections destabilize the water donor arrangement suggesting that it can readily convert to the global minimum. We therefore conclude that there exists only one minimum on the PES in accordance with previous experimental observations. To this end, vibrational averaging and to a lesser extend proper description of intermolecular interactions (BSSE) were found to have a large effect in altering the qualitative features of the ground-state PES of the acetylene-water complex. (c) 2000 American Institute of Physics.
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 infrared (IR) spectra of graphene functionalized with carboxyl (COON) groups are studied in graphene with no surface defects, di-vacancies (DV), and Stone-Wales (SW) defects. Simulated Raman and IR spectra of carboxylated graphene are consistent with available experimental results. Computed vibrational spectra of carboxylated graphene show that the presence of point defects near the functionalization site affect the Raman and IR spectroscopic signatures of the functionalized graphene.
First-principles simulations of charge storage at electrochemical interfaces in supercapacitors
NASA Astrophysics Data System (ADS)
Wood, Brandon
2015-03-01
Supercapacitors store charge via polarization at the electrode-electrolyte interface. Many models of interfacial charge storage focus on the formation of the electric double layer (EDL) in the electrolyte, but it is often assumed that in the electrode, a shift in the Fermi level is the only notable response to interface polarization. In reality, the presence of the interface impacts the fundamental properties of both the electrode and the electrolyte, often in complex and nontrivial ways that are difficult to capture using simple models. I will discuss how including an applied bias potential in first-principles simulations allows one to directly simulate the process of charge storage at the electrode-electrolyte interface, and thereby to unravel the interplay between the electrode and the electrolyte. I will show how these more complex treatments lead to improved descriptions of intrinsic quantum and EDL capacitance contributions in graphene-based supercapacitors, which can be used to suggest engineering strategies for improved electrode materials. I will also discuss how combining theory with in operando X-ray spectroscopy can give insights into nanoscale chemical changes and mesoscale morphological changes in electrodes during charging. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344.
Electronic and optical properties of Au-doped Cu2O: A first principles investigation
NASA Astrophysics Data System (ADS)
Jiang, Zhong-Qian; Yao, Gang; An, Xin-You; Fu, Ya-Jun; Cao, Lin-Hong; Wu, Wei-Dong; Wang, Xue-Min
2014-05-01
The Cu2O and Au-doped Cu2O films are prepared on MgO (001) substrates by pulsed laser deposition. The X-ray photoelectron spectroscopy proves that the films are of Au-doped Cu2O. The optical absorption edge decreases by 1.6% after Au doping. The electronic and optical properties of pure and Au-doped cuprite Cu2O films are investigated by the first principles. The calculated results indicate that Cu2O is a direct band-gap semiconductor. The scissors operation of 1.64 eV has been carried out. After correcting, the band gaps for pure and Au doped Cu2O are about 2.17 eV and 2.02 eV, respectively, decreasing by 6.9%. All of the optical spectra are closely related to the dielectric function. The optical spectrum red shift corresponding to the decreasing of the band gap, and the additional absorption, are observed in the visible region for Au doped Cu2O film. The experimental results are generally in agreement with the calculated results. These results indicate that Au doping could become one of the more important factors influencing the photovoltaic activity of Cu2O film.
First principles calculations of ternary wurtzite Î²-CuGaO2
NASA Astrophysics Data System (ADS)
Suzuki, Issei; Nagatani, Hiraku; Kita, Masao; Iguchi, Yuki; Sato, Chiyuki; Yanagi, Hiroshi; Ohashi, Naoki; Omata, Takahisa
2016-03-01
The electronic structure of Î²-CuGaO2 was studied by first principles calculations and X-ray photoelectron spectroscopy (XPS), and the expected electrical and optical properties of this material were discussed. Density functional theory calculations using the local density approximation with corrections for on-site Coulomb interactions (LDA + U) with U = 5-7 eV reproduced well the experimentally obtained crystal structure and valence-band XPS spectrum. The calculated electronic structure indicates that Î²-CuGaO2 is a direct band gap semiconductor and its conduction band minimum and valence band maximum consist mainly of highly delocalized Ga 4s and Cu 4s states and relatively localized Cu 3d and O 2p states, respectively. The effective electron mass obtained under parabolic approximation is small (me*/m0 = 0.21), similar to common n-type oxide semiconductors, and the effective hole mass is relatively large (mh*/m0 = 1.7-5.1) although p-type conduction is experimentally observed. The direct and allowed band gap and large density of states near the valence band maximum result in a high absorption coefficient of 1 Ã— 105 cm-1 near the absorption edge.
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 ZBâ†’NaCl at a pressure 10 GPa is predicted for ZrH.
Gavrilenko, V. I.; Wu, R. Q.; Downer, M. C.; Ekerdt, J. G.; Lim, D.; Parkinson, P.
2001-04-15
We present calculated second-harmonic-generation (SHG) spectra of the Si(001) surface based on a first-principles description of eigenvalues and eigenvectors using ab initio pseudopotentials. We also present SHG spectra for Ge-covered Si(001). The theoretical results explain all essential features of recent experimental SHG spectra of the Si(001)-(2x1) surface with low coverages of hydrogen and/or germanium, which alter the E{sub 1} resonance in contrasting ways. The strong adatom specificity of the spectra results from redistribution of the adatom-related electronic states on the surface.
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).
Zhao, Li-Juan; Xu, Hong-Guang; Feng, Gang; Wang, Peng; Xu, Xi-Ling; Zheng, Wei-Jun
2016-02-17
We investigate BS2(-) and BSO(-) clusters using photoelectron spectroscopy and theoretical calculations. The electron affinities of BS2 and BSO are measured to be 3.80 ± 0.03 and 3.88 ± 0.03 eV, respectively, higher than those of halogen atoms. Thus, BS2 and BSO can be considered as superhalogens. The comparison of experimental and theoretical results confirmed that the ground state structures of BS2(-), BSO(-), and their neutrals are all linear. Analyses of natural bond orbitals suggest that both BS2(-) and BSO(-) have dual 3c-4e ? hyperbonds. PMID:26847710
Kumar, Ravhi S.; Svane, Axel; Vaitheeswaran, Ganapathy; Kanchana, Venkatakrishnan; Antonio, Daniel; Cornelius, Andrew L.; Bauer, Eric D.; Xiao, Yuming; Chow, Paul
2015-10-19
The crystal structure and the Yb valence of the YbFe_{2}Ge_{2} heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. Furthermore, the measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr_{2}Si_{2}-type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at ~9 GPa, where at low temperature a pressure-induced quantum critical state was reported.
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 âˆ¼32 (30â€‰GPa at 300â€‰K obtained from comparison of Gibbs free energy at 300â€‰K). The transition is identified to be of first order in nature with a volume discontinuity of âˆ¼6.2% 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 âˆ¼19.7mRy/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 âˆ¼112 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.
NASA Astrophysics Data System (ADS)
Kocak, B.; Ciftci, Y. O.; Colakoglu, K.; Deligoz, E.
2010-10-01
First principles calculations are performed to investigate the structural, electronic, elastic, and thermodynamic properties of the less known PrN compound for various space groups: NaCl(Fm3m(2 2 5)), CsCl(Pm3m (2 2 1)), ZB(F43m(2 1 6)), Wc(P6m2(1 8 7)), and CuAu (P4/mmm (1 2 3)). Our calculation indicates that the NaCl type structure is energetically more stable than the other phases. The calculated lattice parameters are consisted with available theoretical and experimental results. Our band structure calculations show that PrN possessess a semi-metallic character for both with and without spin polarized (SP) cases. The calculated elastic constants satisfy the mechanical stability conditions at all considered pressures and the present values are significantly higher than those of the previous results. The related mechanical properties such as Zener anisotropy factor ( A), Poisson’s ratio ( ?), Young’s modulus ( E), and shear modulus ( C) are also computed for NaCl structure. The temperature/pressure-dependent behaviours of bulk modulus, Debye temperature, heat capacity, thermal expansion coeffient, and V/ V0 ratio estimated within the quasi-harmonic Debye model.
Rare-earth vs. heavy metal pigments and their colors from first principles
Tomczak, Jan M.; Pourovskii, Leonid V.; Vaugier, Loig; Georges, Antoine; Biermann, Silke
2013-01-01
Many inorganic pigments contain heavy metals hazardous to health and environment. Much attention has been devoted to the quest for nontoxic alternatives based on rare-earth elements. However, the computation of colors from first principles is a challenge to electronic structure methods, especially for materials with localized f-orbitals. Here, starting from atomic positions only, we compute the colors of the red pigment cerium fluorosulfide as well as mercury sulfide (classic vermilion). Our methodology uses many-body theories to compute the optical absorption combined with an intermediate length-scale modelization to assess how coloration depends on film thickness, pigment concentration, and granularity. We introduce a quantitative criterion for the performance of a pigment. While for mercury sulfide, this criterion is satisfied because of large transition matrix elements between wide bands, cerium fluorosulfide presents an alternative paradigm: the bright red color is shown to stem from the combined effect of the quasi-2D and the localized nature of states. Our work shows the power of modern computational methods, with implications for the theoretical design of materials with specific optical properties. PMID:23302689
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.
First-principles calculation on Î²-SiC(111)/Î±-WC(0001) interface
Jin, Na; Yang, Yanqing E-mail: jinna319@163.com; Li, Jian; Luo, Xian; Huang, Bin; Sun, Qing; Guo, Pengfei
2014-06-14
The Î±-WC(0001) surface and Î²-SiC(111)/Î±-WC(0001) interface were studied by first-principles calculation based on density functional theory. It is demonstrated that the Î±-WC(0001) surface models with more than nine atom-layers exhibit bulk-like interior, wherein the surface relaxations localized within the top three layers are well converged. Twenty-four specific geometry models of SiC/WC interface structures with different terminations and stacking sites were chosen. The calculated work of adhesion and interface energy suggest that the most stable interface structure has the C-C bonding across the interface, yielding the largest work of adhesion and the lowest interface energy. Moreover, the top-site stacking sequence is preferable for the C/C-terminated interface. The effects of the interface on the electronic structures of the C/C-terminated interfaces are mainly localized within the first and second layers of the interface. Calculations of the work of adhesion and interface energy provide theoretical evidence that the mechanical failure may initiate at the interface or in SiC but not in WC.
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.
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.
NASA Astrophysics Data System (ADS)
Tsukamoto, Shigeru; Caciuc, Vasile; Atodiresei, Nicolae; Blügel, Stefan
2012-06-01
In this first-principles study, we present density-functional calculations of the electronic structures and electron transport properties of organic molecular junctions with several anchoring groups containing atoms with different electronegativities, i.e., benzenediboronate (BDB), benzenedicarboxylate (BDC), and dinitrobenzene (DNB) molecular junctions sandwiched between two Cu(110) electrodes. The electronic-structure calculations exhibit a significant difference in the density of states not only at the anchoring groups but also at the aromatic rings of the molecular junctions, suggesting that the electron transport is specific for each system. Our transport calculations show that the BDB and DNB molecular junctions have finite electron transmissions at the zero-bias limit while the BDC molecular junction has a negligible electron transmission. Moreover, for the BDB and DNB systems, the electron transmission channels around the Fermi energy reveal fingerprint features, which provide specific functionalities for the molecular junctions. Therefore, our theoretical results demonstrate the possibility to precisely tune the electron transport properties of molecular junctions by engineering the anchoring groups at the single-atom level.
O 2 dissociation on nitrogen doped carbon nanotubes (10, 0) from first principles simulation
NASA Astrophysics Data System (ADS)
Yang, Shizhong; Zhao, Guang-Lin; Khosravi, Ebrahim
2011-03-01
Reducing the amount of precious platinum (Pt) loading by identifying non-precious metal catalyst is essential for large-scale applications of fuel cells, which provide a cleaning energy technology. Recent experimental, theoretical, and simulation works accelerate the advance in the research area of doped carbon nanotubes acting as an alternate non-precious metal catalyst for dioxygen reduction in the fuel cells. First principles spin polarized density functional theory(DFT) simulations have been performed to understand O2 dissociation on nitrogen doped carbon nanotubes. We have studied nitrogen substitutional doping of carbon nanotubes (CNTs) for dioxygen adsorption, reduction, and dissociation. The calculated results show that nitrogen prefers to stay at the open-edge of short CNTs. Two O2 chemisorption sites are found, the carbon-nitrogen complex (Pauling site) and carbon-carbon long bridge (long bridge) sites. The spin polarized DFT calculations using the nudged elastic band (NEB) method show that O2 dissociation at the Pauling site has a reaction energy barrier of about 0.55 eV. The unique open-edge structure and charge redistribution are crucial to the novel properties of nitrogen-doped CNTs as a new non-precious metal catalyst for fuel cells.
Thermodynamic ground state of MgB{sub 6} predicted from first principles structure search methods
Wang, Hui; Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 ; LeBlanc, K. A.; Gao, Bo; Yao, Yansun; Canadian Light Source, Saskatoon, Saskatchewan S7N 0X4
2014-01-28
Crystalline structures of magnesium hexaboride, MgB{sub 6}, were investigated using unbiased structure searching methods combined with first principles density functional calculations. An orthorhombic Cmcm structure was predicted as the thermodynamic ground state of MgB{sub 6}. The energy of the Cmcm structure is significantly lower than the theoretical MgB{sub 6} models previously considered based on a primitive cubic arrangement of boron octahedra. The Cmcm structure is stable against the decomposition to elemental magnesium and boron solids at atmospheric pressure and high pressures up to 18.3 GPa. A unique feature of the predicted Cmcm structure is that the boron atoms are clustered into two forms: localized B{sub 6} octahedra and extended B{sub âˆž} ribbons. Within the boron ribbons, the electrons are delocalized and this leads to a metallic ground state with vanished electric dipoles. The present prediction is in contrast to the previous proposal that the crystalline MgB{sub 6} maintains a semiconducting state with permanent dipole moments. MgB{sub 6} is estimated to have much weaker electron-phonon coupling compared with that of MgB{sub 2}, and therefore it is not expected to be able to sustain superconductivity at high temperatures.
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 6âˆ’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 study of FeSe epitaxial films on SrTiO3
NASA Astrophysics Data System (ADS)
Liu, Kai; Gao, Miao; Lu, Zhong-Yi; Xiang, Tao
2015-11-01
The discovery of high temperature superconductivity in FeSe films on SrTiO3 substrate has inspired great experimental and theoretical interests. First-principles density functional theory calculations, which have played an important role in the study of bulk iron-based superconductors, also participate in the investigation of interfacial superconductivity. In this article, we review the calculation results on the electronic and magnetic structures of FeSe epitaxial films, emphasizing on the interplay between different degrees of freedom, such as charge, spin, and lattice vibrations. Furthermore, the comparison between FeSe monolayer and bilayer films on SrTiO3 is discussed. Project supported by the National Natural Science Foundation of China (Grant Nos.Â 11190024 and 11404383), the National Basic Research Program of China (Grant No.Â 2011CBA00112), the Fundamental Research Funds for the Central Universities, China, and the Research Funds of Renmin University of China (Grant No.Â 14XNLQ03).
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. PMID:25947542
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.
Thermoelectric properties of AgSbTeâ‚‚ from first-principles calculations
Rezaei, Nafiseh; Akbarzadeh, Hadi; Hashemifar, S. Javad
2014-09-14
The structural, electronic, and transport properties of AgSbTeâ‚‚ 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 Fd3â»m and trigonal R3â»m structures of AgSbTeâ‚‚ 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 0.1â€“0.35 eV for AgSbTeâ‚‚ 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 AgSbTeâ‚‚ as a function of temperature and carrier concentration.
Pressure-induced novel compounds in the Hf-O system from first-principles calculations
NASA Astrophysics Data System (ADS)
Zhang, Jin; Oganov, Artem R.; Li, Xinfeng; Xue, Kan-Hao; Wang, Zhenhai; Dong, Huafeng
2015-11-01
Using first-principles evolutionary simulations, we have systematically investigated phase stability in the Hf-O system at pressure up to 120 GPa. New compounds Hf5O2,Hf3O2 , HfO, and HfO3 are discovered to be thermodynamically stable at certain pressure ranges. Two new high-pressure phases are found for Hf2O : one with space group Pnnm and anti-CaCl2-type structure, another with space group I 41/amd. Pnnm-HfO3 shows interesting structure, simultaneously containing oxide O2 - and peroxide [O-O]2 - anions. Remarkably, it is P 6 ¯2 m -HfO rather than OII-HfO2 that exhibits the highest mechanical characteristics among Hf-O compounds. Pnnm-Hf2O , Imm2-Hf5O2 ,P 3 ¯1 m -Hf2O , and P 4 ¯m 2 -Hf2O3 phases also show superior mechanical properties; theoretically these phases become metastable phases to ambient pressure and their properties can be exploited.
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.
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
First-principles investigation on mechanical properties of Î¶-Ta4C3-x
NASA Astrophysics Data System (ADS)
Yan, Wen-Li; Sygnatowicz, Michael; Shetty, Dinish; Lu, Guang-Hong; Liu, Feng
2015-03-01
As a group of transition metal carbides, tantalum carbides are of great interest due to their high melting temperature and high strength. Among different tantalum carbide phases of different space group and C/Ta atom ratio, the trigonal phase Î¶-Ta4C3-x attracts special attention as high volume fraction of the Î¶ phase is reported to increase the fracture toughness of a tantalum carbide matrix. Using first-principles method, the structural and mechanical properties of Î¶-Ta4C3-x have been investigated. The calculation results show that the weak bonding between Ta atoms in Ta4C3 is further weakened when structural vacancies occupy the carbon sub-lattice in Î¶-Ta4C3-x. The (0 0 1) Ta surface is prominent to appear from surface energy and stacking fault energy calculations, consistent with the observed lamellar substructure during indentation process in fracture toughness measurements. The theoretical fracture toughness is derived from the Griffith relation, in comparison with the experimental results, to explain outstanding questions pertaining to mechanical properties of Î¶-Ta4C3-x. This work is supported by China Scholarship Council (Grant No. 201306020117) and DOE-BES (Grant No. DEFG02-04ER46148).
