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.
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
NASA Astrophysics Data System (ADS)
Pemmaraju, Sri Chaitanya Das; Prendergast, David
2014-03-01
We present two case studies of first-principles theoretical methods applied in conjunction with experimental core-level spectroscopy measurements to investigate the electronic structure and dynamical processes in molecular and interfacial systems relevant to photoelectrochemical (PEC) technologies. In the first, we study the core-level and valence spectroscopies of two zinc(II)-porphyrin based Donor-pi-Acceptor (D-p-A) dyes using the occupancy-constrained excited electron and core-hole (XCH) approach and time-dependent density functional theory (TDDFT) simulations. In the second, we use constrained DFT and TDDFT to interpret measured transient core-level shifts in time-resolved femtosecond x-ray photoelectron spectroscopy, investigating the dynamics of the electron injection process from a N3 dye molecule chemisorbed onto a ZnO substrate. These studies illustrate the utility of first-principles methods in guiding the design of better PEC materials. This work was performed at the Molecular Foundry, LBNL, supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
Doppler effects in a left-handed material: a first-principle theoretical study
Sanshui Xiao; Min Qiu
2005-09-01
The Doppler effects for the reflected wave from a moving media are systemically analyzed in this paper. The theoretical formula for the Doppler shift in the left-handed material, which is described by Drude's dispersion model, is presented. This formula is examined by first-principles numerical experiments, which are in agreement with the theoretical results.
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
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.
Ultrafast Vibrational Echo Spectroscopy of Liquid Water from First-Principles Simulations.
Ojha, Deepak; Chandra, Amalendu
2015-08-27
Vibrational echo spectroscopy has become a powerful technique to study vibrational spectral diffusion in water and aqueous solutions. The dynamics of vibrational spectral diffusion is intimately related to the hydrogen bond fluctuations in liquid water and other hydrogen bonded liquids. Earlier theoretical calculations of vibrational echo spectroscopy of aqueous systems were based on classical molecular dynamics simulations involving empirical force fields of water. In the current work, we have employed the method of ab initio molecular dynamics simulation to calculate the spectral observables of vibrational echo and two-dimensional infrared (2D-IR) spectroscopy of liquid water at room temperature under Condon and cumulant approximations. The time scales extracted from the temporal decay of the frequency-time correlation function (FTCF), short-time slope of three pulse photon echo (SP3E), dynamic line width (DLW), and the slope of nodal line of 2D-IR spectra are found to be in reasonably close agreement with each other which reinforces the assertion that signatures of FTCF can be captured using three pulse photon echo and 2D-IR spectroscopy. PMID:26161933
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
NASA Astrophysics Data System (ADS)
Pagare, Gitanjali; Jain, Ekta; Abraham, Jisha Annie; Sanyal, S. P.
2015-08-01
The structural and elastic properties of aluminum based some intermetallics, AlX (X = Mg, Co, Rh, Pd and Ir) have been investigated using full potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). The exchange-correlation term was treated within generalized gradient approximation (GGA). Grounds state properties i.e. lattice constants (a0), bulk modulus (B) and first-order pressure derivative of bulk modulus (B') are presented. Our calculated results for C11, C12 and C44 have been compared with available previous theoretical data. The Young's modulus (G), shear modulus (E), sound wave velocities and Debye temperature are the first theoretical prediction for these compounds. Using Pugh's rule and Cauchy pressure the ductile/brittle character of these compounds are satisfied.
NASA Astrophysics Data System (ADS)
Muñoz, Francisco; Romero, Aldo H.; Mejía-López, Jose; Morán-López, J. L.
2012-03-01
A theoretical investigation of the adsorption of Mn single atoms and dimers on the (111) surface of Cu, Ag, and Au, within the framework of the density functional theory, is presented. First, the bulk and the clean (111) surface electronic structures are calculated, with results that agree well with previous reports. To understand the adatom-substrate interaction, also the electronic characteristics of the free Mn dimer are determined. Then, the electronic structure of the Mn adatom, chemisorbed on four different surface geometries, is analyzed for the three noble metals. It is found that the most stable geometry, in all three cases, Cu, Ag, and Au, occurs when the Mn atom is chemisorbed on threefold coordinated sites. For the dimer, the lowest-energy configuration corresponds to the molecule lying parallel to the surface. In the three noble metals, the geometry corresponds to both atoms chemisorbed in threefold coordinated sites, but with different local symmetry. It is also found that the magnetic configuration with the lowest energy corresponds to the antiferromagnetic arrangement of Mn atoms, with individual magnetic moments close to 5?B. The ferromagnetic and antiferromagnetic solutions, in the case of a Ag substrate, are close in energy. It is also found that in this case the Mn2 molecule is chemisorbed with very similar energy on various geometries. To study the dynamical motion of the dimer components, we calculated the potential energy barriers for the Mn motion in the various surfaces. In contrast to Cu and Au, this leads to the conclusion that on Ag the Mn dimer moves relatively freely.
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; 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
Yoon, Mina; Weitering, Harm H; Zhang, Zhenyu
2011-01-01
The search for technologically and economically viable storage solutions for hydrogen fuel would benefit greatly from research strategies that involve systematic property tuning of potential storage materials via atomic-level modification. Here, we use first-principles density-functional theory to investigate theoretically the structural and electronic properties of ultrathin Mg films and Mg-based alloy films and their interaction with atomic hydrogen. Additional delocalized charges are distributed over the Mg films upon alloying them with 11.1% of Al or Na atoms. These extra charges contribute to enhance the hydrogen binding strength to the films. We calculated the chemical potential of hydrogen in Mg films for different dopant species and film thickness, and we included the vibrational degrees of freedom. By comparing the chemical potential with that of free hydrogen gas at finite temperature (T) and pressure (P), we construct a hydrogenation phase diagram and identify the conditions for hydrogen absorption or desorption. The formation enthalpies of metal hydrides are greatly increased in thin films, and in stark contrast to its bulk phase, the hydride state can only be stabilized at high P and T (where the chemical potential of free H{sub 2} is very high). Metal doping increases the thermodynamic stabilities of the hydride films and thus significantly helps to reduce the required pressure condition for hydrogen absorption from H{sub 2} gas. In particular, with Na alloying, hydrogen can be absorbed and/or desorbed at experimentally accessible T and P conditions.
NASA Astrophysics Data System (ADS)
Pokrovski, Gleb S.; Roux, Jacques; Ferlat, Guillaume; Jonchiere, Romain; Seitsonen, Ari P.; Vuilleumier, Rodolphe; Hazemann, Jean-Louis
2013-04-01
The molecular structure and stability of species formed by silver in aqueous saline solutions typical of hydrothermal settings were quantified using in situ X-ray absorption spectroscopy (XAS) measurements, quantum-chemical modeling of near-edge absorption spectra (XANES) and extended fine structure spectra (EXAFS), and first-principles molecular dynamics (FPMD). Results show that in nitrate-bearing acidic solutions to at least 200 °C, silver speciation is dominated by the hydrated Ag+ cation surrounded by 4-6 water molecules in its nearest coordination shell with mean Ag-O distances of 2.32 ± 0.02 Å. In NaCl-bearing acidic aqueous solutions of total Cl concentration from 0.7 to 5.9 mol/kg H2O (m) at temperatures from 200 to 450 °C and pressures to 750 bar, the dominant species are the di-chloride complex AgCl2- with Ag-Cl distances of 2.40 ± 0.02 Å and Cl-Ag-Cl angle of 160 ± 10°, and the tri-chloride complex AgCl32- of a triangular structure and mean Ag-Cl distances of 2.60 ± 0.05 Å. With increasing temperature, the contribution of the tri-chloride species decreases from ˜50% of total dissolved Ag in the most concentrated solution (5.9m Cl) at 200 °C to less than 10-20% at supercritical temperatures for all investigated solutions, so that AgCl2- becomes by far the dominant Ag-bearing species at conditions typical of hydrothermal-magmatic fluids. Both di- and tri-chloride species exhibit outer-sphere interactions with the solvent as shown by the detection, using FPMD modeling, of H2O, Cl-, and Na+ at distances of 3-4 Å from the silver atom. The species fractions derived from XAS and FPMD analyses, and total AgCl(s) solubilities, measured in situ in this work from the absorption edge height of XAS spectra, are in accord with thermodynamic predictions using the stability constants of AgCl2- and AgCl32- from Akinfiev and Zotov (2001) and Zotov et al. (1995), respectively, which are based on extensive previous AgCl(s) solubility measurements. These data are thus recommended for chemical equilibrium calculations in mineral-fluid systems above 200 °C. In contrast, our data disagree with SUPCRT-based datasets for Ag-Cl species, which predict large fractions of high-order chloride species, AgCl32- and AgCl43- in high-temperature saline fluids. Comparisons of the structural and stability data of Ag-Cl species derived in this study with those of their Au and Cu analogs suggest that molecular-level differences amongst the chloride complexes such as geometry, dipole moment, distances, and resulting outer-sphere interactions with the solvent may account, at least partly, for the observed partitioning of Au, Ag and Cu in vapor-brine and fluid-melt systems. In hydrothermal environments dominated by fluid-rock interactions, the contrasting affinity of these metals for sulfur ligands and the differences both in chemistry and stability of their main solid phases (Ag sulfides, Cu-Fe sulfides, and native Au) largely control the concentration and distribution of these metals in their economic deposits.
NASA Astrophysics Data System (ADS)
Sanson, A.; Pokrovski, G. S.; Giarola, M.; Mariotto, G.
2015-01-01
The vibrational dynamics of germanium dioxide in the rutile structure has been investigated by using polarized micro-Raman scattering spectroscopy coupled with first-principles calculations. Raman spectra were carried out in backscattering geometry at room temperature from micro-crystalline samples either unoriented or oriented by means of a micromanipulator, which enabled successful detection and identification of all the Raman active modes expected on the basis of the group theory. In particular, the Eg mode, incorrectly assigned or not detected in the literature, has been definitively observed by us and unambiguously identified at 525 \\text{cm}-1 under excitation by certain laser lines, thus revealing an unusual resonance phenomenon. First-principles calculations within the framework of the density functional theory allow quantifying both wave number and intensity of the Raman vibrational spectra. The excellent agreement between calculated and experimental data corroborates the reliability of our findings.