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-principles determination of the effect of boron on aluminum grain boundary cohesion
NASA Astrophysics Data System (ADS)
Zhang, Shengjun; Kontsevoi, Oleg Y.; Freeman, Arthur J.; Olson, Gregory B.
2011-10-01
Despite boron being a common alloying element in aluminum, its segregation into the aluminum grain boundary and its effect on the grain boundary strength have not been studied. Here, the electronic structures of the boron-doped ?5(012)[100] symmetrical tilt grain boundary and (012) free surface systems for aluminum are investigated by means of first-principles calculations using the full-potential linearized augmented plane-wave method with the generalized gradient approximation, within the framework of the Rice-Wang thermodynamic model and the theoretical tensile test approach. We establish that boron has a large driving force to segregate from Al bulk to the symmetrical grain boundary hollow site, and its segregation significantly enhances the grain boundary strength. Through precise calculations on both the grain boundary and free surface environments, it is found that boron is a strong cohesion enhancer in aluminum with a potency of -0.19 eV/atom. An analysis in terms of the relaxed atomic and electronic structures and bonding characters shows that the aluminum-boron bond has mixed covalent and metallic character and is strong in both grain boundary and free surface environments. The strengthening effect of boron is due to creation of additional B-Al bonds across the grain boundary, which are as strong as existing Al-Al transgranular bonds and thus significantly increase grain boundary adhesion and its resistance to tensile stress and cracking.
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 calculation on Î²-SiC(111)/Î±-WC(0001) interface
NASA Astrophysics Data System (ADS)
Jin, Na; Yang, Yanqing; Li, Jian; Luo, Xian; Huang, Bin; Sun, Qing; Guo, Pengfei
2014-06-01
The ?-WC(0001) surface and ?-SiC(111)/?-WC(0001) interface were studied by first-principles calculation based on density functional theory. It is demonstrated that the ?-WC(0001) surface models with more than nine atom-layers exhibit bulk-like interior, wherein the surface relaxations localized within the top three layers are well converged. Twenty-four specific geometry models of SiC/WC interface structures with different terminations and stacking sites were chosen. The calculated work of adhesion and interface energy suggest that the most stable interface structure has the C-C bonding across the interface, yielding the largest work of adhesion and the lowest interface energy. Moreover, the top-site stacking sequence is preferable for the C/C-terminated interface. The effects of the interface on the electronic structures of the C/C-terminated interfaces are mainly localized within the first and second layers of the interface. Calculations of the work of adhesion and interface energy provide theoretical evidence that the mechanical failure may initiate at the interface or in SiC but not in WC.
Electrical contacts to monolayer black phosphorus: A first-principles investigation
NASA Astrophysics Data System (ADS)
Gong, Kui; Zhang, Lei; Ji, Wei; Guo, Hong
2014-09-01
We report first-principles theoretical investigations of possible metal contacts to monolayer black phosphorus (BP). By analyzing lattice geometry, five metal surfaces are found to have minimal lattice mismatch with BP: Cu(111), Zn(0001), In(110), Ta(110), and Nb(110). Further studies indicate Ta and Nb bond strongly with monolayer BP causing substantial bond distortions, but the combined Ta-BP and Nb-BP form good metal surfaces to contact a second layer BP. By analyzing the geometry, bonding, electronic structure, charge transfer, potential, and band bending, it is concluded that Cu(111) is the best candidate to form excellent Ohmic contact to monolayer BP. The other four metal surfaces or combined surfaces also provide viable structures to form metal/BP contacts, but they have Schottky character. Finally, the band bending property in the current-in-plane (CIP) structure where metal/BP is connected to a freestanding monolayer BP, is investigated. By both work function estimates and direct calculations of the two-probe CIP structure, we find that the freestanding BP channel is n type.
First-Principles Calculations of LEEM Reflectivity Spectra of Molybdenum Disulfide
NASA Astrophysics Data System (ADS)
McClain, John; Pohl, Karsten; Tang, Jian-Ming
2015-03-01
We present calculations of the low-energy electron specular reflectivity spectra of systems of a few layers of molybdenum disulfide at general angles of incidence using a newly modified algorithm within our first-principles theoretical approach, which leverages the self-consistent scattering potentials produced by density-functional theory. Our calculated normal-incidence spectra for MoS2 reveal layer-dependent features around 7-8 eV and 15 eV, allowing for a characterization of the number of layers via LEEM reflectivity and thus an in-situ technique for growth monitoring. We have previously described the application of our approach to the off-normal spectra of few-layer graphene, but the lack of mirror symmetry in MoS2 requires a new algorithm for finding degenerate pairs of solutions for the matching procedure. The computed off-normal spectra illustrates the complexity of the electronic structure of MoS2. We also present the way in which our new off-normal algorithm leads naturally to an approach to higher-order diffraction intensity calculations with the wave-matching scheme, along with our results for higher-order diffraction in model systems and progress towards results for real systems. This work is partly supported by a University of New Hampshire Dissertation Year Fellowship.
Two-dimensional arsenic monolayer sheet predicted from first-principles
NASA Astrophysics Data System (ADS)
Pu, Chun-Ying; Ye, Xiao-Tao; Jiang, Hua-Long; Zhang, Fei-Wu; Lu, Zhi-Wen; He, Jun-Bao; Zhou, Da-Wei
2015-03-01
Using first-principles calculations, we investigate the two-dimensional arsenic nanosheet isolated from bulk gray arsenic. Its dynamical stability is confirmed by phonon calculations and molecular dynamics analyzing. The arsenic sheet is an indirect band gap semiconductor with a band gap of 2.21 eV in the hybrid HSE06 functional calculations. The valence band maximum (VBM) and the conduction band minimum (CBM) are mainly occupied by the 4p orbitals of arsenic atoms, which is consistent with the partial charge densities of VBM and CBM. The charge density of the VBM G point has the character of a Ï€ bond, which originates from p orbitals. Furthermore, tensile and compressive strains are applied in the armchair and zigzag directions, related to the tensile deformations of zigzag and armchair nanotubes, respectively. We find that the ultimate strain in zigzag deformation is 0.13, smaller than 0.18 of armchair deformation. The limit compressive stresses of single-layer arsenic along armchair and zigzag directions are -4.83 GPa and -4.76 GPa with corresponding strains of -0.15 and -0.14, respectively. Projected supported by the Henan Joint Funds of the National Natural Science Foundation of China (Grant Nos. U1304612 and U1404608), the National Natural Science Foundation of China (Grant Nos. 51374132 and 11404175), the Special Fund for Theoretical Physics of China (Grant No. 11247222), and Nanyang Normal University Science Foundation, China (Grant Nos. ZX2012018 and ZX2013019).
What about U on surfaces? Extended Hubbard models for adatom systems from first principles.
Hansmann, Philipp; Vaugier, Loïg; Jiang, Hong; Biermann, Silke
2013-03-01
Electronic correlations together with dimensional constraints lead to some of the most fascinating properties known in condensed matter physics. As possible candidates where these conditions are realized, semiconductor (111) surfaces and adatom systems on surfaces have been under investigation for quite some time. However, state-of-the-art theoretical studies on these materials that include many-body effects beyond the band picture are rare. First principles estimates of inter-electronic Coulomb interactions for the correlated states are missing entirely, and usually these interactions are treated as adjustable parameters. In this work, we report on calculations of the interaction parameters for the group IV surface-adatom systems in the ?-phase series of Si(111):C, Si, Sn, Pb. For all systems investigated, the inter-electronic Coulomb interactions are indeed large compared to the kinetic energies of the states in question. Moreover, our study reveals that intersite interactions cannot be disregarded. We explicitly construct an extended Hubbard model for the series of group IV surface-adatom systems on silicon, which can be used for further many-body calculations. PMID:23400014
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 calculations on Mg/Al2CO interfaces
NASA Astrophysics Data System (ADS)
Wang, F.; Li, K.; Zhou, N. G.
2013-11-01
The electronic structure, work of adhesion, and interfacial energy of the Mg(0 0 0 2)/Al2CO(0 0 0 1) interface were studied with the first-principles calculations to clarify the heterogeneous nucleation potential of Al2CO particles in Mg melt. AlO-terminated Al2CO(0 0 0 1) slabs with seven atomic layers were adopted for interfacial model geometries. Results show that the "Over O" stacking interface is more stable than the "Over Al" stacking interface due to the larger interfacial adhesion and stronger mixed ionic/metallic bond formed across the interface. The calculated interfacial energies of Mg/Al2CO depend on the value of Î”Î¼Al + Î”Î¼C, proving Al2CO particles can exist stably in Mg-Al alloys melt and become effective nucleation substrate for Î±-Mg grain under certain conditions. The above calculation and corresponding analysis provide strong theoretical support to the Al2CO nucleus hypothesis from interfacial atomic structure and atomic bonding energy considerations.
NASA Astrophysics Data System (ADS)
Junquera, Javier; Aguado-Puente, Pablo
2007-03-01
The field of ferroelectric thin films is at a momentous stage. Several experimental and theoretical works, both within a phenomenological approach or atomistic models, have been devoted to ascertain the ground state of ferroelectric thin-film capacitors. Using a full first-principles density-functional-theory approach, we have simulated 180^o ferroelectric stripe domains at 0 K in SrRuO3/BaTiO3/SrRuO3 and SrRuO3/PbTiO3/SrRuO3 realistic ferroelectric capacitors epitaxially grown on a SrTiO3 substrate. For a ferroelectic thin-film 2 unit cells thick, the lateral sizes of the domains ranged from 2 to 4 unit cells. The in-plane displacement of the atoms is essential to stabilize the domain structure. A exotic vortex structure, closed by the atoms of the first metallic layer, is found for the polarization. Ph. Ghosez and J. Junquera, http://xxx.lanl.gov/pdf/cond-mat/0605299, and references therein. J. M. Soler et al., J. Phys.: Condens. Matter 14 2745 (2002)
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.
First-principles calculation on dilute magnetic alloys in zinc blend crystal structure
NASA Astrophysics Data System (ADS)
Ullah, Hamid; Inayat, Kalsoom; Khan, S. A.; Mohammad, S.; Ali, A.; Alahmed, Z. A.; Reshak, A. H.
2015-07-01
Ab-initio calculations are performed to investigate the structural, electronic and magnetic properties of spin-polarized diluted magnetic alloys in zinc blende structure. The first-principles study is carried out on Mn doped III-V semiconductors. The calculated band structures, electronic properties and magnetic properties of Ga1-xMnxX (X=P, As) compounds reveal that Ga0.75Mn0.25P is half metallic turned to be metallic with increasing x to 0.5 and 0.75, whereas substitute P by As cause to maintain the half-metallicity nature in both of Ga0.75Mn0.25As and Ga0.5Mn0.5As and tune Ga0.25Mn0.75As to be metallic. Calculated total magnetic moments and the robustness of half-metallicity of Ga0.75Mn0.25P, Ga0.75Mn0.25As and Ga0.5Mn0.5As with respect to the variation in lattice parameters are also discussed. The predicted theoretical evidence shows that some Mn-doped III-V semiconductors can be effectively used in spintronic devices.
First-principles calculations of transition metal solute interactions with hydrogen in tungsten
NASA Astrophysics Data System (ADS)
Kong, Xiang-Shan; Wu, Xuebang; Liu, C. S.; Fang, Q. F.; Hu, Q. M.; Chen, Jun-Ling; Luo, G.-N.
2016-02-01
We have performed systematic first-principles calculations to predict the interaction between transition metal (TM) solutes and hydrogen in the interstitial site as well as the vacancy in tungsten. We showed that the site preference of the hydrogen atom is significantly influenced by the solute atoms, which can be traced to the charge density perturbation in the vicinity of the solute atom. The solute-H interactions are mostly attractive except for Re, which can be well understood in terms of the competition between the chemical and elastic interactions. The chemical interaction dominates the solute-H interaction for the TM solutes with a large atomic volume and small electronegativity compared to tungsten, while the elastic interaction is primarily responsible for the solute-H interaction for the TM solutes with a small atomic volume and large electronegativity relative to tungsten. The presence of a hydrogen atom near the solute atom has a negative effect on the binding of other hydrogen atoms. The large positive binding energies among the solute, vacancy and hydrogen suggest that they would easily form a defect cluster in tungsten, where the solute-vacancy and vacancy-H interaction contribute greatly while the solute-H interaction contributes a little. Our result provides a sound theoretical explanation for recent experimental phenomena of hydrogen retention in the tungsten alloy and further recommends a suitable Wâ€“Reâ€“Ta ternary alloy for possible plasma-facing materials (PFMs) including the consideration of the hydrogen retention.
CernÃ½, Miroslav; RehÃ¡k, Petr; Umeno, Yoshitaka; Pokluda, Jaroslav
2013-01-23
The response of three covalent crystals with a diamond lattice (C, Si and Ge) to uniaxial and a special triaxial (generally nonhydrostatic) loading is calculated from first principles. The lattice deformations are described in terms of variations of bond lengths and angles. The triaxial stress state is simulated as a superposition of axial tension or compression and transverse (both tensile and compressive) biaxial stresses. The biaxial stresses are considered to be adjustable parameters and the theoretical strengths in tension and compression along <100>, <110>, <111> crystallographic directions are calculated as their functions. The obtained results revealed that the compressive strengths are, consistently to fcc metals, almost linear functions of the transverse stresses. Tensile transverse stresses lower the compressive strength and vice versa. The tensile strengths, however, are not monotonic functions of the transverse biaxial stresses since they mostly exhibit maxima for certain values of the transverse stresses (e.g., tensile for <100> and <110> loading of Si and Ge or compressive for <100> loading of C). PMID:23238035
Theoretical background of optical emission spectroscopy for analysis of atmospheric pressure plasmas
NASA Astrophysics Data System (ADS)
Belmonte, Thierry; Noël, Cédric; Gries, Thomas; Martin, Julien; Henrion, Gérard
2015-12-01
This review contains a theoretical background of optical emission spectroscopy and some selected examples of issues in the field of atmospheric plasmas. It includes elements like line broadening, emission of continua and molecules, radiation models, etc. Modernized expressions figuring the terms hidden in global constants where cgs units prevail are given together with restrictions of use. Easy-to-use formulas are provided to give access to essential plasma parameters.
NASA Astrophysics Data System (ADS)
Groh, Sébastien; Alam, Masud
2015-06-01
An approach to derive, from first-principles data, accurate and reliable potentials in the modified embedded-atom method in view of modeling the mechanical behavior of metals is presented in this work and applied to the optimization of a potential representative of lithium (Li). Although the theoretical background of the modified embedded-atom method was considered in this work, the proposed method is general and it can be applied to any other functional form. The main feature of the method is to introduce several path transformations in the material database that are critical for plastic and failure behavior. As part of the potential validation, path transformations different from the ones used for the parameterization procedure are considered. Applied in the case of Li, the material database was enriched with the generalized stacking fault energy curve along the??<1?1?1>??-direction on the {1?1?0}-plane, and with the traction-separation behavior of a {1?0?0}-surface. The path transformations used to enrich the material database were initially derived from first-principles calculations. For validation, the generalized stacking fault energy curves along the??<1?1?1>??-direction on the {1?1?2}- and {1?2?3}-planes were considered for plasticity, while traction-separation behavior of {1?1?0} and {1?1?1}-planes were considered for failure behavior. As part of the validation procedure, the predictions made in the MEAM framework were validated by first-principles data. The final potential accurately reproduced basic equilibrium properties, elastic constants, surface energies in agreement with first-principles predictions, and transition energy between different crystal structures. Furthermore, generalized stacking fault energy curves along the??<1?1?1>??-direction on the {1?1?0}, {1?1?2}, and {1?2?3}-planes, and tensile cohesive stress, characteristic length of fracture, and work of separation of a {1?0?0}, {1?1?0}, and {1?1?1} surfaces obtained in the MEAM framework compared well with first-principles predictions. It also predicts good elastic constants for a crystal structure different than the one used for the fitting of the potential and the other four path transformations. The potential was tested for failure behavior using a full atomistic setup, and in addition of being qualitatively correct, the stress intensity factor for different crack orientations was found to be in agreement with the theory of Rice (1992 J. Mech. Phys. Solids 40 239-71) within an error of 10%. Finally, the optimized Li-MEAM potential is expected to be transferable to different local environments encountered in atomistic simulations of lattice defects.
NASA Astrophysics Data System (ADS)
Bolognesi, P.; Mattioli, G.; O'Keeffe, P.; Feyer, V.; Plekan, O.; Ovcharenko, Y.; Prince, K. C.; Coreno, M.; Amore Bonapasta, A.; Avaldi, L.
2009-11-01
The inner shell ionization of pyrimidine and some halogenated pyrimidines has been investigated experimentally by X-ray photoemission spectroscopy (XPS) and theoretically by density functional theory (DFT) methods. The selected targets-5-Br-pyrimidine, 2-Br-pyrimidine, 2-Cl-pyrimidine, and 5-Br-2-Cl-pyrimidine-allowed the study of the effect of the functionalization of the pyrimidine ring by different halogen atoms bound to the same molecular site, or by the same halogen atom bound to different molecular sites. The theoretical investigation of the inductive and resonance effects in the C(1s) ionization confirms the soundness of the resonance model for a qualitative description of the properties of an aromatic system. Moreover, the combination of the experimental results and the theoretical analysis provides a detailed description of the effects of the halogen atom on the screening of a C(1s) hole in the aromatic pyrimidine ring.
First-principles investigation of vanadium isotope fractionation in solution and during adsorption
NASA Astrophysics Data System (ADS)
Wu, Fei; Qin, Tian; Li, Xuefang; Liu, Yun; Huang, Jen-How; Wu, Zhongqing; Huang, Fang
2015-09-01
Equilibrium fractionation factors of vanadium (V) isotopes among tri- (V(III)), tetra- (V(IV)) and penta-valent (V(V)) inorganic V species in aqueous system and during adsorption of V(V) to goethite are estimated using first-principles calculation. Our results highlight the dependence of V isotope fractionation on valence states and the chemical binding environment. The heavy V isotope (51V) is enriched in the main V species following a sequence of V(III) < V(IV) < V(V). According to our calculations, at 25 °C, the equilibrium isotope fractionation factor between [V5+O2(OH)2]- and [V4+O(H2O)5]2+ (ln ?? V (V)- V (IV)) is 3.9‰, and the equilibrium isotope fractionation factor between [V5+O2(OH)2]- and [V3+(OH)3(H2O)3] (ln ?? V (V)- V (III)) is 6.4‰. In addition, isotope fractionation between +5 valence species [V5+O2(OH)2]- and [V5+O2(H2O)4]+ is 1.5‰ at 25 °C, which is caused by their different bond lengths and coordination numbers (CN). Theoretical calculations also show that light V isotope (50V) is preferentially adsorbed on the surface of goethite. Our work reveals that V isotopes can be significantly fractionated in the Earth's surface environments due to redox reaction and mineral adsorption, indicating that V isotope data can be used to monitor toxic V(V) attenuation processes through reduction or adsorption in natural water systems. In addition, a simple mass balance model suggests that V isotope composition of seawater might vary with change of ambient oxygen levels. Thus our theoretical investigations imply a promising future for V isotopes as a potential new paleo-redox tracer.
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)
Wilson, H. F.
2013-12-01
First-principles atomistic simulation is a vital tool for understanding the properties of materials at the high-pressure high-temperature conditions prevalent in giant planet interiors, but properties such as solubility and phase boundaries are dependent on entropy, a quantity not directly accessible in simulation. Determining entropic properties from atomistic simulations is a difficult problem typically requiring a time-consuming integration over molecular dynamics trajectories. Here I will describe recent advances in first-principles thermodynamic calculations which substantially increase the simplicity and efficiency of thermodynamic integration and make entropic properties more readily accessible. I will also describe the use of first-principles thermodynamic calculations for understanding problems including core solubility in gas giants and superionic phase changes in ice giants, as well as future prospects for combining first-principles thermodynamics with planetary-scale models to help us understand the origin and consequences of compositional inhomogeneity in giant planet interiors.
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.
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
First-principles study of electronic structure and optical properties of Sr(Ti,Zr)O3
NASA Astrophysics Data System (ADS)
Celik, Gulden; Cabuk, Suleyman
2013-03-01
Electronic and optical properties of Sr(Ti,Zr)O3 crystals in the cubic (Pm-3m) and tetragonal (I4/mcm) phase were calculated by the first-principles calculations using the density functional theory and the local density approximation. The band structure of cubic and tetragonal phases show an indirect band gap at (R-?) point and at (M-?) point in the Brillouin zone, respectively. The linear photon-energy dependent dielectric functions and some optical properties such as the absorption coefficient, energy-loss function and reflectivity are calculated for both phases. The optical properties of tetragonal phase of Sr(Ti,Zr)O3 were investigated by theoretical methods for the first time. We have also made some comparisons with the available related experimental and theoretical data.
NASA Astrophysics Data System (ADS)
Lemal, Sébastien; Nguyen, Ngoc; de Boor, Johannes; Ghosez, Philippe; Varignon, Julien; Klobes, Benedikt; Hermann, Raphaël P.; Verstraete, Matthieu J.
2015-11-01
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 1.6 ?B/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 33 meV , 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 38.6 ? V K-1 , slightly lower than the theoretical result. The theoretical conductivity is analyzed as a function of temperature and concentration of charge carriers.
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 = 2â€“12) in the spectral range of the stretching and bending modes of the solute anion (600â€“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 hydrogenmoreÂ Â» bonding to the two terminal P=O groups, these are hydrated before the two Pâ€“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.Â«Â 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 = 2â€“12) in the spectral range of the stretching and bending modes of the solute anion (600â€“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 Pâ€“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.; PicanÃ§o, NÃ¡gela F. M.; Campo, GladÃs S. D. L.; Faria, Jorge L. B.; Instituto de FÃsica/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.
NASA Astrophysics Data System (ADS)
Kumar, Kishor; Bhatt, Samir; Jani, A. R.; Ahuja, B. L.
2015-12-01
We present the first-ever experimental Compton profiles (CPs) of ZrSSe2 and ZrS1.5Se1.5 using 100 mCi 241Am Compton spectrometer. To analyze the experimental momentum densities, we have computed for the first-time the CPs, energy bands and density of states using linear combination of atomic orbitals (LCAO) method. To model the exchange and correlation effects within LCAO approach, we have considered Hartree-Fock (HF), density functional theory (DFT) with revised functional of Perdew-Becke-Ernzerhof (PBEsol) and hybridization of HF and DFT. Going beyond computation of electronic properties using LCAO method, we have also derived electronic and optical properties using the modified Becke-Johnson (mBJ) potential within full potential linearized augmented plane wave (FP-LAPW) method. There is notable decrease in the energy band gap on replacing S by Se atoms in ZrSSe2 to obtain ZrS1.5Se1.5 composition, which is mainly attributed to readjustment of Zr-4d, S-3p and Se-4p states. It is seen that the CPs based on hybridization of HF and DFT show a better agreement with the experimental profiles than those based on individual HF and DFT-GGA-PBEsol schemes. The optical properties computed using FP-LAPW-mBJ method unambiguously depict feasibility of using both the sulphoselenides in photovoltaics and also utility of ZrS1.5Se1.5 in the field of non-linear optics.
NASA Astrophysics Data System (ADS)
Helal, Yaser H.; Neese, Christopher F.; De Lucia, Frank C.; Ewing, Paul R.; Agarwal, Ankur; Craver, Barry; Stout, Phillip J.; Armacost, Michael D.
2015-06-01
Plasmas used in the semiconductor manufacturing industry are of a similar nature to the environments often created for submillimeter spectroscopic study of astrophysical species. At the low operating pressures of these plasmas, submillimeter absorption spectroscopy is a method capable of measuring the abundances and temperatures of molecules, radicals, and ions without disturbing any of the properties of the plasma. These measurements provide details and insight into the interactions and reactions occurring within the plasma and their implications for semiconductor manufacturing processes. A continuous wave, 500 to 750 GHz, absorption spectrometer was designed and used to make measurements of species in semiconductor processing plasmas. Comparisons with expectations from theoretical plasma models provide a basis for validating and improving these models, which is a complex and difficult science itself. Furthermore, these comparisons are an evaluation for the use of submillimeter spectroscopy as a diagnostic tool in manufacturing processes.
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.
Cao, Xiaoxiao; Huang, Yingying; Li, Wenbo; Zheng, Zhaoyang; Jiang, Xue; Su, Yan; Zhao, Jijun; Liu, Changling
2016-01-20
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
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) â†’ Pnma â†’ Cmcm â†’ CsCl type (B2) at hydrostatic pressures of âˆ¼19â€‰GPa, 36â€‰GPa, and 200â€‰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 36â€‰GPa and other up to 50â€‰GPa, on substoichiometric thorium carbide samples with carbon deficiency of âˆ¼20%, 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 âˆ¼33.8â€‰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 300â€‰K and compared these with available experimental data. Further, the behavior of zero pressure bulk modulus, heat capacity and Helmholtz free energy has been examined as a function temperature and compared with the experimental data of Danan [J. Nucl. Mater. 57, 280 (1975)].
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 (Îµ and Î¸) and a cubic Î´ 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 Î¸-TaN is the most ultraincompressible among the consider phases, which suggests that the Î¸ 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 Î¸-TaN is not stable above 53.9 GPa. In addition, the calculated B/G ratio indicated that the Îµ and Î´ phases possess brittle nature in the range of pressure from 0 to 100â€‰GPa. While Î¸ phase is brittleness at low pressure (below 8.2â€‰GPa) and is strongly prone to ductility at high pressure (above 8.2â€‰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
NASA Astrophysics Data System (ADS)
SenGupta, Sumana; Maiti, Nandita; Chadha, Ridhima; Kapoor, Sudhir
2015-10-01
Morpholine is a very interesting cyclic compound which is an ether as well as a secondary amine. The relative distribution of different conformations of morpholine depends on the medium. Using Raman spectroscopy and theoretical calculations, we found that, equatorial chair conformer is predominant in the pure liquid, but in aqueous solution, contribution from the axial conformer increases. The surface-enhanced Raman scattering (SERS) studies in presence of silver nanoparticles showed change in relative intensities of different vibrational modes of morpholine. It was found that the axial chair conformer is preferentially adsorbed vertically through the Nsbnd Ag bond on the nanoparticle surface.
Application of First Principles Ni-Cd and Ni-H2 Battery Models to Spacecraft Operations
NASA Technical Reports Server (NTRS)
Timmerman, Paul; Bugga, Ratnakumar; DiStefano, Salvador
1997-01-01
The conclusions of the application of first principles model to spacecraft operations are: the first principles of Bi-phasic electrode presented model provides an explanation for many behaviors on voltage fading on LEO cycling.
NASA Astrophysics Data System (ADS)
Parreiras, D. E.; Soares, E. A.; Abreu, G. J. P.; Bueno, T. E. P.; Fernandes, W. P.; de Carvalho, V. E.; Carara, S. S.; Chacham, H.; Paniago, R.
2014-10-01
The structural properties of graphene/Ni(111) are investigated by a combination of low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), angle-scanned photoelectron diffraction (PED), and first-principles calculations. XPS data indicate that graphene interacts strongly with the topmost Ni layer. Diffraction data show that graphene is deposited commensurably with the underlying Ni surface atoms. Twelve different graphene preparations were analyzed by LEED and one by PED. Considering the relative position between the carbon and Ni atoms, five experimental sets indicate a top-fcc structure, three show a bridge-top structure, and four have a mixed structure. The analysis of the PED experiment suggests the top-fcc termination. Our first-principles calculations show that the total energies of the top-fcc and bridge-top structures are nearly degenerate, which corroborates the observed bistability of those phases. Moreover, by comparing the structural parameters obtained by the three methods (LEED, PED, and ab initio calculations), excellent agreement is achieved.
NASA Astrophysics Data System (ADS)
Takahashi, Masae; Ishikawa, Yoichi; Ito, Hiromasa
2013-03-01
A weak hydrogen bond (WHB) such as CH-O is very important for the structure, function, and dynamics in a chemical and biological system WHB stretching vibration is in a terahertz (THz) frequency region Very recently, the reasonable performance of dispersion-corrected first-principles to WHB has been proven. In this lecture, we report dispersion-corrected first-principles calculation of the vibrational absorption of some organic crystals, and low-temperature THz spectral measurement, in order to clarify WHB stretching vibration. The THz frequency calculation of a WHB crystal has extremely improved by dispersion correction. Moreover, the discrepancy in frequency between an experiment and calculation and is 10 1/cm or less. Dispersion correction is especially effective for intermolecular mode. The very sharp peak appearing at 4 K is assigned to the intermolecular translational mode that corresponds to WHB stretching vibration. It is difficult to detect and control the WHB formation in a crystal because the binding energy is very small. With the help of the latest intense development of experimental and theoretical technique and its careful use, we reveal solid-state WHB stretching vibration as evidence for the WHB formation that differs in respective WHB networks The research was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant No. 22550003).
NASA Astrophysics Data System (ADS)
Barman, Sonali; Das, G. P.; Kawazoe, Y.
2013-12-01
Size-selected Wn clusters can be deposited firmly on a graphite (0001) surface using a novel technique, where the positive ions (of the same metal atom species) embedded on the graphite surface by ion implantation, act as anchors. The size selected metal clusters can then soft land on this anchored surface m [Hayakawa et al., 2009]. We have carried out a systematic theoretical study of the adsorption of Wn (n = 1-6) clusters on anchored graphite (0001) surface, using state-of-art spin-polarized density functional approach. In our first-principles calculations, the graphite (0001) surface has been suitably modeled as a slab separated by large vacuum layers. Wn clusters bond on clean graphite (0001) surface with a rather weak Van-der-Waals interaction. However, on the anchored graphite (0001) surface, the Wn clusters get absorbed at the defect site with a much larger adsorption energy. We report here the results of our first-principles investigation of this supported Wn cluster system, along with their reactivity trend as a function of the cluster size (n).
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(Îµ)), 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
First principle chemical kinetics in zeolites: the methanol-to-olefin process as a case study.
Van Speybroeck, Veronique; De Wispelaere, Kristof; Van der Mynsbrugge, Jeroen; Vandichel, Matthias; Hemelsoet, Karen; Waroquier, Michel
2014-11-01
To optimally design next generation catalysts a thorough understanding of the chemical phenomena at the molecular scale is a prerequisite. Apart from qualitative knowledge on the reaction mechanism, it is also essential to be able to predict accurate rate constants. Molecular modeling has become a ubiquitous tool within the field of heterogeneous catalysis. Herein, we review current computational procedures to determine chemical kinetics from first principles, thus by using no experimental input and by modeling the catalyst and reacting species at the molecular level. Therefore, we use the methanol-to-olefin (MTO) process as a case study to illustrate the various theoretical concepts. This process is a showcase example where rational design of the catalyst was for a long time performed on the basis of trial and error, due to insufficient knowledge of the mechanism. For theoreticians the MTO process is particularly challenging as the catalyst has an inherent supramolecular nature, for which not only the Brønsted acidic site is important but also organic species, trapped in the zeolite pores, must be essentially present during active catalyst operation. All these aspects give rise to specific challenges for theoretical modeling. It is shown that present computational techniques have matured to a level where accurate enthalpy barriers and rate constants can be predicted for reactions occurring at a single active site. The comparison with experimental data such as apparent kinetic data for well-defined elementary reactions has become feasible as current computational techniques also allow predicting adsorption enthalpies with reasonable accuracy. Real catalysts are truly heterogeneous in a space- and time-like manner. Future theory developments should focus on extending our view towards phenomena occurring at longer length and time scales and integrating information from various scales towards a unified understanding of the catalyst. Within this respect molecular dynamics methods complemented with additional techniques to simulate rare events are now gradually making their entrance within zeolite catalysis. Recent applications have already given a flavor of the benefit of such techniques to simulate chemical reactions in complex molecular environments. PMID:25054453
NASA Astrophysics Data System (ADS)
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 substrates. We have analyzed the mechanisms of the molecular reactions on the class of catalytic surfaces so designed in an effort to optimize materials parameters in the search of optimal catalytic materials. All these studies are likely to bring new perspectives and substantial advancement in the field of high-performance simulations in catalysis and the characterization of nanostructures for energy and environmental applications. Moving to novel materials for electronics applications, I have studied the structural and vibrational properties of mono and bi-layer graphene. I have characterized the lattice thermal conductivity of ideal monolayer and bi-layer graphene, demonstrating that their behavior is similar to that observed in graphite and indicating that the intra-layer coupling does not affect significantly the thermal conductance. I have also calculated the electron-phonon interaction in monolayer graphene and obtained electron scattering rates associated with all phonon modes and the intrinsic resistivity/mobility of monolayer graphene is estimated as a function of temperature. On another project, I have worked on ab initio molecular dynamic studies of novel Phase Change Materials (PCM) for memory and 3D-integration. We characterized high-temperature, sodium | nickel chloride, rechargeable batteries. These batteries are under consideration for hybrid drive systems in transportation applications. As part of our activities to improve performance and reliability of these batteries, we developed an engineering transport model of the component electrochemical cell. To support that model, we have proposed a reaction kinetics expression for the REDOX (reduction-oxidation) reaction at the porous positive electrode. We validate the kinetics expression with electrochemical measurements. A methodology based on the transistor body effect is used to estimate inversion oxide thicknesses (Tinv) in high-kappa/metal gate, undoped, ultra-thin body SOI FINFETs. The extracted Tinvs are compared to independent capacitance voltage (CV) measurements.
Point defects in ZnO: an approach from first principles
NASA Astrophysics Data System (ADS)
Oba, Fumiyasu; Choi, Minseok; Togo, Atsushi; Tanaka, Isao
2011-06-01
Recent first-principles studies of point defects in ZnO are reviewed with a focus on native defects. Key properties of defects, such as formation energies, donor and acceptor levels, optical transition energies, migration energies and atomic and electronic structure, have been evaluated using various approaches including the local density approximation (LDA) and generalized gradient approximation (GGA) to DFT, LDA+U/GGA+U, hybrid Hartree-Fock density functionals, sX and GW approximation. Results significantly depend on the approximation to exchange correlation, the simulation models for defects and the post-processes to correct shortcomings of the approximation and models. The choice of a proper approach is, therefore, crucial for reliable theoretical predictions. First-principles studies have provided an insight into the energetics and atomic and electronic structures of native point defects and impurities and defect-induced properties of ZnO. Native defects that are relevant to the n-type conductivity and the non-stoichiometry toward the O-deficient side in reduced ZnO have been debated. It is suggested that the O vacancy is responsible for the non-stoichiometry because of its low formation energy under O-poor chemical potential conditions. However, the O vacancy is a very deep donor and cannot be a major source of carrier electrons. The Zn interstitial and anti-site are shallow donors, but these defects are unlikely to form at a high concentration in n-type ZnO under thermal equilibrium. Therefore, the n-type conductivity is attributed to other sources such as residual impurities including H impurities with several atomic configurations, a metastable shallow donor state of the O vacancy, and defect complexes involving the Zn interstitial. Among the native acceptor-type defects, the Zn vacancy is dominant. It is a deep acceptor and cannot produce a high concentration of holes. The O interstitial and anti-site are high in formation energy and/or are electrically inactive and, hence, are unlikely to play essential roles in electrical properties. Overall defect energetics suggests a preference for the native donor-type defects over acceptor-type defects in ZnO. The O vacancy, Zn interstitial and Zn anti-site have very low formation energies when the Fermi level is low. Therefore, these defects are expected to be sources of a strong hole compensation in p-type ZnO. For the n-type doping, the compensation of carrier electrons by the native acceptor-type defects can be mostly suppressed when O-poor chemical potential conditions, i.e. low O partial pressure conditions, are chosen during crystal growth and/or doping.
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, Î±{sub c}/Î±{sub a} is nearly constant (1.46) from 300â€‰K to 1500â€‰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 1500â€‰K, the theoretical slope is âˆ’0.0211 GPaÂ·K{sup âˆ’1}, âˆ’0.0155 GPaÂ·K{sup âˆ’1}, and âˆ’0.0384 GPaÂ·K{sup âˆ’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.