Zheng, Zilong; Manna, Arun K; Hendrickson, Heidi P; Hammer, Morgan; Song, Chenchen; Geva, Eitan; Dunietz, Barry D
2014-12-10
We investigate the molecular structure of the solvated complex, [(NC)6Fe-Pt(NH3)4-Fe(CN)6](4-), and related dinuclear and mononuclear model complexes using first-principles calculations. Mixed nuclear complexes in both solution and crystal phases were widely studied as models for charge transfer (CT) reactions using advanced spectroscopical and electrochemical tools. In contrast to earlier interpretations, we find that the most stable gas phase and solvated geometries are substantially different from the crystal phase geometry, mainly due to variance in the underlying oxidation numbers of the metal centers. Specifically, in the crystal phase a Pt(IV) metal center resulting from Fe ? Pt backward electron transfers is stabilized by an octahedral ligand field, whereas in the solution phase a Pt(II) metal complex that prefers a square planar ligand field forms a CT salt by bridging to the iron complexes through long-range electrostatic interactions. The different geometry is shown to be consistent with spectroscopical data and measured CT rates of the solvated complex. Interestingly, we find that the experimentally indicated photoinduced process in the solvated complex is of backward CT (Fe ? Pt). PMID:25424459
NASA Astrophysics Data System (ADS)
Zhao, Yanyuan; Luo, Xin; Zhang, Jun; Wu, Junxiong; Bai, Xuxu; Wang, Meixiao; Jia, Jinfeng; Peng, Hailin; Liu, Zhongfan; Quek, Su Ying; Xiong, Qihua
2014-12-01
Layered materials, such as graphite/graphene, boron nitride, transition metal dichalcogenides, represent materials in which reduced size, dimensionality, and symmetry play critical roles in their physical properties. Here, we report on a comprehensive investigation of the phonon properties in the topological insulator Bi2Te3 and Bi2Se3 two-dimensional (2D) crystals, with the combination of Raman spectroscopy, first-principles calculations, and group theory analysis. Low frequency (<30 c m-1) interlayer vibrational modes are revealed in few-quintuple-layer (QL) B i2T e3/B i2S e3 2D crystals, which are absent in the bulk crystal as a result of different symmetries. The experimentally observed interlayer shear and breathing mode frequencies both show blueshifts, with decreasing thickness in few-QL Bi2Te3 (down to 2QL) and Bi2Se3 (down to 1QL), in agreement with first-principles calculations and a linear chain model, from which the interlayer coupling force constants can be estimated. Besides, an intense ultralow (<12 c m-1) frequency peak is observed in 2-4QL Bi2Te3 , which is tentatively attributed to a substrate-induced interface mode supported by a linear chain model analysis. The high frequency Raman peaks exhibit frequency shifts and broadening from 3D to 2D as a result of the phonon confinement effect. Our studies shed light on a general understanding of the influence of dimensionality and crystal symmetry on the phonon properties in layered materials.
Karre, Rajamallu; Niranjan, Manish K; Dey, Suhash R
2015-05-01
High alloyed ?-phase stabilized titanium alloys are known to provide comparable Young's modulus as that to the human bones (~30 GPa) but is marred by its high density. In the present study the low titanium alloyed compositions of binary Ti-Nb and ternary Ti-Nb-Zr alloy systems, having stable ?-phase with low Young's modulus are identified using first principles density functional framework. The theoretical results suggest that the addition of Nb in Ti and Zr in Ti-Nb increases the stability of the ?-phase. The ?-phase in binary Ti-Nb alloys is found to be fully stabilized from 22 at.% of Nb onwards. The calculated Young's moduli of binary ?-Ti-Nb alloy system are found to be lower than that of pure titanium (116 GPa). For Ti-25(at.%)Nb composition the calculated Young's modulus comes out to be ~80 GPa. In ternary Ti-Nb-Zr alloy system, the Young's modulus of Ti-25(at.%)Nb-6.25(at.%)Zr composition is calculated to be ~50 GPa. Furthermore, the directional Young's moduli of these two selected binary (Ti-25(at.%)Nb) and ternary alloy (Ti-25(at.%)Nb-6.25(at.%)Zr) compositions are found to be nearly isotropic in all crystallographic directions. PMID:25746245
NASA Astrophysics Data System (ADS)
Hirayama, Naomi; Iida, Tsutomu; Funashima, Hiroki; Morioka, Shunsuke; Sakamoto, Mariko; Nishio, Keishi; Kogo, Yasuo; Takanashi, Yoshifumi; Hamada, Noriaki
2015-07-01
We theoretically investigate the impurity doping effects on the structural parameters such as lattice constant, atomic positions, and site preferences of impurity dopants for Al-doped magnesium silicide (Mg2Si) crystal using the first-principles calculation methods. We present comparison between several codes: ABCAP, Quantum Espresso, and Machikaneyama2002 (Akai KKR), which are based on the full-potential linearized augmented plane-wave method, the pseudopotential method, and KKR/GGA Green’s function method, respectively. As a result, any codes used in the present study exhibit qualitative consistency both in the dependence of the lattice constants on the doping concentration and the energetic preference of the Al atom for the following sites; substitutional Si and Mg sites, and interstitial 4b site; in particular, ABCAP, which is based on the all-electron full-potential method, and Quantum Espresso, which is a code of the pseudopotential method, produce closely-resemble calculation results. We also discuss the effects of local atomic displacement owing to the presence of impurities to the structural parameters of a bulk. Using the analytical method considering the local atomic displacement, moreover, we evaluate the formation energy of Na- and B-doped systems as examples of p-type doping in order to examine the possilbility of realizing p-type Mg2Si.
NASA Astrophysics Data System (ADS)
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.
Introductory Training on Theoretical Spectroscopy
Botti, Silvana
Introductory Training on Theoretical Spectroscopy The aims of this tutorial are hence to: - give an overview over the ETSF training possibilties - give an overview of the theoretical points of view properties of finite and infinite systems. Scopes The main scope of this introductory training day
Deumal, Mercè; Bearpark, Michael J; Robb, Michael A; Pontillon, Yves; Novoa, Juan J
2004-12-01
The mechanism of magnetic interactions in the bulk ferromagnet para-(methylthio)phenyl nitronyl nitroxide crystal (YUJNEW) has been theoretically reinvestigated, using only data from ab initio calculations and avoiding any a priori assumptions. We first calculate the microscopic magnetic interactions (JAB exchange couplings) between all unique radical pairs in the crystal, and then generate the macroscopic magnetic properties from the energy levels of the corresponding Heisenberg Hamiltonian. We thus propose a first principles, bottom-up (i.e. micro-to-macro) approach that brings theory and experiment together. We have applied this strategy to study the magnetism of YUJNEW using data from the previously reported 298 and 114 K crystal structures, and also data from a 10 K neutron diffraction structure fully reported in this work. The magnetic topology at 298 K is two-dimensional: noninteracting planes, with three different in-plane JAB pair interactions (+0.24, +0.09, and -0.11 cm(-1)) and one numerically negligible (+0.02 cm(-1)) inter-plane JAB interaction. In contrast, the magnetic topology at 114 and 10 K is three-dimensional, with two non-negligible in-plane JAB constants (+0.11 and +0.07 cm(-1) at 114 K; +0.22 and +0.07 cm(-1) at 10 K) and one inter-plane pair interaction (+0.07 cm(-1) at 114 K; +0.08 cm(-1) at 10 K). Although this three-dimensional magnetic topology is consistent with YUJNEW being a bulk ferromagnet, there is only a qualitative agreement between computed and experimental magnetic susceptibility chiT(T) data at 114 K. However, the experimental chiT(T) curve is quantitatively reproduced at 10 K. The heat capacity curve presents a peak at around 0.12 K, close to the estimated experimental peak (0.20 K). PMID:15532054
Modin, A; Suzuki, M-T; Vegelius, J; Yun, Y; Shuh, D K; Werme, L; Nordgren, J; Oppeneer, P M; Butorin, S M
2015-08-12
Soft x-ray emission and absorption spectroscopic data are reported for the O 1s region of a single crystal of UO2, a polycrystalline NpO2 sample, and a single crystal of PuO2. The experimental data are interpreted using first-principles correlated-electron calculations within the framework of the density functional theory with added Coulomb U interaction (DFT+U). A detailed analysis regarding the origin of different structures in the x-ray emission and x-ray absorption spectra is given and the effect of varying the intra-atomic Coulomb interaction-U for the 5?f electrons is investigated. Our data indicate that O 1s x-ray absorption and emission spectroscopies can, in combination with DFT+U calculations, successfully be used to study 5?f?-shell Coulomb correlation effects in dioxides of light actinides. The values for the Coulomb U parameter in these dioxides are derived to be in the range of 4-5?eV. PMID:26202182
NASA Astrophysics Data System (ADS)
Modin, A.; Suzuki, M.-T.; Vegelius, J.; Yun, Y.; Shuh, D. K.; Werme, L.; Nordgren, J.; Oppeneer, P. M.; Butorin, S. M.
2015-08-01
Soft x-ray emission and absorption spectroscopic data are reported for the O 1s region of a single crystal of UO2, a polycrystalline NpO2 sample, and a single crystal of PuO2. The experimental data are interpreted using first-principles correlated-electron calculations within the framework of the density functional theory with added Coulomb U interaction (DFT+U). A detailed analysis regarding the origin of different structures in the x-ray emission and x-ray absorption spectra is given and the effect of varying the intra-atomic Coulomb interaction-U for the 5?f electrons is investigated. Our data indicate that O 1s x-ray absorption and emission spectroscopies can, in combination with DFT+U calculations, successfully be used to study 5?f?-shell Coulomb correlation effects in dioxides of light actinides. The values for the Coulomb U parameter in these dioxides are derived to be in the range of 4–5?eV.
Neouze, Marie-Alexandra; Kronstein, Martin; Litschauer, Marco; Puchberger, Michael; Coelho, Cristina; Bonhomme, Christian; Gervais, Christel; Tielens, Frederik
2014-11-10
A DFT-based molecular model for imidazolium-silica-based nanoparticle networks (INNs) is presented. The INNs were synthesized and characterized by using small-angle X-ray scattering (SAXS), NMR spectroscopy, and theoretical ab initio calculations. (11)B and (31)P HETCOR CP MAS experiments were recorded. Calculated (19)F?NMR spectroscopy results, combined with the calculated anion-imidazolium (IM) distances, predicted the IM chain density in the INN, which was also confirmed from thermogravimetric analysis/mass spectrometry results. The presence of water molecules trapped between the nanoparticles is also suggested. First considerations on possible ?-? stacking between the IM rings are presented. The predicted electronic properties confirm the photoluminescence emissions in the correct spectral domain. PMID:25241702
NASA Astrophysics Data System (ADS)
Jahn, S.; Schmidt, C.
2008-12-01
Aqueous fluids play an essential role in mass and energy transfer in the lithosphere. Their presence has also a large effect on physical properties of rocks, e.g. the electrical conductivity. Many chemical and physical properties of aqueous fluids strongly depend on the speciation, but very little is known about this fundamental parameter at high pressures and temperatures, e.g. at subduction zone conditions. Here we use a combined approach of first-principles molecular dynamics simulation and Raman spectroscopy to study the molecular structure of aqueous 2~mol/kg MgSO4 fluids up to pressures of 3~GPa and temperatures of 750~°C. MgSO4-H2O is selected as a model system for sulfate bearing subduction zone fluids. The simulations are performed using Car-Parrinello dynamics, a system size of 120 water and four MgSO4 molecules with production runs of at least 10~ps at each P and T. Raman spectra were obtained in situ using a Bassett-type hydrothermal diamond anvil cell with external heating. Both simulation and spectroscopic data show a dynamic co-existence of various associated molecular species as well as dissociated Mg2+ and SO42- in the single phase fluid. Fitting the Raman signal in the frequency range of the ?1-SO42- stretching mode yields the P-T dependence of the relative proportions of different peaks. The latter can be assigned to species based on literature data and related to the species found in the simulation. The dominant associated species found in the P-T range of interest here are Mg-SO4 ion pairs with one (monodentate) and two (bidentate) binding sites. At the highest P and T, an additional peak is identified. At low pressures and high temperature (T>230~°C), kieserite, MgSO4·H2O, nucleated in the experiment. At the same conditions the simulations show a clustering of Mg, which is interpreted as a precursor of precipitation. In conclusion, the speciation of aqueous MgSO4 fluid shows a complex behavior at high P and T that cannot be extrapolated from ambient conditions. The combination of molecular modeling and in situ spectroscopic experiments is a promising approach towards quantitative understanding of geochemical processes in subduction zones.
NASA Astrophysics Data System (ADS)
Wanbayor, Raina; Deák, Peter; Frauenheim, Thomas; Ruangpornvisuti, Vithaya
2011-03-01
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 CO2 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 CO2, 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.