Data set for diffusion coefficients of alloying elements in dilute Mg alloys from first-principles
Zhou, Bi-Cheng; Shang, Shun-Li; Wang, Yi; Liu, Zi-Kui
2015-01-01
Diffusion coefficients of alloying elements in Mg are critical for the development of new Mg alloys for lightweight applications. Here we present the data set of the temperature-dependent dilute tracer diffusion coefficients for 47 substitutional alloying elements in hexagonal closed packed (hcp) Mg calculated from first-principles calculations based on density functional theory (DFT) by combining transition state theory and an 8-frequency model. Benchmark for the DFT calculations and systematic comparison with experimental diffusion data are also presented. The data set refers to â€œDiffusion coefficients of alloying elements in dilute Mg alloys: A comprehensive first-principles studyâ€ by Zhou et al. [1]. PMID:26702419
Data set for diffusion coefficients of alloying elements in dilute Mg alloys from first-principles.
Zhou, Bi-Cheng; Shang, Shun-Li; Wang, Yi; Liu, Zi-Kui
2015-12-01
Diffusion coefficients of alloying elements in Mg are critical for the development of new Mg alloys for lightweight applications. Here we present the data set of the temperature-dependent dilute tracer diffusion coefficients for 47 substitutional alloying elements in hexagonal closed packed (hcp) Mg calculated from first-principles calculations based on density functional theory (DFT) by combining transition state theory and an 8-frequency model. Benchmark for the DFT calculations and systematic comparison with experimental diffusion data are also presented. The data set refers to "Diffusion coefficients of alloying elements in dilute Mg alloys: A comprehensive first-principles study" by Zhou et al. [1]. PMID:26702419
First-Principles Investigation of 180^circ Domain Walls in BaTiO_3.
NASA Astrophysics Data System (ADS)
Padilla, Jorge; Zhong, Weiqing; Vanderbilt, David
1996-03-01
We present a first-principles study of 180^circ ferroelectric domain walls in tetragonal BaTiO_3. The theory is based on an effective Hamiltonian that has previously been determined from first-principles ultrasoft-pseudopotential calculations.( W. Zhong, D. Vanderbilt, and K. M. Rabe, Phys. Rev. B. 52), 6301 (1995). Statistical properties are investigated using Monte Carlo simulations. We compute the domain-wall energy, free energy, and thickness, analyze the behavior of the ferroelectric order parameter in the interior of the domain wall, and study its spatial fluctuations. An abrupt reversal of the polarization is found, unlike the gradual rotation typical of the ferromagnetic case.
NASA Astrophysics Data System (ADS)
Rey, M.; Nikitin, A. V.; Tyuterev, V.
2014-06-01
Knowledge of near infrared intensities of rovibrational transitions of polyatomic molecules is essential for the modeling of various planetary atmospheres, brown dwarfs and for other astrophysical applications 1,2,3. For example, to analyze exoplanets, atmospheric models have been developed, thus making the need to provide accurate spectroscopic data. Consequently, the spectral characterization of such planetary objects relies on the necessity of having adequate and reliable molecular data in extreme conditions (temperature, optical path length, pressure). On the other hand, in the modeling of astrophysical opacities, millions of lines are generally involved and the line-by-line extraction is clearly not feasible in laboratory measurements. It is thus suggested that this large amount of data could be interpreted only by reliable theoretical predictions. There exists essentially two theoretical approaches for the computation and prediction of spectra. The first one is based on empirically-fitted effective spectroscopic models. Another way for computing energies, line positions and intensities is based on global variational calculations using ab initio surfaces. They do not yet reach the spectroscopic accuracy stricto sensu but implicitly account for all intramolecular interactions including resonance couplings in a wide spectral range. The final aim of this work is to provide reliable predictions which could be quantitatively accurate with respect to the precision of available observations and as complete as possible. All this thus requires extensive first-principles quantum mechanical calculations essentially based on three necessary ingredients which are (i) accurate intramolecular potential energy surface and dipole moment surface components well-defined in a large range of vibrational displacements and (ii) efficient computational methods combined with suitable choices of coordinates to account for molecular symmetry properties and to achieve a good numerical convergence. Because high-resolution ab initio spectra predictions for systems with N>4 atoms is a very challenging task, the major issue is to minimize the cost of computations and the loss of accuracy during calculations. To this end, a truncation-reduction technique for the Hamiltonian operator as well as an extraction-compression procedure for the basis set functions will be introduced and discussed in detail. We will give a review on the recent progress in computational methods as well as on existing experimental and theoretical databases 4,5,6,7,8,9. This presentation will be focused on highly symmetric molecules such as methane and phosphine, with the corresponding applications at low-T in relation with Titan's atmosphere and at high-T with the production of theoretical line lists for astrophysical opacity calculations10. The study of isotopic Hâ†’D and 12Câ†’13C substitutions will be also addressed and carried out by means of symmetry and coordinate transformations11. Finally we hope this work will help refining studies of currently available analyses which are not yet finalized. The modeling of non-LTE emissions accounting for contribution of many fundamental and hot bands could also be possible. Support from PNP (French CNRS national planetology program) is acknowledged.
NASA Astrophysics Data System (ADS)
Tabatabaei, Maryam; Shodja, Hossein M.
2015-12-01
Although, the evaluation of the nanohardness of amorphous silicon (a-Si) has been the subject of a few experimental works but, to date, it has not been addressed theoretically yet. In this work, first principles Kohn-Sham density functional theory (DFT)-based molecular dynamics (MD) in combination with Mohr-Coulomb criterion is employed to calculate the ideal shear strength of the damped MD annealed a-Si sample containing dangling and floating bonds which are pertinent to the threefold- and fivefold-coordinated defects, respectively, as well as distorted tetrahedral bonds. The stress state beneath the nanoindenter is triaxial, and is accounted for properly. The calculated values of nanohardness are in reasonable agreement with those values measured experimentally. Consideration of the electronic charge distribution under the state of triaxial tension test reveals that the yield phenomenon in a-Si is accompanied by the transformation of a threefold-coordinated Si atom to a fourfold-coordinated.
NASA Astrophysics Data System (ADS)
Gouda, Mohammed K.; Nakamura, Koichi; A. H. Gepreel, Mohamed
2015-06-01
Theoretical deformation response of hypothetical ?-titanium alloys was investigated using first-principles calculation technique under periodic boundary conditions. Simulation was carried out on hypothetical 54-atom supercell of Ti-X (X = Cr, Mn, Fe, Zr, Nb, Mo, Al, and Sn) binary alloys. The results showed that the strength of Ti increases by alloying, except for Cr. The most effective alloying elements are Nb, Zr, and Mo in the current simulation. The mechanism of bond breaking was revealed by studying the local structure around the alloying element atom with respect to volume change. Moreover, the effect of alloying elements on bulk modulus and admissible strain was investigated. It was found that Zr, Nb, and Mo have a significant effect to enhance the admissible strain of Ti without change in bulk modulus.
NASA Astrophysics Data System (ADS)
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.
First-principles Study of Structural Properties of MgxZn1-xO ternary alloys
NASA Astrophysics Data System (ADS)
Gao, J.; Zhao, G. J.; Liang, X. X.; Song, T. L.
2015-01-01
First-principles calculations have been carried out to investigate the structural and electronic properties of MgxZn1-xO ternary alloys. The calculations are performed using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory(DFT). We conclude that the structural properties of these materials, in particular the composition dependence on the lattice constant and the band gap is found to be linear. The a-axis length in the lattice gradually increases, while the c-axis length decreases with the increase in Mg doping concentration, and be corresponded with the Vegard's law linear rule. The lattice parameters of the MgxZn1-xO ternary alloys are consistent with experimental data and other theoretical results. We found in the conduction band portion, the Mg 2p 2s states are moved to high energy region as the Mg content increases, so the band gap increases.
NASA Astrophysics Data System (ADS)
Liu, Qi-Jun; Liu, Zheng-Tang; Feng, Li-Ping; Tian, Hao
2012-12-01
On the plane-wave ultrasoft pseudopotential technique based on the first-principles density functional theory (DFT), we calculated the structural, elastic, electronic and optical properties of the seven different phases of SrZrO3. The obtained ground-state properties are in good agreement with previous experiments and calculations, which indicate that the most stable phase is orthorhombic Pnma structure. Seven phases of SrZrO3 are mechanically stable with cubic, tetragonal and orthorhombic structures. The mechanical and thermodynamic properties have been obtained by using the Voigt-Reuss-Hill approach and Debye-Grüneisen model. The electronic structures and optical properties are obtained and compared with the available experimental and theoretical data.
NASA Astrophysics Data System (ADS)
Jain, Richa Naja; Chakraborty, Brahmananda; Ramaniah, Lavanya M.
2015-06-01
The electronic structure and hydrogen storage capability of Yttrium-doped BNNTs has been theoretically investigated using first principles density functional theory (DFT). Yttrium atom prefers the hollow site in the center of the hexagonal ring with a binding energy of 0.8048eV. Decorating by Y makes the system half-metallic and magnetic with a magnetic moment of 1.0µB. Y decorated Boron-Nitride (8,0) nanotube can adsorb up to five hydrogen molecules whose average binding energy is computed as 0.5044eV. All the hydrogen molecules are adsorbed with an average desorption temperature of 644.708 K. Taking that the Y atoms can be placed only in alternate hexagons, the implied wt% comes out to be 5.31%, a relatively acceptable value for hydrogen storage materials. Thus, this system can serve as potential hydrogen storage medium.
NASA Astrophysics Data System (ADS)
Zhao, Y. N.; Han, H. L.; Yu, Y.; Xue, W. H.; Gao, T.
2009-02-01
We study the electronic band structure, dielectric, and lattice dynamical properties of FeSi, RuSi and OsSi from first principles using density functional theory. We performed the band structure calculations for these compounds to confirm they are indirect band-gap semiconductors. Density functional perturbation theory is employed to evaluate the Born effective charge tensors, the dielectric permittivity tensors, the phonon frequencies at the ?-point, and the phonon dispersion curves as well as the corresponding density of states. These calculated results are in reasonable agreement with the experimental data and theoretical values available. From the present calculation we also predict that there exist longitudinal-transverse optical (LO-TO) splittings by 2-32 cm- 1 for these three compounds, which were not observed in previous experiments.
Gouda, Mohammed K. Gepreel, Mohamed A. H.; Nakamura, Koichi
2015-06-07
Theoretical deformation response of hypothetical Î²-titanium alloys was investigated using first-principles calculation technique under periodic boundary conditions. Simulation was carried out on hypothetical 54-atom supercell of Tiâ€“X (Xâ€‰=â€‰Cr, Mn, Fe, Zr, Nb, Mo, Al, and Sn) binary alloys. The results showed that the strength of Ti increases by alloying, except for Cr. The most effective alloying elements are Nb, Zr, and Mo in the current simulation. The mechanism of bond breaking was revealed by studying the local structure around the alloying element atom with respect to volume change. Moreover, the effect of alloying elements on bulk modulus and admissible strain was investigated. It was found that Zr, Nb, and Mo have a significant effect to enhance the admissible strain of Ti without change in bulk modulus.
NASA Astrophysics Data System (ADS)
Song, Chi; Li, Dong-Dong; Xu, Yi-Chun; Pan, Bi-Cai; Liu, Chang-Song; Wang, Zhi-Guang
2014-05-01
The corrosion of steels in liquid metal lead (Pb) and bismuth (Bi) is a critical challenge in the development of accelerator driven systems (ADS). Using a first-principles method with a slab model, we theoretically investigate the interaction between the Pb (Bi) atom and the iron (Fe) (100) surface to assess the fundamental corrosion properties. Our investigation demonstrates that both Pb and Bi atoms favorably adsorb on the (100) surface. Such an adsorption decreases the energy required for the dissociation of an Fe atom from the surface, enhancing the dissolution tendency significantly. The segregation of six common alloying elements (Cr, Al, Mn, Ni, Nb, and Si) to the surface and their impacts on the corrosion properties are also considered. The present results reveal that Si seems to have a relatively good performance to stabilize the surface and alleviate the dissolving trend caused by Pb and Bi.
First-principles study of the properties of oxygen-deficient LaNiO3-x structures
NASA Astrophysics Data System (ADS)
Malashevich, Andrei; Ismail-Beigi, Sohrab
2013-03-01
There has been a great deal of recent interest and activity regarding rare earth nickelates in bulk form, as superlattices, and as thin films. The parent nickelate in these cases is typically LaNiO3, which in bulk form is a paramagnetic metal. In addition, due to its relatively good lattice match to other perovskites, it also serves as an electrode in functional oxide film devices. However, it is known that the conductivity of LaNiO3 in any form is strongly affected by the presence of oxygen vacancies. Here, we present a first-principles study of a variety of oxygen-deficient LaNiO3-x structures. We describe our theoretical results for the atomic-scale geometry and energetics of the vacancies (formation and aggregation), their mobilities, and their electronic structure.
NASA Astrophysics Data System (ADS)
Di Stefano, Davide; Mrovec, Matous; ElsÃ¤sser, Christian
2015-12-01
The diffusion coefficients of interstitial hydrogen in bulk Fe and Ni crystals have been calculated over a wide range of temperatures employing first-principles methods based on density functional theory. Quantum mechanical effects have been included by means of the semiclassical transition state theory and the small-polaron model of Flynn and Stoneham. Our results show that to include such effects is crucial for a quantitative simulation of H diffusion in bcc Fe even at room temperature, while in the case of fcc Ni this is less important. The comparison with other theoretical approaches as well as with experimental studies emphasizes the main advantages of the present approach: it is quantitatively accurate and computationally efficient.
NASA Astrophysics Data System (ADS)
Hellman, Anders; Iandolo, Beniamino; Wickman, BjÃ¶rn; GrÃ¶nbeck, Henrik; Baltrusaitis, Jonas
2015-10-01
The oxygen evolution reaction on hydroxyl- and oxygen-terminated hematite was investigated using first-principle calculations within a theoretical electrochemical framework. Both pristine hematite and hematite containing oxygen vacancies were considered. The onset potential was determined to be 1.79 V and 2.09 V vs. the reversible hydrogen electrode (RHE) for the pristine hydroxyl- and oxygen-terminated hematite, respectively. The presence of oxygen vacancies in the hematite surface resulted in pronounced shifts of the onset potential to 3.09 V and 1.83 V, respectively. Electrochemical oxidation measurements conducted on thin-film hematite anodes, resulted in a measured onset potential of 1.66 V vs. RHE. Furthermore, the threshold potential between the hydroxyl- and oxygen-terminated hematite was determined as a function of pH. The results indicate that electrochemical water oxidation on hematite occurs on the oxygen-terminated hematite, containing oxygen vacancies.
Prediction of MAX phases, VN +1SiCN (N=1,2), from first-principles theory
NASA Astrophysics Data System (ADS)
Fang, C. M.; Ahuja, R.; Eriksson, O.
2007-01-01
We have investigated the phase stability of two MAX phases, V3SiC2 and V2SiC, by means of first-principles total-energy calculations within the generalized-gradient approximation and the projector-augmented wave method. The theoretical bulk modulus of V3SiC2 is 219GPa, which is ˜17% larger than that of Ti3SiC2 (187GPa ). The total-energy calculations show that V2SiC is stable with a formation energy of about 0.27eV?f.u. and that V3SiC2 is metastable (only 0.02eV?f.u. is required to stabilize this phase from its competing phases). We suggest that both these two MAX compounds should be possible to synthesize as stable (or metastable) phases using, e.g., thin-film deposition.
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)
Lakel, S.; Okbi, F.; Ibrir, M.; Almi, K.
2015-03-01
We have performed first-principles calculations to investigate the behavior under hydrostatic pressure of the structural, elastic and lattice dynamics properties of aluminum phosphide crystal (AlP), in both zinc-blende (B3) and nickel arsenide (B8) phases. Our calculated structural and electronic properties are in good agreement with previous theoretical and experimental results. The elastic constants, bulk modulus (B), shear modulus (G), and Young's modulus (E), Born effective charge and static dielectric constant ?0, were calculated with the generalized gradient approximations and the density functional perturbation theory (DFPT). Our results in the pressure behavior of the elastic and dielectric properties of both phases are compared and contrasted with the common III-V materials. The Born effective charge ZB decreases linearly with pressure increasing, while the static dielectric constant decreases quadratically with the increase of pressure.
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 Îµ{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 IIIâ€“V materials. The Born effective charge ZB decreases linearly with pressure increasing, while the static dielectric constant decreases quadratically with the increase of pressure.
NASA Astrophysics Data System (ADS)
Wan, L. F.; Nishimatsu, T.; Beckman, S. P.
2012-05-01
The elastic and dielectric properties of the four experimentally known phases of KNbO3 (KNO) are investigated by first-principles methods. The atomic structure is reported along with the Born effective charge tensor to reveal the relation between Nb-O bonds hybridization and ferroelectric structural distortion. The dielectric, elastic, and piezoelectric properties of each phase are presented and compared to results in the literature. The computed structures are found to match experiment to an accuracy of approximately 2%. Although there have been very few experimental studies of single crystal KNO, it is found that the elastic parameters computed for orthorhombic KNO agree with the measured values to better than 25%, which is within the anticipated exchange-correlation error; however, the computed piezoelectric coefficient differ from the experimental values by as much as 50%, which suggests that the disagreement may not be solely due to the theoretical approximations.
Buck, D.R.
2000-09-12
Theoretical simulations and ultrafast pump-probe laser spectroscopy experiments were used to study photosynthetic pigment-protein complexes and antennae found in green sulfur bacteria such as Prosthecochloris aestuarii, Chloroflexus aurantiacus, and Chlorobium tepidum. The work focused on understanding structure-function relationships in energy transfer processes in these complexes through experiments and trying to model that data as we tested our theoretical assumptions with calculations. Theoretical exciton calculations on tubular pigment aggregates yield electronic absorption spectra that are superimpositions of linear J-aggregate spectra. The electronic spectroscopy of BChl c/d/e antennae in light harvesting chlorosomes from Chloroflexus aurantiacus differs considerably from J-aggregate spectra. Strong symmetry breaking is needed if we hope to simulate the absorption spectra of the BChl c antenna. The theory for simulating absorption difference spectra in strongly coupled photosynthetic antenna is described, first for a relatively simple heterodimer, then for the general N-pigment system. The theory is applied to the Fenna-Matthews-Olson (FMO) BChl a protein trimers from Prosthecochloris aestuarii and then compared with experimental low-temperature absorption difference spectra of FMO trimers from Chlorobium tepidum. Circular dichroism spectra of the FMO trimer are unusually sensitive to diagonal energy disorder. Substantial differences occur between CD spectra in exciton simulations performed with and without realistic inhomogeneous distribution functions for the input pigment diagonal energies. Anisotropic absorption difference spectroscopy measurements are less consistent with 21-pigment trimer simulations than 7-pigment monomer simulations which assume that the laser-prepared states are localized within a subunit of the trimer. Experimental anisotropies from real samples likely arise from statistical averaging over states with diagonal energies shifted by in homogeneous broadening and as such, are quite sensitive to diagonal energy disorder. The experimental anisotropies exhibit strong oscillations with {approximately}220 fs period for certain wavelengths in one-color absorption difference experiments. The oscillations only appear when the laser pulse spectrum overlaps both of the lowest-energy groups of exciton levels clustered near 815 and 825 nm. Results suggest that the oscillations stem from quantum beating between exciton levels, rather than from coherent nuclear motion.
Predicting Raman Spectra of Aqueous Silica and Alumina Species in Solution From First Principles
NASA Astrophysics Data System (ADS)
Hunt, J. D.; Schauble, E. A.; Manning, C. E.
2006-12-01
Dissolved silica and alumina play an important role in lithospheric fluid chemistry. Silica concentrations in aqueous fluids vary over the range of crustal temperatures and pressures enough to allow for significant mass transport of silica via fluid-rock interaction. The polymerization of silica, and the possible incorporation of alumina into the polymer structure, could afford crystal-like or melt-like sites to otherwise insoluble elements such as titanium, leading to enhanced mobility. Raman spectroscopy in a hydrothermal diamond anvil cell (HDAC) has been used to study silica polymerization at elevated pressure and temperature [Ref. 1, 2], but Raman spectra of expected solutes are not fully understood. We calculated Raman spectra of H4SiO4 monomers, H6Si2O7 dimers, and H6SiAlO_7^- dimers, from first principles using hybrid density functional theory (B3LYP). These spectra take into account the variation in bridging angle (Si-O-Si and Si-O-Al angles) that the dimers will have at a given temperature by calculating a potential energy surface of the dimer as the bridging angle varies, and using a Boltzmann distribution at that temperature to determine relative populations at each geometry. Solution effects can be incorporated by using a polarizable continuum model (PCM), and a potential energy surface has been constructed for the silica dimer using a PCM. The bridging angle variation explains the broadness of the 630 cm^-^1 silica dimer peak observed in HDAC experiments [Ref. 1, 2] at high temperatures. The silica-alumina dimer bridging angle is shown to be stiffer than the silica dimer bridging angle, which results in a much narrower main peak. The synthetic spectrum obtained for the silica-alumina dimer suggests that there may be a higher ratio of complexed alumina to free alumina in solution at highly basic pH than previously estimated [Ref. 3]. References: 1. Zotov, N. and H. Keppler, Chemical Geology, 2002. 184: p. 71-82. 2. Zotov, N. and H. Keppler, American Mineralogist, 2000. 85: p. 600-603. 3. Gout, R., et al., Journal of Solution Chemistry, 2000. 29: p. 1173-1186.
First principles study of structural, electronic and magnetic properties of magnesium
NASA Astrophysics Data System (ADS)
Abdel Rahim, G. P.; RodrÃguez M, J. A.; Moreno-Armenta, M. G.
2016-02-01
We investigated the structural, electronic, and magnetic properties of Mg, in the CS (simple cubic), NiAs (Nickel arsenide), FCC (rock-salt), R (Rhombohedral), Diamond and WZ (wurtzite) phases. Calculations were performed using the first-principles pseudo-potential method within the framework of spin-density functional theory (DFT).
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.
Nonnenberg, Christel; Gaub, Hermann; Frank, Irmgard
2006-07-17
We present first-principles molecular dynamics simulations of azobenzene and a sterically hindered derivative in the first excited state. The restricted open-shell Kohn-Sham (ROKS) approach is employed to describe the motion in the lowest excited state. The rotational pathway is observed in the molecular dynamics simulations for both azobenzene and its azacrown ether capped derivative. PMID:16755639
Amorphous structures of Cu, Ag, and Au nanoclusters from first principles calculations
NASA Astrophysics Data System (ADS)
Oviedo, J.; Palmer, R. E.
2002-12-01
We have carried out first-principles density functional calculations for clusters of the coinage metals containing thirteen atoms (M13, where M=Cu, Ag, or Au). We find that for this geometric "magic number" the low energy isomers are actually disordered, forming almost a continuous distribution as a function of energy.
First-principles calculations of shear moduli for Monte Carlo-simulated Coulomb solids
NASA Technical Reports Server (NTRS)
Ogata, Shuji; Ichimaru, Setsuo
1990-01-01
The paper presents a first-principles study of the shear modulus tensor for perfect and imperfect Coulomb solids. Allowance is made for the effects of thermal fluctuations for temperatures up to the melting conditions. The present theory treats the cases of the long-range Coulomb interaction, where volume fluctuations should be avoided in the Ewald sums.
ERIC Educational Resources Information Center
Bowen, J. Philip; Sorensen, Jennifer B.; Kirschner, Karl N.
2007-01-01
The analysis explains the basis set superposition error (BSSE) and fragment relaxation involved in calculating the interaction energies using various first principle theories. Interacting the correlated fragment and increasing the size of the basis set can help in decreasing the BSSE to a great extent.
ERIC Educational Resources Information Center
Gardner, Joel
2010-01-01
Research has shown that when Merrill's First Principles of Instruction are used as part of an instructional strategy, student learning increases. Several articles describe these principles of instruction, including specific methods for implementing this theory. However, because teachers and designers often have little time to design instruction,â€¦
Nuclear Quantum Effects in Ice Phases and Water from First Principles Calculations
NASA Astrophysics Data System (ADS)
Pamuk, Betul
Despite the simplicity of the molecule, condensed phases of water show many physical anomalies, some of which are still unexplained to date. This thesis focuses on one striking anomaly that has been largely neglected and never explained. When hydrogen (1H) is replaced by deuterium (2 D), zero point fluctuations of the heavy isotope causes ice to expand, whereas in normal isotope effect, heavy isotope causes volume contraction. Furthermore, in a normal isotope effect, the shift in volume should decrease with increasing temperature, while, in ice, the volume shift increases with increasing temperature and persists up to the melting temperature and also exists in liquid water. In this dissertation, nuclear quantum effects on structural and cohesive properties of different ice polymorphs are investigated. We show that the anomalous isotope effect is well described by first principles density functional theory with van der Waals (vdW-DF) functionals within the quasi-harmonic approximation. Our theoretical modeling explains how the competition between the intra- and inter-molecular bonding of ice leads to an anomalous isotope effect in the volume and bulk modulus of ice. In addition, we predict a normal isotope effect when 16O is replaced by 18O, which is experimentally confirmed. Furthermore, the transition from proton disordered hexagonal phase, ice Ih to proton ordered hexagonal phase, ice XI occurs with a temperature difference between 1H and 2D of 6K, in good agreement with experimental value of 4K. We explain, for first time for that this temperature difference is entirely due to the zero point energy. In the second half of this thesis, we expand our study to the other ice phases: ice Ic, ice IX, ice II, ice VIII, clathrate hydrates, and low and high density amorphous ices. We employ the methodology that we have developed to investigate the isotope effect in structures with different configurations. We show that there is a transition from anomalous isotope effect to normal isotope effect in these structures as the density increases. We analyse the bonding mechanism of these structures and make links to the most important anomalies of liquid water.
Theoretical analysis on the dynamic absorption saturation in pulsed cavity ringdown spectroscopy
NASA Astrophysics Data System (ADS)
Lee, J. Y.; Hahn, J. W.
2004-09-01
We perform a theoretical analysis on the transient dynamics of absorption saturation in pulsed cavity ringdown spectroscopy. Based on a coupled rate equation approach, modelling of the dynamic saturation is carried out to account for time-evolving intracavity photon density and absorbing population. Master equations are derived in terms of normalized system parameters, which enables one to systematically study the non-exponential feature of saturated cavity ringdown signals. Erroneous quantification of sample absorbance by the conventional ringdown time analysis is numerically simulated, and the saturated spectral feature showing a Lamb dip is obtained for a Doppler broadened sample. Finally a novel numerical recipe is proposed to handle saturated ringdown signals and retrieve unsaturated spectra, allowing relevant absorption parameters without degrading the measurement signal-to-noise ratio.
FT-Raman, surface-enhanced Raman spectroscopy and theoretical investigations of diclofenac sodium
NASA Astrophysics Data System (ADS)
Iliescu, T.; Baia, M.; Kiefer, W.
2004-03-01
Raman and surface-enhanced Raman (SER) spectroscopies have been applied to the vibrational characterization of diclofenac sodium (DCF-Na). Theoretical calculations (DFT and ab initio) of two DCF-Na conformers have been performed to find the optimized structure and computed vibrational wavenumbers of the most stable one. SER spectra in silver colloid at different pH values have been also recorded and analyzed. Good SER spectra have been obtained in acidic and neutral environments, proving the chemisorption of the DCF-Na molecule on the silver surface. In the investigated pH range the carboxylate anion has been bonded to the silver surface through the lone pair oxygen electrons. The phenyl rings' orientation with respect to the silver surface changed on passing from acidic to neutral pH from a tilted close to flat to a more perpendicular one.
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).
High-level theoretical rovibrational spectroscopy beyond fc-CCSD(T): The C3 molecule.
SchrÃ¶der, 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 (XÌƒ(1)Î£g (+)). 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 1.294â€‰07(10) Ã… in excellent agreement with the ab initio composite value of 1.293â€‰97 Ã…. Theoretical band intensities based on a newly developed electric dipole moment function also suggest that the infrared active (1, 1(1), 0)â†(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
NASA Astrophysics Data System (ADS)
SchrÃ¶der, Benjamin; Sebald, Peter
2016-01-01
An accurate local (near-equilibrium) potential energy surface (PES) is reported for the C3 molecule in its electronic ground state ( X Ëœ 1 Î£g + ). 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 12C and 13C 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) Ã… in excellent agreement with the ab initio composite value of 1.293 97 Ã…. Theoretical band intensities based on a newly developed electric dipole moment function also suggest that the infrared active (1, 11, 0)â†(0, 00, 0) combination band might be observable by high-resolution spectroscopy.
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.
Sarker, Debalaya; Bhattacharya, Saswata; Rodriguez, Raul D; Sheremet, Evgeniya; Kabiraj, D; Avasthi, D K; Zahn, Dietrich R T; Schmidt, H; Srivastava, P; Ghosh, S
2016-02-24
This work is driven by the vision of engineering planar field emitters with ferromagnetic metal-insulator nanocomposite thin films, using swift heavy ion (SHI) irradiation method. FeCo nanoparticles inside SiO2 matrix, when subjected to SHI get elongated. Using this, we demonstrate here a planar field emitter with maximum current density of 550 ?A/cm(2) at an applied field of 15 V/?m. The film, irradiated with 5 × 10(13) ions/cm(2) fluence (5e13) of 120 MeV Au(9+) ions, shows very high electron emitting quantum efficiency in comparison to its unirradiated counterpart. Surface enhanced Raman spectroscopy analysis of unirradiated and 5e13 films further confirms that the field emission (FE) enhancement is not only due to surface protrusions but also depends on the properties of entire matrix. We find experimental evidence of enhanced valence band density of states (VB DOS) for 5e13 film from XPS, which is verified in the electronic structure of a model FeCo cluster from first-principles based calculations combining density functional theory (DFT) and molecular dynamics (MD) simulations. The MD temperature is selected from the lattice temperature profile inside nanoparticles as deduced from thermal spike model. Increasing the irradiation fluence beyond 5e13, results in reduced VB DOS and melting of surface protrusions, thus causing reduction of FE current density. We finally conclude from theoretical analysis that change in fluence alters the co-ordination chemistry followed by the charge distribution and spin alignment, which influence the VB DOS and concurrent FE as evident from our experiment. PMID:26812580
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 Niâ€“Alâ€“Mo system is developed here, incorporating first-principles calculations and reported modeling of the binary Niâ€“Al, Niâ€“Mo and Alâ€“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 Î³-fcc phases are modeled as random atomic solutions, and the Î³Ê¹-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, Î³-fcc and Î³Ê¹-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 Ïˆ-AlMo, Î¸-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 Î³-fcc and Î³Ê¹-Ni3Al phases in the ternary Niâ€“Alâ€“Mo system, the specific Î³-fcc and Î³Ê¹-Ni3Al phase fields are plotted in x(Al)â€“x(Mo)â€“T space for a temperature range 1200â€“1800 K.
Tkatchenko, Alexandre; AlfÃ¨, Dario; Kim, Kwang S
2012-11-13
Supramolecular host-guest systems play an important role for a wide range of applications in chemistry and biology. The prediction of the stability of host-guest complexes represents a great challenge to first-principles calculations due to an interplay of a wide variety of covalent and noncovalent interactions in these systems. In particular, van der Waals (vdW) dispersion interactions frequently play a prominent role in determining the structure, stability, and function of supramolecular systems. On the basis of the widely used benchmark case of the buckyball catcher complex (C60@C60H28), we assess the feasibility of computing the binding energy of supramolecular host-guest complexes from first principles. Large-scale diffusion Monte Carlo (DMC) calculations are carried out to accurately determine the binding energy for the C60@C60H28 complex (26 Â± 2 kcal/mol). On the basis of the DMC reference, we assess the accuracy of widely used and efficient density-functional theory (DFT) methods with dispersion interactions. The inclusion of vdW dispersion interactions in DFT leads to a large stabilization of the C60@C60H28 complex. However, DFT methods including pairwise vdW interactions overestimate the stability of this complex by 9-17 kcal/mol compared to the DMC reference and the extrapolated experimental data. A significant part of this overestimation (9 kcal/mol) stems from the lack of dynamical dielectric screening effects in the description of the molecular polarizability in pairwise dispersion energy approaches. The remaining overstabilization arises from the isotropic treatment of atomic polarizability tensors and the lack of many-body dispersion interactions. A further assessment of a different supramolecular system - glycine anhydride interacting with an amide macrocycle - demonstrates that both the dynamical screening and the many-body dispersion energy are complex contributions that are very sensitive to the underlying molecular geometry and type of bonding. We discuss the required improvements in theoretical methods for achieving "chemical accuracy" in the first-principles modeling of supramolecular systems. PMID:26605594
A First Principles Molecular Dynamics Study Of Calcium Ion In Water
Lightstone, F; Schwegler, E; Allesch, M; Gygi, F; Galli, G
2005-01-28
In this work we report on Car-Parrinello simulations of the divalent calcium ion in water, aimed at understanding the structure of the hydration shell and at comparing theoretical results with a series of recent experiments. Our paper shows some of the progress in the investigation of aqueous solutions brought about by the advent of ab initio molecular dynamics and highlights the importance of accessing subtle details of ion-water interactions from first-principles. Calcium plays a vital role in many biological systems, including signal transduction, blood clotting and cell division. In particular, calcium ions are known to interact strongly with proteins as they tend to bind well to both negatively charged (e.g. in aspartate and glutamate) and uncharged oxygens (e.g. in main-chain carbonyls). The ability of calcium to coordinate multiple ligands (from 6 to 8 oxygen atoms) with an asymmetric coordination shell enables it to cross-link different segments of a protein and induce large conformational changes. The great biochemical importance of the calcium ion has led to a number of studies to determine its hydration shell and its preferred coordination number in water. Experimental studies have used a variety of techniques, including XRD, EXAFS, and neutron diffraction to elucidate the coordination of Ca{sup 2+} in water. The range of coordination numbers (n{sub C}) inferred by X-ray diffraction studies varies from 6 to 8, and is consistent with that reported in EXAFS experiments (8 and 7.2). A wider range of values (6 to 10) was found in early neutron diffraction studies, depending on concentration, while a more recent measurement by Badyal, et al. reports a value close to 7. In addition to experimental measurements, many theoretical studies have been carried out to investigate the solvation of Ca{sup 2+} in water and have also reported a wide range of coordination numbers. Most of the classical molecular dynamics (MD) and QM/MM simulations report n{sub C} in the range of 8 to 10; in general, n{sub C} appears to be highly sensitive to the choice of the ion-water potential used in the calculations. Even ab initio MD simulations have so far obtained conflicting values for n{sub C}. For the structure of the first salvation shell Naor, et al. found n{sub C} = 7 to 8 and a Ca{sup 2+} - oxygen average distance (r{sub Ca-O}) of 2.64 {angstrom}, while Bako, et al. found n{sub C} = 6 and r{sub Ca-O} = 2.45 {angstrom}. In view of the existing controversies, we have carried out extensive Car-Parrinello simulations of Ca{sup 2+} solvation in water, using both a rigid and a flexible water model, up to time scales of 40 ps. Our simulations show variations of coordination numbers from 6, 7 and 8 occurring over intervals of {approx} 0.3/0.4 exchanges/ps, and yielding average coordination numbers of 6.2 and 7 for flexible and rigid water models, respectively. These results are consistent with those reported in recent EXAFS and neutron diffraction experiments. In addition, our calculations show an asymmetric coordination of Ca{sup 2+} to oxygen, similar to the case of Mg{sup 2+}.
NASA Astrophysics Data System (ADS)
Johnston, Karen; Nieminen, Risto M.