The European Theoretical Spectroscopy Facility
NASA Astrophysics Data System (ADS)
Godby, Rex
2007-03-01
The ETSF (www.etsf.eu) is being created as a permanent output of the EU-funded Nanoquanta Network of Excellence (www.nanoquanta.eu, 2004-8), which joins 10 groups and over 100 researchers in research on the theory and simulation of spectroscopy of electrons in matter, and related excited-state electronic properties including quantum transport. The ETSF is intended to contribute significantly to nanoscience and nanotechnology through the development and application of theoretical spectroscopy, involving close collaboration between theorists (the existing Nanoquanta groups together with further theoretical groups) and a new community of experimental and industrial researchers who wish to apply modern theories of spectroscopy. In this talk I shall review some of the scientific output of the project so far, including the development of new ideas and techniques in many-body perturbation theory and time-dependent density-functional theory, and their application to a variety of prototype and actual systems including quantum transport in nanostructures, optical absorption in biological molecules and advanced materials, optical properties of nanoclusters and nanotubes, non-linear optical response, and spectroscopies of complex surfaces. I shall also briefly describe the network's integration activities, including code interoperability and modularity, training of internal and external researchers, and the legal, financial and organizational preparations for the ETSF.
Towards first-principles electrochemistry
Dabo, Ismaila
2008-01-01
This doctoral dissertation presents a comprehensive computational approach to describe quantum mechanical systems embedded in complex ionic media, primarily focusing on the first-principles representation of catalytic ...
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 (EB), binding site, energy gap (Eg), 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
I. Karbovnyk; S. Piskunov; I. Bolesta; S. Bellucci; M. Cestelli Guidi; M. Piccinini; E. Spohr; A. I. Popov
2009-01-01
The influence of temperature on the far-infrared (FIR)\\u000a spectrum of polycrystalline Ag2CdI4 has been investigated by\\u000a Fourier Transform Infrared (FTIR) spectroscopy measurements. The FIR\\u000a phonon spectrum has also been calculated by means of quantum\\u000a mechanical density functional theory (DFT). The comparison of\\u000a calculated and measured spectra allows the fast and reliable\\u000a assignment of the measured phonon modes, and thus clarifies
NASA Astrophysics Data System (ADS)
Persson, P. O. Å.; Rosén, J.; McKenzie, D. R.; Bilek, M. M. M.
2009-09-01
Oxygen incorporation in the Ti2AlC MAX phase and TiC was investigated in the electron microscope using spatially resolved fine-structure electron energy-loss spectroscopy analysis. Corresponding fine structures were calculated within the full-potential-linearized augmented plane-wave framework. In the calculations, oxygen was substituted for aluminum and carbon in Ti2AlC as well as for carbon in TiC, in concentrations of 3.1, 6.2, and 12.5at% . Comparison of calculated and measured spectra shows that oxygen is incorporated on the carbon site in both TiC and Ti2AlC . These findings reveal the existence of MAX phase oxycarbide MA(O,C) alloys and O as a third X element in addition to C and N.
Catalysis from First-Principles
NASA Astrophysics Data System (ADS)
Neurock, Matthew
2001-03-01
Considerable progress has been made over the past few years in advancing both theory and atomic-scale simulation toward understanding surface reactivity. First-principle quantum chemical methods, for example, can be used to predict structural conformations, energies, and even activation barriers. This enables us to elucidate and in some cases quantify the nature of the adsorbate-surface bond strength. The application of these methods to actual catalytic processing, however, is much more challenging in that a number of extrinsic features which describe the local reaction environment must be properly accounted for. These include coverage effects, defect sites, metal support interactions, solvent effect, and surface structure and composition. These extrinsic factors can alter the metal-adsorbate surface interaction and thus markedly affect the measured surface kinetics. We will describe an approach in which we integrate first-principle DFT calculated activation barriers, reaction energies, adsorption energies and lateral interactions into a site-explicit dynamic Monte Carlo algorithm in order to simulate surface catalytic kinetics. The algorithm explicitly tries to treat the effects of the local environment on the local reactivity. We will discuss the application of this method toward modeling different selective oxidation and hydrogenation chemistries over transition metal surfaces.
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
Culturing conceptions: From first principles
NASA Astrophysics Data System (ADS)
Roth, Wolff-Michael; Lee, Yew Jin; Hwang, Sungwon
2008-07-01
Over the past three decades, science educators have accumulated a vast amount of information on conceptions--variously defined as beliefs, ontologies, cognitive structures, mental models, or frameworks--that generally (at least initially) have been derived from interviews about certain topics. During the same time period, cultural studies has emerged as a field in which everyday social practices are interrogated with the objective to understand culture in all its complexity. Science educators have however yet to ask themselves what it would mean to consider the possession of conceptions as well as conceptual change from the perspective of cultural studies. The purpose of this article is thus to articulate in and through the analysis of an interview about natural phenomenon the first principles of such a cultural approach to scientific conceptions. Our bottom-up approach in fact leads us to develop the kind of analyses and theories that have become widespread in cultural studies. This promises to generate less presupposing and more parsimonious explanations of this core issue within science education than if conceptions are supposed to be structures inhabiting the human mind.
Chamber Clearing First Principles Modeling
Loosmore, G
2009-06-09
LIFE fusion is designed to generate 37.5 MJ of energy per shot, at 13.3 Hz, for a total average fusion power of 500 MW. The energy from each shot is partitioned among neutrons ({approx}78%), x-rays ({approx}12%), and ions ({approx}10%). First wall heating is dominated by x-rays and debris because the neutron mean free path is much longer than the wall thickness. Ion implantation in the first wall also causes damage such as blistering if not prevented. To moderate the peak-pulse heating, the LIFE fusion chamber is filled with a gas (such as xenon) to reduce the peak-pulse heat load. The debris ions and majority of the x-rays stop in the gas, which re-radiates this energy over a longer timescale (allowing time for heat conduction to cool the first wall sufficiently to avoid damage). After a shot, because of the x-ray and ion deposition, the chamber fill gas is hot and turbulent and contains debris ions. The debris needs to be removed. The ions increase the gas density, may cluster or form aerosols, and can interfere with the propagation of the laser beams to the target for the next shot. Moreover, the tritium and high-Z hohlraum debris needs to be recovered for reuse. Additionally, the cryogenic target needs to survive transport through the gas mixture to the chamber center. Hence, it will be necessary to clear the chamber of the hot contaminated gas mixture and refill it with a cool, clean gas between shots. The refilling process may create density gradients that could interfere with beam propagation, so the fluid dynamics must be studied carefully. This paper describes an analytic modeling effort to study the clearing and refilling process for the LIFE fusion chamber. The models used here are derived from first principles and balances of mass and energy, with the intent of providing a first estimate of clearing rates, clearing times, fractional removal of ions, equilibrated chamber temperatures, and equilibrated ion concentrations for the chamber. These can be used to scope the overall problem and provide input to further studies using fluid dynamics and other more sophisticated tools.
Infrared Absorption Spectrum of Brushite from First Principles Anna Hirsch,
Curtarolo, Stefano
Infrared Absorption Spectrum of Brushite from First Principles Anna Hirsch, Ido Azuri, Lia Addadi in humans. Several groups have investigated the experimental Fourier transform infrared vibrational spectrum the properties of minerals can be obtained from vibrational spectroscopy. Fourier transform infrared (IR
McCluskey, Matthew
Hydrogen donors in SnO2 studied by infrared spectroscopy and first-principles calculations W. M; revised manuscript received 8 October 2010; published 3 November 2010 Hydrogen is a potentially important source of n-type conductivity in oxide materials. We have investigated hydrogen in tin oxide SnO2
NASA Astrophysics Data System (ADS)
Chang, Yunhee; Kim, Howon; Lee, Eui-Sup; Jang, Won-Jun; Kim, Yong-Hyun; Kahng, Se-Jong
2015-03-01
To microscopically understand the mechanisms of electron-induced NO dissociations, we performed first-principles density-functional theory (DFT) calculations for NO-CoTPP on Au(111). We explain the scanning tunneling microscopy (STM) results that the dissociations of NO were induced by both positive and negative voltage pulses with threshold voltages, +0.68 V and 0.74 V, respectively, at 0.1 nA tunneling current, showing power law relations between tunneling current and dissociation yield. To evaluate first-principles thermodynamics of the NO dissociation, we considered not only adsorption-desorption energetics, zero-point energy, and vibrational free energy at experiment temperature from first-principles, but also the chemical potential of NO gas at the cryogenic ultra-high vacuum condition. Using first-principles thermodynamics for the NO dissociation, we argue that the dissociations are induced with inelastic electron tunneling through molecular orbital resonances.
Electronic and structural properties of ununquadium from first principles
NASA Astrophysics Data System (ADS)
Noffsinger, Jesse; Cohen, Marvin L.
2010-02-01
Using first-principles techniques, the electronic and structural properties of ununquadium are investigated. These properties are compared to the known experimental and theoretical properties of Pb which lies in the same column above Uuq in the periodic table. Within the limits of density-functional theory, we find that solid Uuq is expected to be metallic and crystallize in an fcc structure.
Site-specific electronic structure analysis by channeling EELS and first-principles calculations.
Tatsumi, Kazuyoshi; Muto, Shunsuke; Yamamoto, Yu; Ikeno, Hirokazu; Yoshioka, Satoru; Tanaka, Isao
2006-01-01
Site-specific electronic structures were investigated by electron energy loss spectroscopy (EELS) under electron channeling conditions. The Al-K and Mn-L(2,3) electron energy loss near-edge structure (ELNES) of, respectively, NiAl2O4 and Mn3O4 were measured. Deconvolution of the raw spectra with the instrumental resolution function restored the blunt and hidden fine features, which allowed us to interpret the experimental spectral features by comparing with theoretical spectra obtained by first-principles calculations. The present method successfully revealed the electronic structures specific to the differently coordinated cationic sites. PMID:16867310
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
2015-01-01
17O NMR spectroscopy combined with first-principles calculations was employed to understand the local structure and dynamics of the phosphate ions and protons in the paraelectric phase of the proton conductor CsH2PO4. For the room-temperature structure, the results confirm that one proton (H1) is localized in an asymmetric H-bond (between O1 donor and O2 acceptor oxygen atoms), whereas the H2 proton undergoes rapid exchange between two sites in a hydrogen bond with a symmetric double potential well at a rate ?107 Hz. Variable-temperature 17O NMR spectra recorded from 22 to 214 °C were interpreted by considering different models for the rotation of the phosphate anions. At least two distinct rate constants for rotations about four pseudo C3 axes of the phosphate ion were required in order to achieve good agreement with the experimental data. An activation energy of 0.21 ± 0.06 eV was observed for rotation about the P–O1 axis, with a higher activation energy of 0.50 ± 0.07 eV being obtained for rotation about the P–O2, P–O3d, and P–O3a axes, with the superscripts denoting, respectively, dynamic donor and acceptor oxygen atoms of the H-bond. The higher activation energy of the second process is most likely associated with the cost of breaking an O1–H1 bond. The activation energy of this process is slightly lower than that obtained from the 1H exchange process (0.70 ± 0.07 eV) (Kim, G.; Blanc, F.; Hu, Y.-Y.; Grey, C. P. J. Phys. Chem. C2013, 117, 6504?6515) associated with the translational motion of the protons. The relationship between proton jumps and phosphate rotation was analyzed in detail by considering uncorrelated motion, motion of individual PO4 ions and the four connected/H-bonded protons, and concerted motions of adjacent phosphate units, mediated by proton hops. We conclude that, while phosphate rotations aid proton motion, not all phosphate rotations result in proton jumps. PMID:25732257
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 simulations of nanoelectronic devices
NASA Astrophysics Data System (ADS)
Maassen, Jesse
As the miniaturization of devices begins to reveal the atomic nature of materials, where chemical bonding and quantum effects are important, one must resort to a parameter-free theory for predictions. This thesis theoretically investigates the quantum transport properties of nanoelectronic devices using atomistic first principles. Our theoretical formalism employs density functional theory (DFT) in combination with Keldysh nonequilibrium Green's functions (NEGF). Self-consistently solving the DFT Hamiltonian with the NEGF charge density provides a way to simulate nonequilibrium systems without phenomenological parameters. This state-of-the-art technique was used to study three problems related to the field of nanoelectronics. First, we investigated the role of metallic contacts (Cu, Ni and Co) on the transport characteristics of graphene devices. With Cu, the graphene is simply electron-doped (Fermi level shift of ? -0.7 eV) which creates a unique signature in the conduction profile allowing one to extract the doping level. With Ni and Co, spin-dependent band gaps are formed in graphene's linear dispersion bands, thus leading to the prediction of high spin injection efficiencies reaching 60% and 80%, respectively. Second, we studied how controlled doping distributions in nano-scale Si transistors could suppress OFF-state leakage currents. By assuming the dopants (B and P) are confined in ? 1.1 nm regions in the channel, we discovered large conductance variations (GMAX/G MIN ˜ 105) as a function of the doping location. The largest fluctuations arise when the dopants are in the vicinity of the electrodes. Our results indicate that if the dopants are located away from the leads, a distance equal to 20% of the channel length, the tunneling current can be suppressed by a factor of 2 when compared to the case of uniform doping. Thus, controlled doping engineering is found to suppress device-to-device variations and lower the undesirable leakage current. Finally, we incorporated a dephasing model into our ab initio transport formalism, which was used to study the effect of phase-breaking scattering in three different systems. Our calculations revealed the complex role of dephasing, where conduction increased or decreased depending on the system under consideration. We demonstrated that the backscattering component of this dephasing scheme also allows one to retrieve Ohm's law.