2007-03-01
The adhesion of plastics to ceramics is important for many industrial and technological appliations. It is therefore essential to understand the underlying structure and bonding of the polymer and the surface. The aim of this research is to improve plastic adhesion using a multiscale approach. The first step involves the use of density functional calculations to understand the atomic-scale structure and bonding of polymers on surfaces. The plastic of interest is mainly composed of the polymer bisphenol-A-polycarbonate (BPA-PC). The BPA-PC monomer consists of two phenol groups, one propane group and a carbonic acid group. First-principles calculations of the adsorption of these molecules onto the Si(001)-(2x1) dimer surface will be presented. Finally, the incorporation of first-principles data into a coarse-graining method will be discussed.
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.
Zhou, Fei; Nielson, Weston; Xia, Yi; Ozoli?š, Vidvuds
2014-10-31
First-principles prediction of lattice thermal conductivity ?(L) of strongly anharmonic crystals is a long-standing challenge in solid-state physics. Making use of recent advances in information science, we propose a systematic and rigorous approach to this problem, compressive sensing lattice dynamics. Compressive sensing is used to select the physically important terms in the lattice dynamics model and determine their values in one shot. Nonintuitively, high accuracy is achieved when the model is trained on first-principles forces in quasirandom atomic configurations. The method is demonstrated for Si, NaCl, and Cu(12)Sb(4)S(13), an earth-abundant thermoelectric with strong phonon-phonon interactions that limit the room-temperature ?(L) to values near the amorphous limit. PMID:25396378
NASA Astrophysics Data System (ADS)
Tadano, Terumasa; Tsuneyuki, Shinji
2015-12-01
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 Bi2Te3 based on the Boltzmann transport equation. We also show that the phonon lifetime and the lattice thermal conductivity of the high-temperature phase of SrTiO3 can be estimated by employing the perturbation theory on top of the solution of the self-consistent phonon equation.
First-principles study on elastic constants of C-S-H type minerals
NASA Astrophysics Data System (ADS)
Shahsavari, R.; Buehler, M.; Ulm, F.
2008-12-01
Calcium-Silicate-Hydrate (C-S-H) is the mineral binding phase of all Portland concrete materials, and the principle source of their strength and stiffness. Despite decades of research, the elastic properties of C-S-H mineral crystals are unknown. Here we investigate two natural analogs of C-S-H, tobermorite and jennite, and characterize their mechanical properties by first-principles calculations. First, we calculate their lattice parameters and elastic constants. Second, we show that in contrast to previous suggestions, for natural tobermorite 11 Ã… , the mechanically weakest directions are two inclined regions that form a hinge mechanism. By studying bond length changes under deformation in tobermorite 14 Ã… and jennite, we show that water molecules play a major structural role in defining their elastic properties. Averaged elastic moduli obtained by first-principles calculations of tobermorite 14 Ã… and jennite compare well with corresponding nanoindentation experiment on C-S-H.
Zhou, Fei; Nielson, Weston; Xia, Yi; OzoliÅ†Å¡, Vidvuds
2014-10-01
First-principles prediction of lattice thermal conductivity Îº_{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 Îº_{L} to values near the amorphous limit.
Redox condition in molten salts and solute behavior: A first-principles molecular dynamics study
NASA Astrophysics Data System (ADS)
Nam, Hyo On; Morgan, Dane
2015-10-01
Molten salts technology is of significant interest for nuclear, solar, and other energy systems. In this work, first-principles molecular dynamics (FPMD) was used to model the solute behavior in eutectic LiCl-KCl and FLiBe (Li2BeF4) melts at 773 K and 973 K, respectively. The thermo-kinetic properties for solute systems such as the redox potential, solute diffusion coefficients and structural information surrounding the solute were predicted from FPMD modeling and the calculated properties are generally in agreement with the experiments. In particular, we formulate an approach to model redox energetics vs. chlorine (or fluorine) potential from first-principles approaches. This study develops approaches for, and demonstrates the capabilities of, FPMD to model solute properties in molten salts.
A first-principles study of orthorhombic CN as a potential superhard material.
Tang, Xiao; Hao, Jian; Li, Yinwei
2015-11-01
Using first-principles calculations, we have investigated the structural, electronic, dynamical and mechanical properties of a recently synthesized Pnnm-CN. Phonon dispersion and elastic constant calculations were carried out to demonstrate the dynamical and mechanical stabilities of the Pnnm structure of CN at ambient pressure. The electronic band structure suggests that Pnnm-CN is an insulator with an indirect band gap of about 3.7 eV. First-principles strain-stress relationships at large strains were also simulated to examine the structural and mechanical properties of Pnnm-CN. The established ideal tensile strength of âˆ¼41 GPa in the ã€ˆ100ã€‰ direction suggests that CN is a potential superhard material. The present results provide deep insights for understanding the mechanical properties of CN and thus are helpful to explore the potential industrial applications of CN. PMID:26437847
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 acetylcholinemoreÂ Â» 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.Â«Â less
First-principles study of intrinsic dielectric loss [in oxides] at microwave frequencies
NASA Astrophysics Data System (ADS)
Antons, Armin
2003-03-01
Ceramic dielectric materials play an important role in microwave communication systems such as cellular phones. One of the most important requirements for such materials, along with a high and weakly temperature-dependent dielectric constant, is a low dielectric loss. Here, we demonstrate the feasibility of a fully first-principles approach to computing dielectric loss at microwave frequencies by focusing on the intrinsic losses which arise from anharmonic processes within the crystal, while neglecting extrinsic mechanisms associated with defects such as vacancies, impurity phases, and grain boundaries. Using SrO as our model system, we have computed the loss arising from two-phonon processes using third order force-constant matrices, phonon dispersion relations, phonon eigenvectors, and Born effective charges calculated from first principles using density-functional perturbation theory techniques. Progress in extending the work to SrTiO3 will also be discussed.
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.
Elastic and thermodynamic properties of Fe3Ga from first-principles calculations
NASA Astrophysics Data System (ADS)
Lin, Ya-Ning; Li, Lin-Ling; Yan, Xiang-Hong; Zhang, Ya-Ping; Zhang, Dong-yun; Zhang, Peng
2016-03-01
First-principles calculations within the framework of density functional theory (DFT) are performed to investigate the elastic and thermodynamic properties of DO3-type Fe3Ga alloy. The obtained lattice constants and the bulk modulus are in good agreement with available experimental data. In terms of the calculated formation energy and Poisson's ratio, the Fe3Ga alloy is mechanically stable and exhibit a negative Poisson's ratio of -0.81 along the <110> direction. The thermodynamic properties such as the Gibbs free energy, thermal expansion, and the specific heat are obtained by the first-principles phonon calculations with the quasiharmonic approximation method. The predicted coefficient of linear thermal expansion and specific heat may provide a helpful reference for experimental work.
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. PMID:25651435
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
First-principles theory of 250 000-atom coherent alloy microstructure
NASA Astrophysics Data System (ADS)
Wolverton, C.
2000-05-01
Microstructural issues in alloys such as precipitation have largely been outside the realm of first-principles electronic structure calculations due to the length scales involved in precipitation microstructure (typically nanometres to micrometres) and the inherent thermodynamic/statistical nature of the problem. Here, we show that modern, first-principles total energy calculations can be combined with a mixed-space cluster expansion approach (a generalized real/reciprocal space Ising model) and Monte Carlo simulations to yield a method capable of describing equilibrium coherent precipitate shapes in alloys with system sizes up to 250 000 atoms. Both the (anisotropic) interfacial free energies and the coherency strain between precipitate and matrix are accounted for in this method as well as the short-range atomic-scale ordering of the solid solution. Illustrations of the technique are given for several famous examples of coherent precipitation in aluminium alloys: Al-Mg, Al-Cu and Al-Ni.
Ground state structure of BaZrO3 : A comparative first-principles study
NASA Astrophysics Data System (ADS)
Bili?, Ante; Gale, Julian D.
2009-05-01
First-principles calculations, based on density-functional theory, are exploited to investigate the nature of the ground-state structure of barium zirconate. The experimentally observed simple-cubic structure is found to be dynamically unstable against an antiferrodistortive transformation. This instability manifests itself through imaginary frequency modes along the whole R-M edge of the Brillouin zone. The computations predict an orthorhombic crystal structure of the material, only slightly distorted from the cubic lattice, with an eight times larger unit cell and alternate ZrO6 octahedra slightly rotated in opposite directions around the Cartesian axes. The apparent disagreement with some of the previous first-principles results regarding the nature of the ground-state structure is considered in detail. The neglect of the barium 5s2 and 5p6 electrons in the valence configuration of Ba is found to be responsible for the previously reported erroneous results.
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.
New Stable Phases in the Re-B System: A First-principles Study
NASA Astrophysics Data System (ADS)
Zhao, Xin; Nguyen, Manh Cuong; Wang, Cai-Zhuang; Ho, Kai-Ming; Iowa State University Team
2014-03-01
We studied rhenium borides using genetic algorithm in combination with first-principles calculations and revealed several new stable phases in the Re-B system. The structures obtained from our genetic algorithm search are energetically much superior to those proposed in the literature. Two new phases of Re2B were found to be thermodynamically stable at different pressures, which possibly explains the recent experimental observations (Solid State Sciences 25 85-92 (2013)). ReB is stable against decomposition reactions below 10 GPa and ReB3 is stable above 22 GPa. A C2/m structure was discovered for ReB4 to have lower energy than the R-3m structure reported earlier (J. Alloys Compd. 573 20-26 (2013)). Elastic properties from first-principles calculations indicate that the structures we report in this work are mechanically stable and promising targets as new ultra-hard materials.
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.
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
Large-scale first-principles molecular dynamics for electrochemical systems with O(N) methods
NASA Astrophysics Data System (ADS)
Ohwaki, Tsukuru; Otani, Minoru; Ikeshoji, Tamio; Ozaki, Taisuke
2012-04-01
A method for large-scale first-principles molecular dynamics (MD) simulations on electrochemical systems has been developed by combining the effective screening medium (ESM) method with O(N) density functional theory (DFT). This implementation has been significantly simplified by the introduction of neutral atom potentials, which minimizes the modifications to existing DFT code. In order to demonstrate ability of this implementation, it has been applied to an electrochemical system consisting of a H-Si(111) electrode, which is a candidate anode for high-capacity Li-ion secondary batteries, and a propylene carbonate (PC) solvent to simulate how PC molecules in the vicinity of the electrode surface respond to an imposed electric field. The large-scale MD simulation clearly demonstrates that the combination of the ESM and O(N) DFT methods provides a useful tool for first-principles investigation of complicated electrochemical systems such as high-capacity batteries.
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.
NASA Astrophysics Data System (ADS)
Gunst, Tue; Markussen, Troels; Stokbro, Kurt; Brandbyge, Mads
2016-01-01
We present density functional theory calculations of the phonon-limited mobility in n -type monolayer graphene, silicene, and MoS2. The material properties, including the electron-phonon interaction, are calculated from first principles. We provide a detailed description of the normalized full-band relaxation time approximation for the linearized Boltzmann transport equation (BTE) that includes inelastic scattering processes. The bulk electron-phonon coupling is evaluated by a supercell method. The method employed is fully numerical and does therefore not require a semianalytic treatment of part of the problem and, importantly, it keeps the anisotropy information stored in the coupling as well as the band structure. In addition, we perform calculations of the low-field mobility and its dependence on carrier density and temperature to obtain a better understanding of transport in graphene, silicene, and monolayer MoS2. Unlike graphene, the carriers in silicene show strong interaction with the out-of-plane modes. We find that graphene has more than an order of magnitude higher mobility compared to silicene in the limit where the silicene out-of-plane interaction is reduced to zero (by substrate interaction, clamping, or similar). If the out-of-plane interaction is not actively reduced, the mobility of silicene will essentially be zero. For MoS2, we obtain several orders of magnitude lower mobilities compared to graphene in agreement with other recent theoretical results. The simulations illustrate the predictive capabilities of the newly implemented BTE solver applied in simulation tools based on first-principles and localized basis sets.
Pyro- and piezoelectric properties of polar polymers from the first principles
NASA Astrophysics Data System (ADS)
Nakhmanson, Serge; Buongiorno Nardelli, Marco; Bernholc, Jerry
2003-03-01
Using large-scale ab initio computer simulations and the Berry-phase formalism we compute polarization and piezoelectric stress constants in various crystalline phases of polyvinylidene fluoride (PVDF) and its trifluoroethylene copolymer, P(VDF/TrFE). Our results allow for a rigorous evaluation of the quality of simple ``field summation'' models for computing pyro- and piezoelectric properties of PVDF and its copolymers. First principles evaluations of other polymer properties are in progress.
First-principles study of He point-defects in HCP rare-earth metals
Li, Yang; Chen, Ru; Peng, SM; Long, XG; Wu, Z.; Gao, Fei; Zu, Xiaotao
2011-05-01
He defect properties in Sc, Y, Gd, Tb, Dy, Ho, Er and Lu were studied using first-principles calculations based on density functional theory. The results indicate that the formation energy of an interstitial He atom is smaller than that of a substitutional He atom in all hcp rare-earth metals considered. Furthermore, the tetrahedral interstitial position is more favorable than an octahedral position for He defects. The results are compared with those from bcc and fcc metals.
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 solidsâ€”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.
Phase stability of ZnO from first-principles calculations (abstract only).
Wröbel, Jan; Piechota, Jacek
2008-02-13
First-principles calculations of the phonon dispersion relations and the phonon density of states for ZnO polymorphs: wurtzite, zinc-blende, rocksalt structures, and as yet the experimentally undiscovered CsCl structure, are presented. All the phases were exposed to pressures ranging from 0 to 20 GPa. The pressure-temperature phase diagram of ZnO was constructed and compared to experimental data, where available. PMID:21693882
NASA Astrophysics Data System (ADS)
Liu, X. X.; Liu, L. Z.; Wu, X. L.; Chu, Paul K.
2015-07-01
The defect states and optical absorption enhancement induced by twin boundaries in silicon are investigated by first-principle calculation. The defect states in the forbidden bands are identified and based on the established electronic structures, the dielectric functions and absorption coefficients are derived. An important result of our calculations is that visible light absorption by the twinning configuration is enhanced significantly, indicating that twinning structures possibly play an important role in silicon-based photovoltaic devices.
NASA Astrophysics Data System (ADS)
Bannikov, V. V.; Shein, I. R.; Ivanovskii, A. L.
2010-05-01
First-principles FLAPW-GGA calculations for six possible polymorphs of ruthenium mononitride RuN indicate that the most stable structure is that of zinc blende rather than the rock salt structure recently reported for synthesized RuN samples. The elastic, electronic properties and the features of chemical bonds of zinc-blende RuN polymorph were investigated and discussed in detail.
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.
Trapping of multiple hydrogen atoms in a tungsten monovacancy from first principles
Ohsawa, Kazuhito; Yagi, Masatoshi; Goto, Junya; Yamakami, Masahiro; Yamaguchi, Masatake
2010-11-01
The configuration of multiple hydrogen atoms trapped in a tungsten monovacancy is investigated using first-principles calculations. Unlike previous computational studies, which have reported that hydrogen in bcc metal monovacancies occupies octahedral interstitial sites, it is found that the stable sites shift toward tetrahedral interstitial sites as the number of hydrogen atoms increases. As a result, a maximum of twelve hydrogen atoms can become trapped in a tungsten monovacancy.
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.
First-principles aluminum database: Energetics of binary Al alloys and compounds
NASA Astrophysics Data System (ADS)
Wolverton, C.; Ozoli?š, V.
2006-04-01
Using an extensive series of first-principles density functional calculations, we have constructed a “first-principles aluminum database” of thermodynamic properties of binary Al-based alloys with 26 different solute elements, X . In all cases, first-principles results are critically compared to experimental data for observed Al-rich ordered compounds, dilute impurities in Al, disordered Al-X solid solutions, and pure elements, X , in a variety of structure types. We find the following: (i) In all Al-X systems, first-principles formation enthalpies of ordered compounds are in excellent agreement with experimental data. Impurity energetics for X in Al also agree rather well with thermodynamically assessed values from the COST507 database. (ii) Formation enthalpies of ordered compounds and energies of dilute impurities for elements from the 3d series are most negative at the beginning and the end of the series, and reach a maximum near the middle of the series for Cr. The ordering tendency decreases dramatically in the Al-Cu system with filled d bands. (iii) The special quasirandom structure approach has been used to obtain mixing energies of disordered solid solutions across the whole composition range for all systems. We find that mixing energies follow the same general trend across the 3d series as the ordered and impurity formation enthalpies. Asymmetry in the mixing energies is also similar in all systems, giving less negative mixing energies for Al-rich compositions. (iv) Calculation of the solubility enthalpy, which is the difference in the formation energy per solute atom between the ordered and dilute solid solution phases, shows that the observed low solubility in Al-X systems is due to very negative values of the ordered formation enthalpies in comparison with those for the dilute solid solution.
First principles total energy study of NbCr{sub 2} + V Laves phase ternary system
Ormeci, A.; Chen, S.P.; Wills, J.M.; Albers, R.C.
1999-04-01
The C15 NbCr{sub 2} + V Laves phase ternary system is studied by using a first-principles, self-consistent, full-potential total energy method. Equilibrium lattice parameters, cohesive energies, density of states and formation energies of substitutional defects are calculated. Results of all these calculations show that in the C15 NbCr{sub 2} + V compounds, V atoms substitute Cr atoms only.
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 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.
First-principles model of the dielectric response of ultrathin perovskite films
NASA Astrophysics Data System (ADS)
Rabe, Karin; Ghosez, Philippe
2001-03-01
Using a recently developed first-principles model [1] for the structural energetics of ultrathin PbTiO3 films, we investigate the thickness dependence of the dielectric response of perfect single-crystal films. The dependence on field direction and on the mechanical and electrical boundary conditions are examined, and the effects of surface relaxations discussed. 1. Ph. Ghosez and K. M. Rabe, Appl. Phys. Lett. 76, 2767 (2000).
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.
Phonon thermal transport in strained and unstrained graphene from first principles
NASA Astrophysics Data System (ADS)
Lindsay, L.; Li, Wu; Carrete, Jesús; Mingo, Natalio; Broido, D. A.; Reinecke, T. L.
2014-04-01
A rigorous first principles Boltzmann-Peierls equation (BPE) for phonon transport approach is employed to examine the lattice thermal conductivity, ?L, of strained and unstrained graphene. First principles calculations show that the out-of-plane, flexural acoustic phonons provide the dominant contribution to ?L of graphene for all strains, temperatures, and system sizes considered, supporting a previous prediction that used an optimized Tersoff empirical interatomic potential. For the range of finite system sizes considered, we show that the ?L of graphene is relatively insensitive to strain. This provides validation for use of the BPE approach to calculate ?L for unstrained graphene, which has recently been called into question. The temperature and system size dependence of the calculated ?L of graphene is in good agreement with experimental data. The enhancement of ?L with isotopic purification is found to be relatively small due to strong anharmonic phonon-phonon scattering. This work provides insight into the nature of phonon thermal transport in graphene, and it demonstrates the power of first principles thermal transport techniques.
Phase transitions and magnetostructural coupling in ZnCr2O4 from first principles
NASA Astrophysics Data System (ADS)
Eklund, Carl-Johan; Fennie, Craig J.; Rabe, Karin M.
2009-03-01
In the spinel structure oxide ZnCr2O4, a phase transition is observed from the high-temperature cubic phase to a low-temperature low-symmetry phase, reported as tetragonal^1 or orthorhombic.^2 Building on a previous first-principles analysis of the zone-center phonons and spin-phonon coupling,^3 we construct a first-principles effective Hamiltonian to investigate this transition. The local modes included are the Cr displacements, distortions of the Zn-centered tetrahedra, and the homogeneous strain. The magnetostructural coupling of these degrees of freedom to the spins of the Cr^3+ ions is included in the effective Hamiltonian parameterization and first-principles determination using a symmetry analysis. The role of the magnetostructural coupling in the phase transition will be analyzed and discussed. 1. S. H. Lee et al., J. Phys. Cond. Matt. 19, 145259 (2007) 2. V. N. Glazkov et al., http://arxiv.org/abs/0807.0546 3. C. J. Fennie and K. M. Rabe, Phys. Rev. Lett. 96, 205505 (2006)
A theoretical and observational study of the X-ray spectroscopy of isolated neutron stars
NASA Astrophysics Data System (ADS)
Mori, Kaya
2003-10-01
This thesis consists of a theoretical study of neutron star atmospheres and its application to recent X-ray data on the isolated neutron star 1E1207.4-5209, which shows absorption features. Although the study of pulsars has been carried out mostly in the radio band for many years, spectroscopy study by recent X-ray telescopes provides a powerful tool for describing the interior of neutron stars. In particular, a unique determination of mass and radius from a single neutron star severely constrains the equation of state and composition of the core region of neutron stars. In the first part of my thesis, theoretical work on neutron star atmosphere models is presented. Intensive study of atomic structure in strong magnetic fields typical of neutron stars was undertaken. A novel atomic calculation which computes transition energies and oscillator strengths with sufficient accuracy to analyze high energy resolution data on Chandra and XMM-Newton was performed. In the second part of my thesis, a methodology was established to identify spectral features from strongly- magnetized dense plasmas. Based on the application of the atomic code and methodology to the Chandra data from the isolated neutron star 1E1207.4-5209, it was shown that the observed absorption features are due to Helium-like Oxygen or Neon at B ˜ 1012 G. This led to a simultaneous determination of the surface composition, the magnetic field strength and, more importantly, the gravitational redshift. Further constraints on the atmosphere, as well as the neutron star equation of state, can be inferred under the assumption that the neutron star mass is near the canonical value of 1.4 M? . In that case Neon is ruled out, and the atmosphere is unambiguously Oxygen. The corresponding gravitational redshift for the Oxygen atmosphere constrains the equation of state of the interior of 1E1207.4-5209 to be quite stiff.
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 ?OD (change in the optical density). The cascading signals, however, will produce stimulated Raman signal of ˜10-2 ?OD, as has been observed experimentally. Experiments probing deuterochloroform find significant sidebands of the CCl3 bend, which has an E type symmetry, shifted from the A1 type C-D and C-Cl stretching modes, despite the fact that third-order anharmonic coupling between these modes is forbidden by symmetry. Experiments probing a 50:50 mixture of chloroform and d-chloroform find equivalent intensity signals of low-frequency CDCl3 modes as sidebands shifted from both the C-D stretch of CDCl3 and the C-H stretch of CHCl3. Such intermolecular sidebands are allowed in the cascade mechanism, but are expected to be extremely small in the fifth-order frequency modulation mechanism. Each of these observations indicates that the observed signals are due to cascading third-order Raman signals.
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 â€First-principles simulations of high-speed combustion and detonation.â€ 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 broader applicability to petascale simulations of high speed combustion and detonation phenomena in reacting gases, and to high speed viscous gaseous flows in general. Project activities included three major steps â€“ (1) development of physical and numerical models, (2) code validation, and (3) demonstration simulation of flame acceleration and DDT in a long pipe.
First-principles analysis of the STM image heights of styrene on Si(100)
NASA Astrophysics Data System (ADS)
Bevan, K. H.; Zahid, F.; Kienle, D.; Guo, H.
2007-07-01
We report on theoretical investigations of scanning tunneling spectroscopy (STM) image heights on Si(100). Calculations are performed using density functional theory (DFT) within the Keldysh nonequilibrium Greenâ€™s function (NEGF) formalism. The nonequilibrium potential drop between Si(100) and a STM tip is determined self-consistently. This potential drop is found to play an important role in the calculated image height characteristics of adsorbed hydrocarbons by lowering the vacuum barrier and shifting molecular levels. Numerical data collected for image heights of styrene against a hydrogen passivated Si(100) background are found to agree quantitatively with the corresponding experimental results. We also present a comparison between results obtained by the NEGF-DFT formalism and the Tersoff-Hamann approximation, showing that nonequilibrium analysis can be important in the study of STM image heights of molecules.
Zhang, Jian; Zhou, Bin; Sun, Zhen-Rong; Wang, Xue-Bin
2015-02-01
Proposed in theory and then their existence confirmed, anion-? interactions have been recognized as new and important non-covalent binding forces. Despite extensive theoretical studies, numerous crystal structural identifications, and a plethora of solution phase investigations, anion-? interaction strengths that are free from complications of condensed-phase environments have not been directly measured in the gas phase. Herein we present a joint photoelectron spectroscopic and theoretical study on this subject, in which tetraoxacalix[2]arene[2]triazine 1, an electron-deficient and cavity self-tunable macrocyclic, was used as a charge-neutral molecular host to probe its interactions with a series of anions with distinctly different shapes and charge states (spherical halides Cl(-), Br(-), I(-), linear thiocyanate SCN(-), trigonal planar nitrate NO3(-), pyramidic iodate IO3(-), and tetrahedral sulfate SO4(2-)). The binding energies of the resultant gaseous 1?:?1 complexes (1·Cl(-), 1·Br(-), 1·I(-), 1·SCN(-), 1·NO3(-), 1·IO3(-) and 1·SO4(2-)) were directly measured experimentally, exhibiting substantial non-covalent interactions with pronounced anion-specific effects. The binding strengths of Cl(-), NO3(-), IO3(-) with 1 are found to be strongest among all singly charged anions, amounting to ca. 30 kcal mol(-1), but only about 40% of that between 1 and SO4(2-). Quantum chemical calculations reveal that all the anions reside in the center of the cavity of 1 with an anion-? binding motif in the complexes' optimized structures, where 1 is seen to be able to self-regulate its cavity structure to accommodate anions of different geometries and three-dimensional shapes. Electron density surface and charge distribution analyses further support anion-? binding formation. The calculated binding energies of the anions and 1 nicely reproduce the experimentally estimated electron binding energy increase. This work illustrates that size-selective photoelectron spectroscopy combined with theoretical calculations represents a powerful technique to probe anion-? interactions and has potential to provide quantitative guest-host molecular binding strengths and unravel fundamental insights in specific anion recognitions. PMID:25515705
High pressure behavior of phlogopite using neutron diffraction and first principle simulations
NASA Astrophysics Data System (ADS)
Chheda, T. D.; Mookherjee, M.; dos Santos, A. M.; Molaison, J.; Manthilake, G. M.; Chantel, J.; Mainprice, D.
2013-12-01
Hydrous phases play an important role in the deep water cycle by transporting water into the Earth's interior. Upon, reaching their thermodynamic stability, these hydrous phases decompose and release the water. A part of the water is cycled back to the arc, thus completing the deep water cycle, the remaining water is partitioned into dense hydrous phases and nominally anhydrous phases. Hence, in order to understand the role the hydrous phases in the deep water cycle, it is important to constrain the effect of pressure, temperature, and chemistry on the thermodynamic stability of the hydrous phases. In addition, it is important to constrain the elasticity of these hydrous phases to test whether they can explain the distinct geophysical observations such as lower bulk sound velocities and elastic anisotropy. Phlogopite is a potassium bearing mica that is stable in the hydrated crust and metasomatized mantle up to pressures of ~9 GPa, i.e., base of the upper mantle. We investigated the response of the crystal structure, lattice parameters and unit-cell volume of a natural phlogopite upon compression. We conducted in situ neutron diffraction studies at high-pressures using Paris-Edinburgh press at the Spallation Neutrons and Pressure Diffractometer (SNAP), Oak Ridge National Laboratory. All the experiments were conducted at room temperatures and pressures up to 10 GPa were explored. The equation of state parameters from our experiments could be explained by a finite strain formulation with V0= 487 Ã…3, K0 = 49 GPa, K' = 4.1. In addition, we have used first principle simulations based on density functional theory to calculate the equation of state and elasticity. The predicted equation of state is in good agreement with the experiments, with V0= 519 Ã…3, K0 = 45.8 GPa and K'= 6.9. The full elastic constant tensor shows significant anisotropy with the principal elastic constants at theoretical V0: C11= 181 GPa, C22= 185 GPa, C33= 62 GPa, the shear elastic constants- C44= 14 GPa, C55=20 GPa, C66= 68 Ga, and C46 = -6 GPa; the off diagonal elastic cosntants C12= 48 GPa, C13= 12 GPa, C23 = 12 GPa, C15 = -16 GPa, C25 = -5 GPa, and C35 = -1 GPa. We also note that the shear elastic constants for phlogopite are significantly low and it also has a high VP/VS ratio (~2 km/sec). Phlogopite bearing hydrated crust could explain the low velocity layers in the top 6-8 km of the subducting slabs. Acknowledgements TC and MM are supported by US NSF grant #EAR1250477 and also acknowledge computing resources (EAR130015) from XSEDE (OCI-1053575).
NASA Astrophysics Data System (ADS)
Yamada, A.; Nanbu, S.; Kasai, Y.; Ozima, M.
2009-12-01
Mass-independently fractionated oxygen isotope were reported on metal particles extracted from Apollo lunar soils [1, 2], but these origins are still unknown. Since the substantial fraction of Earth-escaping O+ flux (Earth Wind, EW hereafter), comparable to the amount of the anomalous oxygen implanted on the metal particles, could reach the lunar surface [3], Ozima et al. [4] suggested that EW may be responsible to the anomalous oxygen. The purpose is to test this EW hypothesiss, we study oxygen isotopic ratios of O+ at the upper atmosphere. From quantum chemical calculations of photo-dissociation of O2, we show the results in mass-independent isotopic fractionation of oxygen, thereby in conformity with the EW hypothesis. First principles reaction dynamics simulations were performed to compute the photolysis rate for the B3?u- ? X3?g- electronic transition, for Schumann-Runge band. With the assumption of the Born-Oppenheimer approximation, we performed the wave-packet dynamics for the nuclei-motion in the potential energy curves determined by the first step calculation. Quantum chemical program package [5] was used for the first step calculation, and the quantum dynamics was carried out by our own program package. Assuming the quantum yield of the corresponding photolysis is unity, the photo-absorption cross section can be correlated with the photolysis rate. Therefore, following the time dependent approach, the autocorrelation function (A(t) = ) was numerically computed by the second step calculation. Finally, the theoretical spectrum as a function of wavelength of excitation light was estimated by the Fourier transform of the autocorrelation function A(t) [6]. Calculated absorption cross sections for C16O showed similar wavelength dependence with experiment [7], although the absolute magnitude was yet to be calibrated for a quantitative comparison. Assuming Boltzmann distribution at 1200 K, we estimated enrichment factors defined as ??(?)/?16(?) - 1 (i = 17, 18) using the above calculated cross sections. Assuming SMOW for the initial oxygen isotopic composition, the isotopic ratios of O atom dissociated from O2 are ?17O = 5.62‰, ?18O = 3.53‰, ?17O = 3.8‰, suggesting large mass-independent isotopic fractionation in photo-dissociation of CiO. Numerical values of isotopic fractionation (e.g. ?17O) can be obtained by solving photochemical reaction equations in the thermosphere conditions (>100 km) with the above estimated dissociation rates, where effective O+ pickup is likely to take place. We are currently working on the latter problem with hopes that this would test the EW hypothesis. References: [1] Ireland et al., 2006, Nature, 440:776. [2] Hashizume & Chaussidon, 2009, GCA, 73:3038. [3] Seki et al., 2001, Science, 291:1939. [4] Ozima et al., 2008, PNAS, 105:17654. [5] Werner & Knowles, http://www.molpro.net. [6] Heller, 1978, J. Chem. Phys., 68:2066. [7] Ackermann et al., 1970, Planet. Space Sci., 18:1639.
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.
Theoretical study of core-loss electron energy-loss spectroscopy at graphene nanoribbon edges.
Fujita, N; Hasnip, P J; Probert, M I J; Yuan, J
2015-08-01
A systematic study of simulated atomic-resolution electronic energy-loss spectroscopy (EELS) for different graphene nanoribbons (GNRs) is presented. The results of ab initio studies of carbon [Formula: see text] core-loss EELS on GNRs with different ribbon edge structures and different hydrogen terminations show that theoretical core-loss EELS can distinguish key structural features at the atomic scale. In addition, the combination of polarized core-loss EELS with symmetry resolved electronic partial density of states calculations can be used to identify the origins of all the primary features in the spectra. For example, the nature of the GNR edge structure (armchair, zigzag, etc) can be identified, along with the degree of hydrogenation. Hence it is possible to use the combination of ab initio calculations with high resolution, high energy transmission core-loss EELS experiments to determine the local atomic arrangement and chemical bonding states (i.e. a structural fingerprint) in GNRs, which is essential for future practical applications of graphene. PMID:26173149
NASA Astrophysics Data System (ADS)
Sun, Tao; Gawad, Shady; Bernabini, Catia; Green, Nicolas G.; Morgan, Hywel
2007-09-01
Measurements of the dielectric (or impedance) properties of cells can be used as a general characterization and diagnostic tool. In this paper, we describe a novel impedance spectroscopy technique for the analysis of single biological cells in suspension. The technique uses maximum length sequences (MLS) for periodic excitation signal in a microfluidic impedance cytometer. The method allows multi-frequency single cell impedance measurements to be made in a short time period (ms). Spectral information is obtained in the frequency domain by applying a fast M-sequence transform (FMT) and fast Fourier transform (FFT) to the time domain response. Theoretically, the impedance is determined from the transfer function of the system when the MLS is a current excitation. The order of the MLS and sampling rate of A/D conversion are two factors that determine the bandwidth and spectral accuracy of the technique. Experimentally, the applicability of the technique is demonstrated by characterizing the impedance spectrum of red blood cells (RBCs) in a microfluidic cytometer. The impedance is measured within 1 ms at 512 discrete frequencies, evenly distributed in the range from 976.56 Hz to 500 kHz. The measured spectrum shows good agreement with simulations.
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.
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.
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.
Crystalline LiN5 Predicted from First-Principles as a Possible High-Energy Material.
Peng, Feng; Yao, Yansun; Liu, Hanyu; Ma, Yanming
2015-06-18
The search for stable polymeric nitrogen and polynitrogen compounds has attracted great attention due to their potential applications as high-energy-density materials. Here we report a theoretical prediction of an interesting LiN5 crystal through first-principles calculations and unbiased structure searching techniques. Theoretical calculations reveal that crystalline LiN5 is thermodynamically stable at pressures above 9.9 GPa, and remains metastable at ambient conditions. The metastability of LiN5 stems from the inherent stability of the N5(-) anions and strong anion-cation interactions. It is therefore possible to synthesize LiN5 by compressing solid LiN3 and N2 gas under high pressure and quench recover the product to ambient conditions. To the best of our knowledge, this is the first time that stable N5(-) anions are predicted in crystalline states. The weight ratio of nitrogen in LiN5 is nearly 91%, placing LiN5 as a promising high-energy material. The decomposition of LiN5 is expected to be highly exothermic, releasing an energy of approximately 2.72 kJÂ·g(-1). The present results open a new avenue to synthesize polynitrogen compounds and provide a key perspective toward the understanding of novel chemical bonding in nitrogen-rich compounds. PMID:26266618
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).
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.
Systematic studies of the methylation of DNA base molecules from first-principles
NASA Astrophysics Data System (ADS)
Lee, Geunjung; Yoon, Young-Gui
2012-05-01
We have studied selected DNA base molecules, such as guanine, cytosine, adenine, and thymine, and methylated DNA base molecules from first-principles. The impact of methylation and the rearrangement of the hydrogen atoms of the DNA base molecules on the geometries and the electronic properties are explained with the bond orders, the steric effects, and the molecular topologies. A DNA base molecule without methylation and a corresponding methylated DNA base molecule with the same bond order are expected to have similar HOMO-LUMO energy gaps.
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.
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.
First-Principles Study on ?-SiC/BNNT Core/shell Nanocable
NASA Astrophysics Data System (ADS)
Zou, X. C.; Ouyang, J.; Wu, M. S.; Liu, G.; Lei, X. L.; Ouyang, C. Y.; Xu, B.