Electronic absorption spectra from first principles
NASA Astrophysics Data System (ADS)
Hazra, Anirban
Methods for simulating electronic absorption spectra of molecules from first principles (i.e., without any experimental input, using quantum mechanics) are developed and compared. The electronic excitation and photoelectron spectra of ethylene are simulated, using the EOM-CCSD method for the electronic structure calculations. The different approaches for simulating spectra are broadly of two types---Frank-Condon (FC) approaches and vibronic coupling approaches. For treating the vibrational motion, the former use the Born-Oppenheimer or single surface approximation while the latter do not. Moreover, in our FC approaches the vibrational Hamiltonian is additively separable along normal mode coordinates, while in vibronic approaches a model Hamiltonian (obtained from ab initio electronic structure theory) provides an intricate coupling between both normal modes and electronic states. A method called vertical FC is proposed, where in accord with the short-time picture of molecular spectroscopy, the approximate excited-state potential energy surface that is used to calculate the electronic spectrum is taken to reproduce the ab initio potential at the ground-state equilibrium geometry. The potential energy surface along normal modes may be treated either in the harmonic approximation or using the full one-dimensional potential. Systems with highly anharmonic potential surfaces can be treated and expensive geometry optimizations are not required, unlike the traditional FC approach. The ultraviolet spectrum of ethylene between 6.2 and 8.7 eV is simulated using vertical FC. While FC approaches for simulation are computationally very efficient, they are not accurate when the underlying approximations are unreasonable. Then, vibronic coupling model Hamiltonians are necessary. Since these Hamiltonians have an analytic form, they are used to map the potential energy surfaces and understand their topology. Spectra are obtained by numerical diagonalization of the Hamiltonians. The photoelectron spectrum corresponding to the four lowest ionized states of ethylene are simulated using various methods. For the ground ionized state, Duschinsky rotation among the symmetric normal modes is found to play an important role. The FC and vibronic models are refined to capture this effect by locating the center of the Taylor expansion of the potential surfaces used in the models to be the symmetry constrained optimized geometry of the ionized state.
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
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.
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.
First-principles transversal DNA conductance deconstructed
Zhang, Xiaoguang; Krstic, Predrag; Zikic, Radomir; Wells, Jack C; Fuentes-Cabrera, Miguel A
2006-01-01
First-principles calculation of the transverse conductance across DNA fragments placed between gold nanoelectrodes, reveals that such conductance describes electron tunneling that depends critically on geometrical rather than electronic-structure properties. By factoring the first-principles result into two simple and approximately independent tunneling factors, we show that the conductances of the A, C, G, and T fragments differ only because of their sizes: the larger is the DNA base, the smaller is the distance that separates the electrode from the corresponding molecule, and the larger is its conductance. Because the geometrical factors are difficult to control in an experiment, the DC-current measurements across DNA may not be a convenient approach to DNA sequencing.
First principle thousand atom quantum dot calculations
Wang, Lin-Wang; Li, Jingbo
2004-03-30
A charge patching method and an idealized surface passivation are used to calculate the single electronic states of IV-IV, III-V, II-VI semiconductor quantum dots up to a thousand atoms. This approach scales linearly and has a 1000 fold speed-up compared to direct first principle methods with a cost of eigen energy error of about 20 meV. The calculated quantum dot band gaps are parametrized for future references.
Theoretical study of single-molecule spectroscopy and vibrational spectroscopy in condensed phases
Yang, Shilong, 1975-
2005-01-01
In this thesis, theoretical models and computer simulations are employed to study several problems of single-molecule spectroscopy and vibrational spectroscopy in condensed phases. The first part of the thesis concentrates ...
Theoretical methods in fluorescence correlation spectroscopy
NASA Astrophysics Data System (ADS)
Torres, Tedman Anthony
2009-12-01
Fluorescence correlation spectroscopy (FCS) is a valuable tool in the study of reaction kinetics, diffusion, and photo-physics of bio-molecules. FCS utilizes correlation-function analysis of fluorescence intensity fluctuations from a small number of laser-excited molecules in a confocal optical system. The theoretical foundation of FCS was established in 1974 by Madge and Elson (ME). From this foundation, equations necessary to fit experimental correlations and extract parameters of interest are obtained. It can be shown that ME theory does not yield steady-state solutions, contradicting observed continuous intensity fluctuations from solutions in thermodynamic equilibrium. In this work, the contradiction is corrected through the application of stochastic process theory (SPT). To accomplish this, the master equation for a chemical reaction can be written; manipulations permit derivation of all equations necessary for FCS analysis and solve the contradiction. This new approach dispenses with the assumptions required in ME theory to write the reaction/diffusion equations and conditions on correlations at zero lag-time. These can be derived through SPT whereas ME methodology requires their assumption. It can be shown for non-linear reactions (at least for the types of non-linear, non-reversible reactions presented in this work) that the zero-time correlation conditions and correlation-functions from ME theory and SPT yield divergent results, converging for vanishing non-linearity. The SPT technique furnishes other possibilities not available in the ME technique. First, the SPT approach yields an efficient means of calculating spatial correlation-functions. While such functions have been derived previously, the approach in this work provides a means to obtain them in a direct and logical manner. Second, a power spectrum can be written in terms of the white noise driving the system. This allows one to easily derive the integrals for the FCS power spectrum for use in analysis of the data. Also, a method is presented to eliminate the effect of diffusion in experimental correlation-functions of conformational reaction studies by means of auto/cross-correlation ratios. This method eliminates the need to utilize a separate diffusion-characterizing sample in a second experiment. Changes in confocal volume shape between experiments are also eliminated, providing more reliable parameter extraction.
First-principles calculation of elastic moduli of early-late transition metal alloys
NASA Astrophysics Data System (ADS)
Huhn, William Paul; Widom, Michael; Cheung, Andrew M.; Shiflet, Gary J.; Poon, S. Joseph; Lewandowski, John
2014-03-01
Motivated by interest in the elastic properties of high-strength amorphous metals, we examine the elastic properties of select crystalline phases. Using first-principles methods, we calculate elastic moduli in various chemical systems containing transition metals, specifically early (Ta,W) and late (Co,Ni). Theoretically predicted alloy elastic properties are verified for Ni-Ta by comparison with experimental measurements using resonant ultrasound spectroscopy. Comparison of our computed elastic moduli with effective medium theories shows that alloying leads to enhancement of bulk moduli relative to averages of the pure elements and considerable deviation of predicted and computed shear moduli. Specifically, we find an enhancement of bulk modulus relative to effective medium theory and propose a candidate system for high-strength, ductile amorphous alloys. Trends in the elastic properties of chemical systems are analyzed using force constants, electronic densities of state, and crystal overlap Hamilton populations. We interpret our findings in terms of the electronic structure of the alloys.
Positron lifetimes in solids from first principles calculations
Sterne, P.A.; O'Brien, J.C.; Howell, R.H. (Lawrence Livermore National Lab., CA (United States)); Kaiser, J.H. (Texas Univ., Arlington, TX (United States). Dept. of Physics)
1991-08-07
We present a first principles method for calculating positron lifetimes in solids, based on self-consistent calculations using the Linear Muffin-Tin Orbital method. Local density approximations are used for both electron-electron and electron-positron interactions. Results are presented for a variety of elemental metals and vacancies to demonstrate the reliability of this approach. Theoretical calculations of positron lifetimes can be used to interpret experimental data. As an examples of this, we interpret our experimental lifetime data for the oxide superconductor Ba{sub 1-x}K{sub x}BiO{sub 3} using calculations based on this method. 12 refs., 3 figs.
Liu, Hsiang-Lin; Hsueh, Hung-Chung; Lin, I-Nan; Yang, Ming-Ti; Lee, Wei-Chung; Chen, Yi-Chun; Chia, Chia-Ta; Cheng, Hsiu-Fung
2011-06-01
La(Mg(0.5)Ti(0.5))O(3) (LMT) ceramics were prepared by either the solid-state reaction (LMT)(SS) or the citric-acid chemical method (LMT)(CA). A combination of Raman scattering, infrared reflectivity, and first-principles calculations was carried out to elucidate the correlation between lattice dynamics and the dielectric properties of these materials. Twelve Raman-active phonons are observed in both samples, displaying similar frequency positions. Interestingly, the A(g) phonon (g(11) mode) of (LMT)(SS) at about 717 cm(-1) involving the oxygen octahedron breathing vibrations demonstrates a narrower linewidth, suggesting its better crystallinity. Furthermore, an infrared-active u(2) phonon band due to the vibrations of O(I) and O(II) layers, which possesses the largest oscillator strength, exhibits stronger intensity for (LMT)(SS), as compared with those for (LMT)(CA). Additionally, the Q × f values (the product of dielectric Q values and measurement frequency) of (LMT)(SS) estimated from either microwave cavity or infrared spectroscopic measurements are larger than those of (LMT)(CA). These results indicate that the better coherence of lattice vibrations in (LMT)(SS) leads to its higher Q × f value, providing evidence for a strong connection between optical spectroscopic behavior and microwave dielectric characteristics in these materials. PMID:21576769
Origin of Spinel Nanocheckerboards via First Principles
NASA Astrophysics Data System (ADS)
Kornbluth, Mordechai; Marianetti, Chris A.
2015-06-01
Self-organizing nanocheckerboards have been experimentally fabricated in Mn-based spinels but have not yet been explained with first principles. Using density-functional theory, we explain the phase diagram of the ZnMnxGa2 -xO4 system and the origin of nanocheckerboards. We predict total phase separation at zero temperature and then show the combination of kinetics, thermodynamics, and Jahn-Teller physics that generates the system's observed behavior. We find that the {011 } surfaces are strongly preferred energetically, which mandates checkerboard ordering by purely geometrical considerations.
Boron Fullerenes: A First-Principles Study
2008-01-01
A family of unusually stable boron cages was identified and examined using first-principles local-density functional method. The structure of the fullerenes is similar to that of the B12icosahedron and consists of six crossing double-rings. The energetically most stable fullerene is made up of 180 boron atoms. A connection between the fullerene family and its precursors, boron sheets, is made. We show that the most stable boron sheets are not necessarily precursors of very stable boron cages. Our finding is a step forward in the understanding of the structure of the recently produced boron nanotubes.
Ivanov, Sergei D; Witt, Alexander; Marx, Dominik
2013-07-01
Infrared spectroscopy is a powerful technique to unravel the structure and dynamics of molecular systems of ever increasing complexity. For isolated molecules in the gas phase theoretical approaches that directly rely on solving the Schrödinger equation, either approximately or quasi-exactly, are well established. A distinctly different approach to compute infrared spectra can be based on advanced molecular dynamics, itself being based on classical Newtonian dynamics, in conjunction with concurrent first principles electronic structure calculations. At variance with traditional methods, which are formulated in terms of the Schrödinger representation of quantum mechanics, the molecular dynamics approach stems from Heisenberg's representation and thus relies on computing thermal expectation values of time-correlation functions. Crucial in addition to generating the spectra themselves is their decomposition in terms of modes, which can be assigned to correlated atomic motion. This ab initio molecular dynamics route to compute infrared spectra, and its recent extension to quasiclassical techniques relying on approximate path integral dynamics, is covered in the review part of this Perspective. The usefulness of this unconventional approach, which can be generalized beyond infrared spectroscopy, is demonstrated in detail by applying the full machinery in computing and assigning the infrared spectra of protonated methane and its isotopologues. This particular molecule is often considered to be the most prominent member of the class of floppy or fluxional molecules. CH5(+) has been a longstanding challenge for theoretical infrared spectroscopy because it undergoes intricate large-amplitude motion, which is also reviewed. Molecular dynamics based infrared spectroscopy is general and can be applied to diverse systems such as molecular complexes in the gas phase, chromophores in biomolecular environments, and solute-solvent systems in the liquid phase. PMID:23666315
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.
First principles potential for the cytosine dimer.
Manukyan, Artür; Tekin, Adem
2015-06-14
We developed a new first principles potential for the cytosine dimer. The ab initio calculations were performed with a DFT-SAPT combination of the symmetry-adapted perturbation method and density functional theory, and fitted to a model site-site functional form. The model potential was used to predict cluster structures up to cytosine hexamers. The global cluster structure optimizations showed that the new potential is able to reproduce some of the 2D filament structures. Moreover many new non-planar cytosine cluster structures were also discovered. Interaction energies of these clusters were compared with B3LYP-D, MP2, SCS-MP2, SCS-MI-MP2 and AMBER. It has been shown that the model agrees well with all ab initio methods, especially for the cytosine hexamer. The model potential outperforms the AMBER force field and therefore it can be exploited to study more challenging larger systems. PMID:25971940
Theoretical standards in x-ray spectroscopies
Not Available
1992-01-01
We propose to extend our state-of-the-art, ab initio XAFS (X-ray absorption fine structure) codes, FEFF. Our current work has been highly successful in achieving accurate, user-friendly XAFS standards, exceeding the performance of both tabulated standards and other codes by a considerable margin. We now propose to add the capability to treat more complex materials. This includes multiple-scattering, polarization dependence, an approximate treatment of XANES (x-ray absorption near edge structure), and other improvements. We also plan to adapt FEFF to other spectroscopies, e.g. photoelectron diffraction (PD) and diffraction anomalous fine structure (DAFS).
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.
First Principles Studies of Complex Oxides for Energy Applications
NASA Astrophysics Data System (ADS)
Rebola, Alejandro Federico
In this thesis, I present results and analyses of first principles calculations based on Density Functional Theory (DFT) to examine the structural, electronic and lattice properties of two complex oxide systems: the misfit-layered thermoelectric Ca3Co4O9 (CCO) and a polar interface containing the solid oxide electrolyte LaGaO3 (LGO). In Chapter 1, I present a general introduction and the motivation for the study of these systems. This is followed by a brief summary of the fundamental ideas in DFT and the different theoretical methods employed in my calculations in Chapter 2. In Chapter 3, I present results and discussion related to the electronic structure of CCO calculated by means of increasing order Fibonacci approximants. In Chapter 4, I focus on the lattice and thermal properties of this material, in particular, the calculation of the thermal conductivity by combining first principles results with the Boltzmann transport equation. In Chapter 5, I propose and analyze the computational design of a polar interface in order to increase the ionic conductivity of LGO. Finally, in Chapter 6 I present a summary and the conclusions of my studies.
Molecular electronics: insight from first-principles transport simulations.
Paulsson, Magnus; Frederiksen, Thomas; Brandbyge, Mads
2010-01-01
Conduction properties of nanoscale contacts can be studied using first-principles simulations. Such calculations give insight into details behind the conductance that is not readily available in experiments. For example, we may learn how the bonding conditions of a molecule to the electrodes affect the electronic transport. Here we describe key computational ingredients and discuss these in relation to simulations for scanning tunneling microscopy (STM) experiments with C60 molecules where the experimental geometry is well characterized. We then show how molecular dynamics simulations may be combined with transport calculations to study more irregular situations, such as the evolution of a nanoscale contact with the mechanically controllable break-junction technique. Finally we discuss calculations of inelastic electron tunnelling spectroscopy as a characterization technique that reveals information about the atomic arrangement and transport channels. PMID:21137708
First Principles Dynamics of Photoexcited DNA and RNA Bases
Hudock, Hanneli R.; Levine, Benjamin G.; Thompson, Alexis L.; Martinez, Todd J.
2007-12-26
The reaction dynamics of excited electronic states in nucleic acid bases is a key process in DNA photodamage. Recent ultrafast spectroscopy experiments have shown multi-component decays of excited uracil and thymine, tentatively assigned to nonadiabatic transitions involving multiple electronic states. Using both quantum chemistry and first principles quantum molecular dynamics methods we show that a true minimum on the bright S{sub 2} electronic state is responsible for the first step which occurs on a femtosecond timescale. Thus the observed femtosecond decay does not correspond to surface crossing as previously thought. We suggest that subsequent barrier crossing to the minimal energy S{sub 2}/S{sub 1} conical intersection is responsible for the picosecond decay.
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.
First Principle Modeling of Energetic Storm Particles
NASA Astrophysics Data System (ADS)
Shiota, Daikou; Kataoka, Ryuho; Sugiyama, Tooru; Kusano, Kanya
The origin and cause of solar energetic particles (SEPs) has been one of the most important topics in the field of space weather research. Energetic storm particle (ESP) event is a subset of SEP events, which is characterized as proton flux enhancement of the relatively low energy range (¡10 MeV) associated with interplanetary shock arrivals at the Earth. Here we perform the first principle modeling of ESP and quantitatively compare the results with in-situ observations, using hybrid plasma simulation. In this paper we focus on the coronal mass ejection event on 13 Dec 2006. As the input parameters for the simulation, fundamental shock parameters of the plasma beta, Alfven mach, and shock angle are given from a global solar wind simulation of Kataoka et al. (2009). As a result, it is found that proton flux and spectral index of ESP event can be quantitatively reproduced by the interlocked simulation, which is driven by the realistic shock parameters without injection particles. We suggest that ESPs can be originated from thermal solar wind plasma accelerated by the passage of interplanetary shocks.
First principles study of semiconductor nanostructures
NASA Astrophysics Data System (ADS)
Buda, F.; Saitta, A. M.; Fiumara, G.; Giaquinta, P. V.; Fasolino, A.
1996-03-01
The increasing interest in low-dimensional semiconductors structures is mainly motivated by the search for materials with tunable optical properties of evident technological relevance. The discovery of visible photoluminescence in porous silicon a few years ago has given momentum to this field of research. The quantum confinement effects of charge carriers in silicon nanostructures seem to be the clue to explain the optical properties of porous silicon. We study hydrogenated silicon quantum wires by first principles electronic structure calculations based on Density Functional Theory. We use the ab initio molecular dynamics (Car-Parrinello) method in order to determine the equilibrium geometry for these nanostructures. We compute the energy gap and the imaginary part of the dielectric function, which is related to the absorption coefficient, as a function of the diameter of the wire. We investigate also the possible relevance of the orientation of the wire in determining the optical and electronic properties of the porous material. Another promising direction for the search of optically active systems, obtained by self-limiting processes, is the growth of small semiconductor clusters in host materials. We show ab initio molecular dynamics results for III-V hydrogenated microclusters with the purpose of identifying stable and optically active structures suitable for inclusion into zeolite cages.
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
Lewis, Laurent J.
Electronic and Optical Properties of Si/SiO2 Superlattices from First Principles : Role ABSTRACT The observation of intense luminescence in Si/SiO2 superlattices (SLs) has lead to new theoretical research on silicon-based materials. We have performed first-principles calculations using three Si/SiO2 SL
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.
First-principles modeling of Li-air battery materials
NASA Astrophysics Data System (ADS)
Radin, Maxwell; Siegel, Donald
2011-03-01
Of the many possible battery chemistries, the so-called ``Li-air'' system is noteworthy in that its theoretical capacity (~ 5 kWh/kg, including mass of oxygen) exceeds that of any electrochemical system. Perhaps more importantly, the simplified composition of its air cathode -- involving only the inlet of oxygen from the atmosphere -- has the potential to provide cost benefits in comparison to the Li-ion systems of today. Although the first rechargeable Li-air battery was demonstrated by Abraham and Jiang 14 years ago, its performance in many dimensions remains poor, and relatively little computational work has been done to elucidate performance-limiting phenomena. This talk will introduce the basic properties and main performance issues associated with Li-air batteries. Opportunities for first-principles modeling to assist in overcoming these obstacles will be highlighted.
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 ˜20 A. Optimal double-walled boron nanotubes with inter-walled bonds formed are metallic and always more stable than single-walled ones. For Mg boride nanostructures, certain Mg boride sheets prefer to curve themselves into nanotubes, which is explained via Mg-Mg interactions governed by the charge state of Mg. In addition, optimal Mg boride sheet structures are explored with a genetic algorithm. Phase diagrams for Mg boride sheet structures are constructed and stable phases under boron-rich environments are identified. Curvature effects on the phase diagram of Mg boride nanotubes are also discussed. As a natural extension to boron sheets, layered boron crystals based on boron sheets are then presented and are shown to be stable under high pressure. Finally, this thesis ends with an investigation of hydrogen-storage properties of pristine and metal doped boron nanostructures.
First principles theory of piezoelectricity in ferroelectrics
NASA Astrophysics Data System (ADS)
Cohen, Ronald
2000-03-01
Piezoelectricity is the strain response of a material under an applied macroscopic electric field, or conversely, the development of macroscopic polarization due to macroscopic strain. The latter is the key to computing electromechanical response from first-principles. Changes in the macroscopic polarization can be computed using the Berry's phase approach [1] within standard density functional methods. There are two contributions to the piezoelectric response. The first is due to the transverse effective charges of the ions, and their displacements in response to lattice strain. The second is the clamped contribution, the change in polarization under homogeneous strain. We have used the Linearized Augmented Plane Wave (LAPW) method within the LDA and GGA to compute the piezoelectrics constants of PbTiO3 and PZT 50/50 [2]. For PbTiO3 we find good agreement with experiment, though there is a wide spread among experimental measurements. The ionic displacement contribution is large due to the large effective charges, and the homogeneous contribution is smaller and of opposite sign. For PZT we find a much lower response than observed in ceramics, suggesting either significant extrinsic contributions to the piezoelectric response, even at low temperatures, or very anisotropic piezoelectric response. A model for the new giant piezoelectric response materials [3] will be presented, based on computations of finite field response in BaTiO3 [4]. This work was done in collaboration with H. Fu, H. Krakauer, G. Saghi-Szabo. This research is supported by the Office of Naval Research. Computations were performed on the Cray SV1 at the Carnegie Institution of Washington, supported by NSF and the Keck Foundation. [1] R.D. King-Smith and D. Vanderbilt, Phys. Rev. B, 47, 1651, 1993; R. Resta, Rev. Mod. Phys., 66, 899, 1994. [2] G. Saghi-Szabo, R.E. Cohen, and H. Krakauer, Phys. Rev. Lett. 80, 4321, 1998; 59, 12771, 1999. [3] S.E. Park and T.R. Shrout, J. Appl. Phys., 82, 1804, 1997. [4] H. Fu and R.E. Cohen, in press.