2013-09-01
In this paper, we studied the structural and electronic properties of core/shell nanocables composed of cubic silicon carbide nanowires (?-SiCNW) and boron nitride nanotubes (BNNT) using first-principles pseudopotential plane wave method within density functional theory. Our results show that the ?-SiC/BNNT heterojunction structures are metallic, which primarily originates from the contributions of the BNNTs and the surfaces of SiCNWs. The BNNTs exhibit metallic characters after the SiC nanowires are inserted. The transition of the BNNTs is attributed to the charge transfer between BNNTs and SiCNWs.
Carrier compensation in semi-insulating CdTe: First-principles calculations
Du, Mao-Hua; Singh, David J
2008-01-01
Carrier compensation in semi-insulating CdTe has been attributed to the compensation of surplus shallow acceptors by deep donors, usually assumed to be Te antisites. However, our first-principles calculations show that intrinsic defects should not have a significant effect on the carrier compensation due either to lack of deep levels near midgap or to low defect concentration. We demonstrate that an extrinsic defect, OTe-H complex, may play an important role in the carrier compensation in CdTe because of its amphoteric character and reasonably high concentration. Our findings have important consequences for improving device performance in CdTe-based radiation detectors and solar cells.
First-principles study of water on copper and noble metal (110) surfaces
Ren, Jun; Meng, Sheng
2008-02-01
Water structure and dissociation kinetics on a model open metal surface: Cu(110), has been investigated in detail based on first-principles electronic structure calculations. We revealed that in both monomer and overlayer forms, water adsorbs molecularly, with a high tendency for diffusion and/or desorption rather than dissociation on clean surfaces at low temperature. Studying water on other noble metal (110) surfaces confirms that Cu(110) is the borderline between intact and dissociative water adsorption, differing in energy by only 0.08 eV. This may lead to promising applications in hydrogen generation and fuel cells.
Elastic properties of Ca-based metallic glasses predicted by first-principles simulations
Widom, M.; Sauerwine, B.; Cheung, A.M.; Poon, S.J.; Tong, P.; Louca, D.; Shiflet, G.J.
2012-07-11
First-principles simulations of Ca-based metallic glass-forming alloys yield sample amorphous structures whose structures can be compared to experiment and whose properties can be analyzed. In an effort to understand and control ductility, we investigate the elastic moduli. Calculated Poisson ratios depend strongly on alloying elements in a manner that correlates with ionicity (charge transfer). Consequently, we predict that alloying Ca with Mg and Zn should result in relatively ductile glasses compared to alloying with Ag, Cu, or Al. Experimental observations validate these predictions.
Liquid–liquid phase transition in compressed hydrogen from first-principles simulations
Scandolo, Sandro
2003-01-01
The properties of compressed liquid hydrogen, the most abundant fluid in the universe, have been investigated by means of first-principles molecular dynamics at pressures between 75 and 175 GPa and temperatures closer to the freezing line than so far reported in shock-wave experiments. Evidence for a liquid–liquid transition between a molecular and a dissociated phase is provided. The transition is accompanied by a 6% increase in density and by metallization. This finding has important implications for our understanding of the interiors of giant planets and supports predictions of a quantum fluid state at low temperatures. PMID:12626753
First-principles study of enhancement of transport properties of silica melt by water.
Karki, Bijaya B; Stixrude, Lars
2010-05-28
First-principles molecular dynamics simulations show that water (8.25 wt%) dramatically affects the transport properties of SiO2 liquid increasing the diffusivity and decreasing the viscosity by an order of magnitude. At 3000 K, the diffusivity of Si, O, and H, and the viscosity vary anomalously with pressure. Highly mobile protons make the hydrous liquid a potential superionic conductor. The predicted dynamical changes are associated with structural depolymerization and water speciation, which changes from being dominated by hydroxyls at low pressure to extended structures at high pressure. PMID:20867116
Hydrogen effect on shearing and cleavage of Al: A first-principles study
NASA Astrophysics Data System (ADS)
Apostol, F.; Mishin, Y.
2011-09-01
We report on first-principles calculations of the effect of a (111) hydrogen layer embedded in Al on generalized stacking fault energies and cleavage energy for different choices of the slip and cleavage planes. It is shown that the H layer softens Al against shear by reducing the stable and unstable stacking fault energies relative to pure Al. This finding points to a possible enhancement of plasticity of Al by H. The H layer also reduces the cleavage energy on the (111) plane. The reductions in the cleavage energy and unstable stacking fault energy compensate each other and produce only a moderate change in the Rice criterion of ductile versus brittle fracture.
First principles calculation of the energy and structure of two solid surface phases on Ir?100?
NASA Astrophysics Data System (ADS)
Ge, Q.; King, D. A.; Marzari, N.; Payne, M. C.
1998-12-01
The structure and energetics of the hexagonal reconstruction of Ir{100} have been determined with first principles density functional theory calculations based on the local-density approximation with the generalised-gradient correction. The results reproduced the experimentally determined surface buckling and show the presence of some lateral displacement of the reconstructed (1×5) phase with respect to the ideal hexagonal close packed structure. The (1×5) phase is found to be 0.06 eV/(1×1 area) more stable than the (1×1) phase. The reconstruction is analysed by examining surface bonding.
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 (?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-based properties of DT are essential for designing ICF ignition targets. Future work on first-principles studies of ICF ablator materials is also discussed.
First principles calculation of the effect of Coulomb collisions in partially ionized gases
Donkó, Z.
2014-04-15
Coulomb collisions, at appreciable ratios (?) of the electron to the neutral particle density, influence significantly the electron kinetics in particle swarms and in plasmas of gas discharges. This paper introduces a combination of Molecular Dynamics and Monte Carlo simulation techniques, to provide a novel, approximation-free, first principles calculation method for the velocity distribution function of electrons, and related swarm characteristics, at arbitrary ?. Simulation results are presented for electrons in argon gas, for density ratios between zero and 10{sup ?1}, representing the limits of a negligible electron density and an almost complete Maxwellization of the velocity distribution function, respectively.
Dielectric Properties of Ice and Liquid Water from First-Principles Calculations
NASA Astrophysics Data System (ADS)
Lu, Deyu; Gygi, François; Galli, Giulia
2008-04-01
We present a first-principles study of the static dielectric properties of ice and liquid water. The eigenmodes of the dielectric matrix ? are analyzed in terms of maximally localized dielectric functions similar, in their definition, to maximally localized Wannier orbitals obtained from Bloch eigenstates of the electronic Hamiltonian. We show that the lowest eigenmodes of ?-1 are localized in real space and can be separated into groups related to the screening of lone pairs, intra-, and intermolecular bonds, respectively. The local properties of the dielectric matrix can be conveniently exploited to build approximate dielectric matrices for efficient, yet accurate calculations of quasiparticle energies.
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.
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.
First principles study of structural, electronic and magnetic properties of Mn2CoAs
NASA Astrophysics Data System (ADS)
Berri, Saadi; Ibrir, M.; Maouche, D.; Bensalem, R.
2014-06-01
We have performed first-principle calculations of the structural, electronic and magnetic properties of Mn2CoAs Heusler alloy, using full-potential linearized augmented plane wave (FP-LAPW) scheme within the GGA. Features such as the lattice constant, the bulk modulus and its pressure derivative are reported. The electronic band structures and density of states of the Mn2CoAs compound show that the spin-up electrons are metallic, but the spin-down bands have a gap of 0.48 eV, resulting in stable half-metallic ferrimagnetic behavior with a magnetic moment of 4.00 ?B.
First-principles study of Ti intercalation between graphene and Au surface
NASA Astrophysics Data System (ADS)
Kaneko, T.; Imamura, H.
2011-06-01
We investigate the effects of Ti intercalation between graphene and Au surface on binding energy and charge doping by using the first-principles calculations. We show that the largest binding energy is realized by the intercalation of single mono-layer of Ti. We also show that electronic structure is very sensitive to the arrangement of metal atoms at the interface. If the composition of the interface layer is Ti0.33Au0.67 and the Ti is located at the top site, the Fermi level lies closely at the Dirac point, i.e., the Dirac cone of the ideal free-standing graphene is recovered.
Half metallic ferromagnetism in alkali metal nitrides MN (M = Rb, Cs): A first principles study
Murugan, A. Rajeswarapalanichamy, R. Santhosh, M. Sudhapriyanga, G.; Kanagaprabha, S.
2014-04-24
The structural, electronic and elastic properties of two alkali metal nitrides (MN: M= Rb, Cs) are investigated by the first principles calculations based on density functional theory using the Vienna ab-initio simulation package. At ambient pressure the two nitrides are stable in ferromagnetic state with CsCl structure. The calculated lattice parameters are in good agreement with the available results. The electronic structure reveals that these materials are half metallic in nature. A pressure-induced structural phase transition from CsCl to ZB phase is observed in RbN and CsN.
First Principles Calculations of Oxygen Adsorption on the UN(001) Surface
Zhukovskii, Yuri F.; Bocharov, Dmitry; Kotomin, Eugene Alexej; Evarestov, Robert; 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 (001) 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(001) surface are simulated here combining the two first principles calculation methods based on the plane wave basis set and that of the localized orbitals.
First-principles study of migration mechanisms and diffusion of carbon in GaN
NASA Astrophysics Data System (ADS)
Kyrtsos, Alexandros; Matsubara, Masahiko; Bellotti, Enrico
2015-09-01
Carbon related defects are readily incorporated in GaN due to its abundance during growth both with MBE and CVD techniques. Employing first-principles calculations we compute the migration barrier of neutral carbon interstitials in the wurtzite GaN crystal. The Minimum Energy Path (MEP) and the migration barriers of these defects are obtained using the Nudged Elastic Band (NEB) method with the climbing image modification (CI-NEB). In addition, the Dimer method is used to verify the results. The results yield a quantitative description of carbon diffusion in the crystal allowing for the determination of the most probable migration paths.
First-principles study of the work function of nitrogen doped molybdenum (110) surface
NASA Astrophysics Data System (ADS)
Black-Schaffer, Annica M.; Cho, Kyeongjae
2006-12-01
The electronic properties of nitrogen doped Mo(110) surfaces were investigated using the first-principles pseudopotential method within the local density approximation in order to determine the effect of doping on the work function. Nitrogen doping was modeled by adsorbing N in both surface and subsurface positions. Surface adsorption of nitrogen was found to increase the work function by as much as 2eV due to the negative surface dipole induced by the electronegativity of nitrogen. Subsurface doping of nitrogen is energetically similar to surface adsorption, but has a small effect on the work function and only when within the first two to three surface Mo layers.
NASA Astrophysics Data System (ADS)
Lazic, Predrag; Sipahi, Guilherme; Kawakami, Roland; Zutic, Igor
2013-03-01
Recent experimental advances in graphene suggest intriguing opportunities for novel spintronic applications which could significantly exceed the state-of-the art performance of their conventional charge-based counterparts. However, for reliable operation of such spintronic devices it is important to achieve an efficient spin injection and large magnetoresistive effects. We use the first principles calculations to guide the choice of a ferromagnetic region and its relative orientation to optimize the desired effects. We propose structures which could enable uniform spin injection, one of the key factors in implementing scalable spintronic circuits. Supported by NSF-NRI, SRC, ONR, Croatian Ministry of Science, Education, and Sports, and CCR at SUNY UB.
Electronic structures and mechanical properties of iron borides from first principles
NASA Astrophysics Data System (ADS)
Gou, Yanpeng; Fu, Zhao; Liang, Yongcheng; Zhong, Zheng; Wang, Shiming
2014-06-01
The structural properties, mechanical behaviors and electronic structures of FeB4 and FeB2 have been studied systematically by first-principles calculations considering the strong correlation effect. Our results show that FeB4 is incompressible and hard, but the recently reported superhard feature [Phys. Rev. Lett. 111 (2013) 157002] is not supported by the present calculations. Interestingly, we find that FeB2 rivals FeB4 in hardness. By analyzing their crystal geometries, band structures and density of states, we elucidate the underlying origins of the related physical properties.
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.
First-principles theory of quantum well resonance in double barrier magnetic tunnel junctions
Wang, Y.; Lu, Zhong-Yi; Zhang, Xiaoguang; Han, Prof. X. F.
2006-01-01
Quantum well (QW) resonances in Fe(001)/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions are calculated from the first-principles. By including the Coulomb blockade energy due to the finite size islands of the Fe middle layer, we confirm that the oscillatory differential resistance observed in a recent experiment, T. Nozaki et al, Phys. Rev. Lett. {\\bf 96}, 027208 (2006), originates from the QW resonances from the $\\Delta_1$ 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.
First-principle path integral study of DNA under hydrodynamic flows
NASA Astrophysics Data System (ADS)
Yang, Shilong; Witkoskie, James B.; Cao, Jianshu
2003-08-01
We use the worm-like chain as a first-principles model to study single molecule experiments of double stranded DNA subject to constant plug, elongational, and shear flows. The steady-state configurations of the polymer correspond to a locally defined potential and result in a path integral description of the canonical partition function. The parameters of this model are consistent with previous theory and experimental measurements. The time averaged mean extension reproduces experimental results and compares well with computationally more expensive Brownian dynamics simulations of reduced models.
First-principles study of lithium adsorption and diffusion on graphene: the effects of strain
NASA Astrophysics Data System (ADS)
Hao, Feng; Chen, Xi
2015-10-01
Large strain is produced within graphene sheets, which serve as a critical component in lithium-ion batteries, due to the expansion of the electrodes. First-principles calculations are therefore employed to investigate the interaction of Li with strained single-layer graphene. It is found that tensile strain enhances Li binding on graphene and significantly reduces the formation energy of divacancies. In addition, Li diffusion through graphene with defects is facilitated by tensile strain, whereas diffusion parallel to the plane of pristine graphene is slightly hindered.
Ground state of doped cuprates from first-principles quantum Monte Carlo calculations
NASA Astrophysics Data System (ADS)
Wagner, Lucas K.
2015-10-01
The author reports on high-fidelity simulations of charge carriers in the high-Tc cuprate materials using quantum Monte Carlo techniques applied to the first-principles Hamiltonian. With this high accuracy technique, the doped ground state is found to be a spin polaron, in which charge is localized through a strong interaction with the spin. This spin polaron has calculated properties largely similar to the phenomenology of the cuprates, and may be the object which forms the Fermi surface and charge inhomogeneity in these materials. The spin polaron has some unique features that should be visible in x-ray, EELS, and neutron experiments.
First-principles studies of the geometry and energetics of the Si36 cluster
NASA Astrophysics Data System (ADS)
Sun, Q.; Wang, Q.; Jena, P.; Waterman, S.; Kawazoe, Y.
2003-06-01
First-principles studies of the geometry and energetics of the Si36 cluster are performed by searching 17 structural isomers including cage, wire, and stuffed fullerene. It is found that the tricapped-trigonal-prism unit is not the structural unit for Si36, and the stable structure is identified to have a more spherical geometry derived from the stuffed fullerene configurations. This is in agreement with the experiment which suggested a structural transition from the elongated geometry to a more spherical one for the medium sized silicon cluster [Hudgins et al., J. Chem. Phys. 111, 7865 (1999); Bergeron et al., J. Chem. Phys. 117, 3219 (2002)].
Hu, Suxing X.; Goncharov, V. N.; Boehly, T. R.; McCrory, R. L.; Skupsky, S.; Collins, Lee A.; Kress, Joel David; 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 deuteriumâ€“tritium (DT) mixtures and ablator materials, such as the equation of state (EOS), thermal conductivity, opacity, and stopping power, were usually estimated by models in hydrocodes used for ICF simulations. In these models, many-body and quantum effects were only approximately taken intomoreÂ Â» 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 â€œpathâ€ to ignition. These FP methods include the pathintegral Monte Carlo (PIMC) and quantum-molecular dynamics (QMD) simulations, which treat electrons with many-body quantum theory. The first-principles equation-of-state (FPEOS) table, thermal conductivities (KQMD), and first principles opacity table (FPOT) 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 are essential for designing ICF ignition targets. We also discuss future work on first-principles studies of ICF ablator materials.Â«Â less
Hu, Suxing X.; Goncharov, V. N.; Boehly, T. R.; McCrory, R. L.; Skupsky, S.; Collins, Lee A.; Kress, Joel David; 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 deuteriumâ€“tritium (DT) mixtures and ablator materials, such as the equation of state (EOS), thermal conductivity, opacity, and stopping power, were usually estimated by models in hydrocodes 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 â€œpathâ€ to ignition. These FP methods include the pathintegral Monte Carlo (PIMC) and quantum-molecular dynamics (QMD) simulations, which treat electrons with many-body quantum theory. The first-principles equation-of-state (FPEOS) table, thermal conductivities (^{K}QMD), and first principles opacity table (FPOT) 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 are essential for designing ICF ignition targets. We also discuss future work on first-principles studies of ICF ablator materials.
First-Principles Study of Multilayer Lithium and Graphene Compounds for Battery Applications
NASA Astrophysics Data System (ADS)
Buldum, Alper; Tetiker, Gulcin
2013-03-01
Recently, graphene and graphene based materials have attracted great interest for energy storage applications such as rechargable Li-ion batteries and supercapacitors. However, recent experiments showed that these materials may have different electrochemical mechanism compared to graphite. Porosity and presence of single or few layers of graphene play important roles in carbon based anode materials in lithium ion batteries. In this work, a study of different multilayer lithium-graphene compounds is performed using first-principles density-functional theory(DFT). Relaxed structures are determined, adsorption energies, density of states and charge density are calculated. Possible multilayer structures for energy storage are discussed.
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.%.
Fullerene-like CS x: A first-principles study of synthetic growth
NASA Astrophysics Data System (ADS)
Goyenola, C.; Gueorguiev, G. K.; Stafström, S.; Hultman, L.
2011-04-01
Fullerene-Like (FL) Sulpho-Carbide (CSx) compounds have been addressed by first principles calculations. Geometry optimization and cohesive energy results are presented for the relative stability of precursor species such as C2S, CS2, and C2S2 in isolated form. The energy cost for structural defects, arising from the substitution of C by S is also reported. Similar to previously synthesized FL-CNx and FL-CPx compounds, the pentagon, the double pentagon defects as well as the Stone-Wales defects are confirmed as energetically feasible in CSx compounds.