First Principles Simulations of THz Spectra of Acephate: Insight Into the Phonon Signatures
Yiming Zhang; Xihong Peng; Yunqing Chen; Saroj Nayak; X.-C. Zhang
2007-01-01
Acephate is an insecticide that kills insects by disrupting nervous system functions. THz spectroscopy offers a unique tool for detecting trace amount of these materials. Using a combination of solid state first principles simulations and gas phase quantum mechanical modeling we have studied phonon spectra of acephate compound. This talk will present a detailed vibrational spectra analysis over a wide
Ethanol adsorbed on ice: A first-principles study C. Thierfelder*
Schmidt, Wolf Gero
Ethanol adsorbed on ice: A first-principles study C. Thierfelder* Centre of Theoretical Chemistry of ethanol molecules on the ice Ih basal plane. Apart from smooth surfaces, also substrate models simulation for ethanol adsorbed on ice using the TIP4P potential16 for the water-water interaction. Recent
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…
FIRST-PRINCIPLES MOLECULAR DYNAMICS Roberto Car1
Giannozzi, Paolo
1.4 FIRST-PRINCIPLES MOLECULAR DYNAMICS Roberto Car1 , Filippo de Angelis2 , Paolo Giannozzi3, Massachusetts Institute of Technology, Cambridge, MA, USA Ab initio or first-principles methods have emerged. Car et al. elements such as hydrogen; classical or ab initio path integral approaches can
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 study of O defects in CdSe
NASA Astrophysics Data System (ADS)
T-Thienprasert, J.; Limpijumnong, S.; Du, M.-H.; Singh, D. J.
2012-08-01
Recently, the vibrational signatures related to oxygen defects in oxygen-doped CdSe were measured using ultrahigh resolution Fourier transform infrared (FTIR) spectroscopy by Chen et al.(2008) [1]. They observed two absorption bands centered at ?1991.77 and 2001.3 cm-1, which they attributed to the LVMs of OCd, in the samples grown with the addition of CdO and excess Se. For the samples claimed to be grown with even more excess Se, three high-frequency modes (1094.11, 1107.45, and 1126.33) were observed and assigned to the LVMs of OSe-VCd complex. In this work, we explicitly calculated the vibrational signatures of OCd and OSe-VCd complex defects based on first principles approach. The calculated vibrational frequencies of OCd and OSe-VCd complex are inconsistent with the frequencies observed by Chen et al., indicating that their observed frequencies are from other defects. Potential defects that could explain the experimentally observed modes are suggested.
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.
Electronic and vibrational properties of nickel sulfides from first principles.
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. PMID:18067373
First Principles Modelling of Oxygen Impurities in UN Nuclear Fuels
Kotomin, Eugene Alexej; Mastrikov, Yuri A.
2008-07-15
We report results of first principles VASP supercell calculations of O impurity in UN fuels placed either at an interstitial tetrahedral position or as a substitution for a host N ion. In the latter case O perfectly fits into N site producing no lattice distortion. Such the O substitutional impurity only slightly affects the formation energies of U and N vacancies nearby. In both interstitial and substitutional positions O atom attracts the additional electron density and transforms into the negatively charged ion. Oxygen incorporation into pre-existing N vacancy is energetically more favourable than into the interstitial position. The O impurities produce an additional peak at the low energy side of N contribution to the DOS calculated for uranium mononitride which could be used for the O identification by means of the UPS spectroscopy. We compare also the DOS calculated for UN and hypothetical isostructural UO. Both O solution and incorporation energies are negative, indicating that O penetration into UN fuel is the energetically favourable. The migration energy of the interstitial O ion is estimated as 2.8 eV.
NASA Astrophysics Data System (ADS)
Bian, G.; Wang, X.; Miller, T.; Chiang, T.-C.; Kowalczyk, P. J.; Mahapatra, O.; Brown, S. A.
2014-11-01
The electronic structure of Bi(110) thin films as a function of film thickness is investigated by first-principles calculations, angle-resolved photoemission spectroscopy, and scanning tunneling microscopy. Energy minimization in the calculation reveals significant atomic relaxation and rebonding at the surface. The calculated surface energy for the relaxed structures indicates that films consisting of odd numbers of atomic layers are inherently unstable and tend to bifurcate into film domains consisting of neighboring even numbers of atomic layers. This theoretical trend agrees with experimental observations. The results can be explained by the presence of unsaturated pz dangling bonds on the surfaces of films of odd-numbered atomic layers only. These pz dangling bonds form a Dirac-cone feature near the Fermi level at the M ¯ point as a consequence of the interplay of mirror symmetry and spin-orbit coupling. Films consisting of even numbers of atomic layers exhibit a band gap at M ¯ instead.
First principles characterization of silicate sites in clay surfaces.
Alvim, Raphael S; Miranda, Caetano R
2015-02-21
Aluminosilicate clays like Montmorillonite (MMT) and Muscovite Mica (MT) have siloxane cavities on the basal plane. The hydroxyl groups localized in these cavities and van der Waals (vdW) forces contribute significantly to adsorption processes. However, the basal sites are found to be difficult to characterize experimentally. Here, (001) surfaces of MMT and MT clays were investigated using first-principles calculations to understand how these silicate surface sites are influenced by hydroxyl groups and the effective role of inner layer vdW interactions. Based on density-functional theory (DFT) within the generalized gradient approximation (GGA), different types of exchange-correlation functionals were tested to check the effect of vdW dispersion correction. Noncontact atomic force microscopy (nc-AFM), X-ray absorption spectroscopy (XAS) in the near-edge region and solid-state nuclear magnetic resonance (SS-NMR) spectroscopy were simulated. In both clays, the oxygen surface sites are directly affected by the intralayer interaction through hydroxyl groups. Our results indicated that the chemical environment of the hydroxyl groups is distinct in the MMT and MT structures. The vdW correction was essential for a better description of the surface oxygen sites and correctly describes the similarity between both clays. Particularly, the bulk apical oxygen sites in the MT structure are less influenced by vdW interaction. Compared to MMT, the silicon surface sites of MT are more sensitive to the intralayer changes in Si-Oapical-Al and with less effect of the hydroxyl groups. These results provide a clear understanding of influence of the siloxane cavity on the oxygen and silicon surface sites in aluminosilicates. PMID:25592132
NMR characterization of hydrocarbon adsorption on calcite surfaces: a first principles study.
Bevilaqua, Rochele C A; Rigo, Vagner A; Veríssimo-Alves, Marcos; Miranda, Caetano R
2014-11-28
The electronic and coordination environment of minerals surfaces, as calcite, are very difficult to characterize experimentally. This is mainly due to the fact that there are relatively few spectroscopic techniques able to detect Ca(2+). Since calcite is a major constituent of sedimentary rocks in oil reservoir, a more detailed characterization of the interaction between hydrocarbon molecules and mineral surfaces is highly desirable. Here we perform a first principles study on the adsorption of hydrocarbon molecules on calcite surface (CaCO3 (101¯4)). The simulations were based on Density Functional Theory with Solid State Nuclear Magnetic Resonance (SS-NMR) calculations. The Gauge-Including Projector Augmented Wave method was used to compute mainly SS-NMR parameters for (43)Ca, (13)C, and (17)O in calcite surface. It was possible to assign the peaks in the theoretical NMR spectra for all structures studied. Besides showing different chemical shifts for atoms located on different environments (bulk and surface) for calcite, the results also display changes on the chemical shift, mainly for Ca sites, when the hydrocarbon molecules are present. Even though the interaction of the benzene molecule with the calcite surface is weak, there is a clearly distinguishable displacement of the signal of the Ca sites over which the hydrocarbon molecule is located. A similar effect is also observed for hexane adsorption. Through NMR spectroscopy, we show that aromatic and alkane hydrocarbon molecules adsorbed on carbonate surfaces can be differentiated. PMID:25429955
NMR characterization of hydrocarbon adsorption on calcite surfaces: A first principles study
NASA Astrophysics Data System (ADS)
Bevilaqua, Rochele C. A.; Rigo, Vagner A.; Veríssimo-Alves, Marcos; Miranda, Caetano R.
2014-11-01
The electronic and coordination environment of minerals surfaces, as calcite, are very difficult to characterize experimentally. This is mainly due to the fact that there are relatively few spectroscopic techniques able to detect Ca2+. Since calcite is a major constituent of sedimentary rocks in oil reservoir, a more detailed characterization of the interaction between hydrocarbon molecules and mineral surfaces is highly desirable. Here we perform a first principles study on the adsorption of hydrocarbon molecules on calcite surface (CaCO3 ( {10bar 14} )). The simulations were based on Density Functional Theory with Solid State Nuclear Magnetic Resonance (SS-NMR) calculations. The Gauge-Including Projector Augmented Wave method was used to compute mainly SS-NMR parameters for 43Ca, 13C, and 17O in calcite surface. It was possible to assign the peaks in the theoretical NMR spectra for all structures studied. Besides showing different chemical shifts for atoms located on different environments (bulk and surface) for calcite, the results also display changes on the chemical shift, mainly for Ca sites, when the hydrocarbon molecules are present. Even though the interaction of the benzene molecule with the calcite surface is weak, there is a clearly distinguishable displacement of the signal of the Ca sites over which the hydrocarbon molecule is located. A similar effect is also observed for hexane adsorption. Through NMR spectroscopy, we show that aromatic and alkane hydrocarbon molecules adsorbed on carbonate surfaces can be differentiated.
Carbon nanotubes as gas sensors: a first-principles study
NASA Astrophysics Data System (ADS)
Picozzi, Silvia; Lozzi, Luca; Santucci, Sandro; Cantalini, Carlo; Valentini, Luca; Delley, Bernard
2003-03-01
Carbon nanotubes have recently been proposed as chemical sensors(J.Kong et al.), Science 287, 622 (2000)., due to their fast response and high sensitivity towards environmental gaseous molecules. However, the chemical and physical interactions between molecules and sensing nanotubes are not yet completely understood. Within this framework, first principles calculations within the density functional theory have been performed for simple molecules (such as NO_2, CO and O_3) adsorbed on (10,0) carbon nanotubes (CNT), using the Dmol^3 code(B. Delley, J. Chem. Phys. 113, 7756 (2000)). The effects of the different gases on the CNTs are discussed in terms of binding energies, charge transfer and density of states. Our findings indicate that NO2 and O3 molecules are charge acceptors, whereas CO is a charge donor. The CNT density of states is sensitive to the adsorption of NO2 and O_3, with a high peak close to the CNT valence band maximum, leading to an enhanced p-type conductivity. On the other hand, the CO adsorption does not alter the CNT electronic properties. Our theoretical results are in excellent consistency with experimental changes of the tube conductivity upon different gas exposure.
Semiconducting Graphene on Silicon from First-Principles Calculations.
Dang, Xuejie; Dong, Huilong; Wang, Lu; Zhao, Yanfei; Guo, Zhenyu; Hou, Tingjun; Li, Youyong; Lee, Shuit-Tong
2015-08-25
Graphene is a semimetal with zero band gap, which makes it impossible to turn electric conduction off below a certain limit. Transformation of graphene into a semiconductor has attracted wide attention. Owing to compatibility with Si technology, graphene adsorbed on a Si substrate is particularly attractive for future applications. However, to date there is little theoretical work on band gap engineering in graphene and its integration with Si technology. Employing first-principles calculations, we study the electronic properties of monolayer and bilayer graphene adsorbed on clean and hydrogen (H)-passivated Si (111)/Si (100) surfaces. Our calculation shows that the interaction between monolayer graphene and a H-passivated Si surface is weak, with the band gap remaining negligible. For bilayer graphene adsorbed onto a H-passivated Si surface, the band gap opens up to 108 meV owing to asymmetry introduction. In contrast, the interaction between graphene and a clean Si surface is strong, leading to formation of chemical bonds and a large band gap of 272 meV. Our results provide guidance for device designs based on integrating graphene with Si technology. PMID:26213346
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 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.
Leptogenesis from first principles in the resonant regime
NASA Astrophysics Data System (ADS)
Garny, Mathias; Kartavtsev, Alexander; Hohenegger, Andreas
2013-01-01
The lepton asymmetry generated by the out-of-equilibrium decays of heavy Majorana neutrinos with a quasi-degenerate mass spectrum is resonantly enhanced. In this work, we study this scenario within a first-principle approach. The quantum field theoretical treatment is applicable for mass splittings of the order of the width of the Majorana neutrinos, for which the enhancement is maximally large. The non-equilibrium evolution of the mixing Majorana neutrino fields is described by a formal analytical solution of the Kadanoff-Baym equations, that is obtained by neglecting the back-reaction. Based on this solution, we derive approximate analytical expressions for the generated asymmetry and compare them to the Boltzmann result. We find that the resonant enhancement obtained from the Kadanoff-Baym approach is smaller compared to the Boltzmann approach, due to additional contributions that describe coherent transitions between the Majorana neutrino species. We also discuss corrections to the masses and widths of the degenerate pair of Majorana neutrinos that are relevant for very small mass splitting, and compare the approximate analytical result for the lepton asymmetry with numerical results.
Ionization potentials of semiconductors from first-principles
NASA Astrophysics Data System (ADS)
Jiang, Hong; Shen, Yu-Chen
2013-10-01
The ionization potential is the key to determine the absolute positions of valence and conduction bands of a semiconductor with respect to the vacuum level, which play a crucial role in physical and chemical properties of surfaces and interfaces. In spite of its far-reaching significance, theoretical determination of ionization potentials has not attained as much attention as that of band gaps. In this work, a set of prototypical semiconductors are considered to establish the performance of the state-of-the-art first-principles approaches. We have shown that in general Kohn-Sham density functional theory with local density approximation or generalized gradient approximation (LDA/GGA) significantly underestimates the ionization potentials of semiconductors. When the quasi-particle correction from many-body perturbation theory in the GW approximation is taken into account, the agreement between theory and experiment can be greatly improved. We have made a critical comparison between two GW correction schemes, one taking into account the GW correction to the valence band maximum (VBM) of the bulk system, and the other based on the assumption that the LDA/GGA gives correct band gap center (BGC). Our study shows that the VBM scheme is better founded theoretically and leads to closer agreement with experiment practically than the BGC scheme. For semiconductors with shallow semicore states, for which the band gaps from the GW approach also exhibit significant errors, there is still significant discrepancy between GW and experiment, indicating the necessity to go beyond the standard GW approach for these materials.
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.
First-principles studies of multiferroic and magnetoelectric materials
Fang, Yue-Wen; Tong, Wen-Yi; Zhu, Wan-Jiao; Shen, Xin; Gong, Shi-Jing; Wan, Xian-Gang; Duan, Chun-Gang
2015-01-01
Multiferroics are materials where two or more ferroic orders coexist owing to the interplay between spin, charge, lattice and orbital degrees of freedom. The explosive expansion of multiferroics literature in recent years demon-strates the fast growing interest in this field. In these studies, the first-principles calculation has played a pioneer role in the experiment explanation, mechanism discovery and prediction of novel multiferroics or magnetoelectric materials. In this review, we discuss, by no means comprehensively, the extensive applications and successful achievements of first-principles approach in the study of multiferroicity, magnetoelectric effect and tunnel junc-tions. In particular, we introduce some our recently developed methods, e.g., the orbital selective external potential (OSEP) method, which prove to be powerful tools in the finding of mechanisms responsible for the intriguing phe-nomena occurred in multiferroics or magnetoelectric materials. We also summarize first-principles studies o...
Transport properties at nano scales via first principles studies
Chun Zhang
2004-01-01
There are two main difficulties for the first principles study of transport properties at the nano scale. The first is that many-body interactions need to be taken into account for the infinite system without periodic boundary conditions. The other is that the system is usually in a non-equilibrium state. Both of these two difficulties are beyond the ability of conventional
First Principles Studies of Water and Ice on Oxide Surfaces
Alavi, Ali
. First Principles Studies of Water and Ice on Oxide Surfaces Xiaoliang Hu Department of Chemistry of water (and ice) with oxide surfaces has been studied with first prin- ciples density functional theory of rocksalt (alkaline earth metal) oxide surfaces was examined with a view to better understanding
Generating Representative ISP Topologies From First-Principles
Byers, John W.
configurations, into the model. Our work synthesizes these two lines by designing a topology generation mechanism, network design, optimization. 1. INTRODUCTION Topology modeling has significant impact on other aspectsGenerating Representative ISP Topologies From First-Principles Chong Wang Computer Science
Generating Representative ISP Topologies From FirstPrinciples
Generating Representative ISP Topologies From FirstPrinciples Chong Wang Computer Science that underlie the growth and evolution of network topologies are basic ques- tions that impact capacity planning, forecasting, and pro- tocol research. Early topology generation work focused on generating network
COMMUNICATION First Principles Prediction of Protein Folding Rates
Goddard III, William A.
COMMUNICATION First Principles Prediction of Protein Folding Rates Derek A. Debe and William A studies have demonstrated that many small, single-domain proteins fold via simple two-state kinetics. We, diffusive search for the native tertiary topology. To estimate the rates of folding for various proteins via
Optical properties of porous silicon - A first-principles study
F. Buda; J. Kohanoff; M. Parrinello
1992-01-01
It is shown that first-principles electronic structure calculations of silicon wires with diameters up to 1.5 nm support the idea that quantum confinement and surface effects are responsible for the luminescence in porous silicon. Instead of the indirect gap of crystalline bulk silicon, the band structure of these wires exhibits a direct gap at k = 0. The imaginary part
Zhu, G.; Lewandowski, A.
2012-11-01
A new analytical method -- First-principle OPTical Intercept Calculation (FirstOPTIC) -- is presented here for optical evaluation of trough collectors. It employs first-principle optical treatment of collector optical error sources and derives analytical mathematical formulae to calculate the intercept factor of a trough collector. A suite of MATLAB code is developed for FirstOPTIC and validated against theoretical/numerical solutions and ray-tracing results. It is shown that FirstOPTIC can provide fast and accurate calculation of intercept factors of trough collectors. The method makes it possible to carry out fast evaluation of trough collectors for design purposes. The FirstOPTIC techniques and analysis may be naturally extended to other types of CSP technologies such as linear-Fresnel collectors and central-receiver towers.
NASA Astrophysics Data System (ADS)
Yangthaisong, A.
2013-06-01
Through first-principles pseudopotential calculations based on density functional theory, the electronic structure and lattice vibrational properties of Pbnm orthorhombic SrHfO3 were investigated in the framework of standard functional approximation and density functional perturbation theory, respectively. The calculated equilibrium lattice constants of Pbnm orthorhombic SrHfO3 are in good agreement with available experimental and theoretical results. The results show that Pbnm orthorhombic SrHfO3 is an insulator with a direct band gap of 3.9 eV and 4.0 eV within the calculations using local density approximation (LDA) and generalized gradient approximation (GGA), respectively. Use of the screened exchange local density approximation (sX-LDA) as a functional in a successive band calculation has also been performed. The band gap is predicted to be 6.7 eV within sX-LDA, somewhat higher than the gap values of 6.1 ± 0.1 eV and 6.5 eV obtained from recent x-ray photoelectron spectroscopy. The phonon dispersion curves of Pbnm orthorhombic SrHfO3 were also calculated. All-positive phonon frequencies were observed in the whole Brillouin zone, indicating stability of the Pbnm orthorhombic SrHfO3 structure. In addition, the infrared-active and Raman-active vibrational modes of SrHfO3 were calculated and compared with available theoretical and experimental investigations.
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.
Using NMR to Validate First-Principles Granular Flow Equations
D. Candela; C. Huan; K. Facto; R. Wang; R. W. Mair; R. L. Walsworth
2005-10-23
Nuclear magnetic resonance (NMR) experiments are described for two granular-flow systems, the vibrofluidized bed and the gas-fluidized bed. Using pulsed field gradient, magnetic resonance imaging, and hyperpolarized gas NMR, detailed information is obtained for the density and motions of both grains and interstitial gas. For the vibrofluidized bed, the granular temperature profile is measured and compared with a first-principles formulation of granular hydrodynamics. For the gas-fluidized bed, dynamic correlations in the grain density are used to measure the bubble velocity and hyperpolarized xenon gas NMR is used to measure the bubble-emulsion exchange rate. A goal of these measurements is to verify in earth gravity first-principles theories of granular flows, which then can be used to make concrete predictions for granular flows in reduced gravity.
First-principles study of complex material systems
NASA Astrophysics Data System (ADS)
He, Lixin
This thesis covers several topics concerning the study of complex materials systems by first-principles methods. It contains four chapters. A brief, introductory motivation of this work will be given in Chapter 1. In Chapter 2, I will give a short overview of the first-principles methods, including density-functional theory (DFT), planewave pseudopotential methods, and the Berry-phase theory of polarization in crystallines insulators. I then discuss in detail the locality and exponential decay properties of Wannier functions and of related quantities such as the density matrix, and their application in linear-scaling algorithms. In Chapter 3, I investigate the interaction of oxygen vacancies and 180° domain walls in tetragonal PbTiO3 using first-principles methods. Our calculations indicate that the oxygen vacancies have a lower formation energy in the domain wall than in the bulk, thereby confirming the tendency of these defects to migrate to, and pin, the domain walls. The pinning energies are reported for each of the three possible orientations of the original Ti--O--Ti bonds, and attempts to model the results with simple continuum models are discussed. CaCu3Ti4O12 (CCTO) has attracted a lot of attention recently because it was found to have an enormous dielectric response over a very wide temperature range. In Chapter 4, I study the electronic and lattice structure, and the lattice dynamical properties, of this system. Our first-principles calculations together with experimental results point towards an extrinsic mechanism as the origin of the unusual dielectric response.
First-principles approach to lattice-mediated magnetoelectric effects.
Iñiguez, Jorge
2008-09-12
We present a first-principles scheme for the computation of the linear magnetoelectric response of magnetic insulators. We focus on the lattice-mediated part of the response, which we argue can be expected to be dominant in materials displaying strong magnetoelectric couplings. We apply our method to Cr2O3 and show that its low-temperature response has a significant lattice character. PMID:18851322
Temperature-dependent magnetoelectric effect from first principles.
Mostovoy, Maxim; Scaramucci, Andrea; Spaldin, Nicola A; Delaney, Kris T
2010-08-20
We show that nonrelativistic exchange interactions and spin fluctuations can give rise to a linear magnetoelectric effect in collinear antiferromagnets at elevated temperatures that can exceed relativistic magnetoelectric responses by more than 1 order of magnitude. We show how symmetry arguments, ab initio methods, and Monte Carlo simulations can be combined to calculate temperature-dependent magnetoelectric susceptibilities entirely from first principles. The application of our method to Cr2O3 gives quantitative agreement with experiment. PMID:20868128
Holzwarth, Natalie
Modeling the interface of Li metal and Li solid electrolytes from first principles Nicholas Lepley battery electrolytes. Simplified theoretical models often fail to agree with experimental observations of the stability of electrode electrolyte interfaces. An example of this disagreement is the thiophosphate
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.
Mechanical and Vibrational Properties of ZnS with Wurtzite Structure: A First-Principles Study
NASA Astrophysics Data System (ADS)
Yu, You; Chen, Chun-Lin; Zhao, Guo-Dong; Zheng, Xiao-Lin; Zhu, Xing-Hua
2014-10-01
We perform a first-principles study of the mechanical and vibrational properties of ZnS with a wurtzite structure. The calculated elastic constants by using a pseudopotential plane-wave method agree well with the experimental data and with the previous theoretical works. Based on the elastic constants and their related parameters, the crystal mechanical stability is discussed. Calculations of the zone-center optical-mode frequencies including longitudinal-optical/transverse-optical splitting, by using the density functional perturbation theory, are reported. All optical modes are identified, especially B1 modes, and agree with Raman measurements.
NASA Astrophysics Data System (ADS)
Barras, Faye; Schneider, Guenter
2011-10-01
Electronic band structures based on first principles density functional theory are reported for functionalized anthradithiophene (ADT) derivatives, such as ADT-TES-F (donor) and ADT-TIPS-CN (acceptor).The addition of side groups such as triethylsilylethynyl (TES) to the ADT backbone induces a change in the morphology from a herringbone to a planar crystal structure in which improved intermolecular ?-orbital overlap increases carrier mobility. A comparison of the band gaps and effective masses deduced from theoretical calculations may indicate which side groups promote increased optical response and electrical conduction. We compare our results to available experimental results such as photoluminescence and photocurrent measurements.
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
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.
Computation of Mössbauer isomer shifts from first principles.
Zwanziger, J W
2009-05-13
Computation of the observables of a Mössbauer spectrum, primarily the isomer shift, from a first-principles approach is described. The framework used is density functional theory using the projector augmented wave formalism (DFT PAW), which enables efficient computation even of many-electron solids such as SnCl(2). The proper PAW version of the isomer shift is derived and shown to be correct through comparison of computed shifts and experiment in a variety of compounds based on tin, germanium and zinc. The effects of pressure are considered as well as motional effects including the Lamb-Mössbauer factor and the second-order Doppler shift. PMID:21825488
First principle study of unzipped boron nitride nanotubes
NASA Astrophysics Data System (ADS)
Azadi, Sam; Moradian, Rostam
2010-01-01
Systematic first principle calculations have been used to explain the dangling bonds behaviour in the rolling up of a boron nitride nanoribbon (BNNR) to construct a single-walled boron nitride nanotube (BNNT). We found in armchair BNNR two degenerate dangling bonds split and move up to higher energies due to symmetry breaking of system. While in zigzag BNNR changing the topology of system does not affect on metallic features of the band structure, but in unzipped BNNT case a metallic-semimetallic phase transition occurs. Considering the width dependent electronic properties of hydrogen passivated armchair BNNRs, exhibit zigzag behaviour of energy gap in agreement with previous results.
First-Principles Study of Titanium Dioxide: Rutile and Anatase
NASA Astrophysics Data System (ADS)
Mikami, Masayoshi; Nakamura, Shinichiro; Kitao, Osamu; Arakawa, Hironori; Gonze, Xavier
2000-08-01
The atomic and electronic structures of two phases of titanium dioxide, anatase and rutile, have been investigated by a first-principles pseudopotential method based on local density approximation in density functional theory. The calculated band structure, equilibrium lattice constants, and bulk modulus of rutile are consistent with experimental data and with other calculations. The calculated structure of anatase is also close to experimental data. The calculated bulk modulus of anatase is found to be smaller than that of rutile, presumably due to the sparsity of anatase. The band structure of anatase is given in comparison with that found in previous works. The energetics between the two phases is also discussed.
First-principles homogenization theory for periodic metamaterials
NASA Astrophysics Data System (ADS)
Alù, Andrea
2011-08-01
We derive from first principles an accurate homogenized description of periodic metamaterials made of magnetodielectric inclusions, highlighting and overcoming relevant limitations of standard homogenization methods. We obtain closed-form expressions for the effective constitutive parameters, pointing out the relevance of inherent spatial dispersion effects, present even in the long-wavelength limit. Our results clarify the limitations of quasistatic homogenization models, restore the physical meaning of homogenized metamaterial parameters, and outline the reasons behind magnetoelectric coupling effects that may arise also in the case of centersymmetric inclusions.
First-principles study of lithium intercalated bilayer graphene
NASA Astrophysics Data System (ADS)
Zhou, JingJing; Zhou, WeiWei; Guan, ChunMei; Shen, JingQin; Ouyang, ChuYing; Lei, MinSheng; Shi, SiQi; Tang, WeiHua
2012-08-01
Lithium intercalated bilayer graphene has been investigated using first-principles density functional theory calculations. Results show that there exist AB and AA stacking sequences for bilayer graphene in which the latter is more favorable for the Li storage and the former will evolve into the latter with the intercalation of Li ions. The relationship between the interlayer distance of two graphene sheets and the intercalated capacity of Li ions is discussed. It is found that structural defect is identified to store Li ions more favorably than pristine bilayer graphene and an isolated C atom vacancy in bilayer graphene can capture three Li ions between two graphene sheets.
Spin crossover in ferropericlase from first-principles molecular dynamics.
Holmström, E; Stixrude, L
2015-03-20
Ferropericlase, (Mg,Fe)O, is the second-most abundant mineral of Earth's lower mantle. With increasing pressure, the Fe ions in the material begin to collapse from a magnetic to nonmagnetic spin state. We present a finite-temperature first-principles phase diagram of this spin crossover, finding a broad pressure range with coexisting magnetic and nonmagnetic ions due to favorable enthalpy of mixing of the two. Furthermore, we find the electrical conductivity of the mineral to reach semimetallic values inside Earth. PMID:25839305
Comparative study of Ti and Ni clusters from first principles
Lee, B; Lee, G W
2007-08-20
Icosahedral clusters in Ti and Ni are studied with first-principles density functional calculations. We find significant distortion on the Ti icosahedron caused by the strong interaction between surface atoms on the icosahedron but not between the center atom and surface atoms, whereas no such distortion is observed on Ni clusters. In addition, distortion becomes more severe when atoms are added to the Ti13 cluster resulting in short bonds. Such distorted icosahedra having short bonds are essentially to explain the structure factor of Ti liquid obtained in experiment.
First-principles studies of low tolerance factor perovskites
NASA Astrophysics Data System (ADS)
Kang, Sung Gu; Fennie, Craig J.
2014-03-01
Most perovskites form in the non-polar Pnma structure, however, materials found in the polar subgroup of this structure, e.g., space group Pna21, are rare. Here we study from first principles the structural and vibrational properties of twelve materials that span a wide range of tolerance factors (MgSnO3, ZnSnO3, MgTiO3, ZnTiO3, MgGeO3, ZnGeO3, CdSnO3, CaSnO3, CdTiO3, CaTiO3, CdGeO3, and CaGeO3) . We illustrate how low tolerance factor materials that have been artificially constrained to the Pnma structure do in fact display ferroelectric instabilities. Insight is gained by further studying the energetics for each material in the ilmenite, lithium niobate, and perovskite structures over a wide pressure range. Our first-principles results are shown to correlate with physical descriptors, such as tolerance factor, ionic radii, and electronegativity. The rationalized rules from our data analysis will guide to design the new ferroelectric/functional materials.
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.
Quantitative relaxation dynamics of supercooled liquids from first principles
NASA Astrophysics Data System (ADS)
Janssen, Liesbeth; Mayer, Peter; Reichman, David
2015-03-01
Understanding the liquid-to-glass transition remains one of the deepest unsolved problems in condensed matter science. Here we present a novel first-principles framework, referred to as generalized mode-coupling theory (GMCT), which can predict the microscopic dynamics of glass-forming systems with near-quantitative accuracy using only simple static information as input. The theory is based on the well-established standard mode-coupling theory (MCT) of the glass transition, but rigorously incorporates higher-order dynamic density correlations neglected in standard MCT. We demonstrate that GMCT can accurately describe the dynamics of quasi-hard spheres over an unprecedentedly large time and density domain, supporting the view that activated glassy behavior is inherently dynamic in origin. Finally, our schematic results show that GMCT is capable of predicting novel types of glass transitions, including type-A, type-B, and avoided transitions, as well as different types of relaxation-time scaling behaviors. This suggests that GMCT may constitute the first microscopic, first-principles theory that can account for different fragilities in glass-forming materials.
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.
Electronic Stopping Power in LiF from First Principles
Pruneda, J. M. [Instituto de Ciencia de Materiales de Barcelona (ICMAB-CSIC) Campus de Bellaterra, 08193 Barcelona (Spain); Department of Physics, University of California, Berkeley, California 94720 (United States); Sanchez-Portal, D. [Donostia International Physics Center, Paseo Manuel de Lardizabal 4, 20018 San Sebastian (Spain); Centro de Fisica de Materiales, Centro Mixto CSIC-UPV/EHU, Apartado 1072, 20080 San Sebastian (Spain); Arnau, A.; Juaristi, J. I. [Donostia International Physics Center, Paseo Manuel de Lardizabal 4, 20018 San Sebastian (Spain); Centro Mixto CSIC-UPV/EHU, Facultad de Quimica, Apartado 1072, 20080 San Sebastian (Spain); Departamento de Fisica de Materiales, Facultad de Quimica, Apartado 1072, 20080 San Sebastian (Spain); Artacho, Emilio [Donostia International Physics Center, Paseo Manuel de Lardizabal 4, 20018 San Sebastian (Spain); Department of Earth Sciences, Downing Street, University of Cambridge, Cambridge CB2 3EQ (United Kingdom)
2007-12-07
Using time-dependent density-functional theory we calculate from first principles the rate of energy transfer from a moving proton or antiproton to the electrons of an insulating material, LiF. The behavior of the electronic stopping power versus projectile velocity displays an effective threshold velocity of {approx}0.2 a.u. for the proton, consistent with recent experimental observations, and also for the antiproton. The calculated proton/antiproton stopping-power ratio is {approx}2.4 at velocities slightly above the threshold (v{approx}0.4 a.u.), as compared to the experimental value of 2.1. The projectile energy loss mechanism is observed to be extremely local.
First-principles simulations of electrostatic interactions between dust grains
NASA Astrophysics Data System (ADS)
Itou, H.; Amano, T.; Hoshino, M.
2014-12-01
We investigated the electrostatic interaction between two identical dust grains of an infinite mass immersed in homogeneous plasma by employing first-principles N-body simulations combined with the Ewald method. We specifically tested the possibility of an attractive force due to overlapping Debye spheres (ODSs), as was suggested by Resendes et al. [Phys. Lett. A 239, 181-186 (1998)]. Our simulation results demonstrate that the electrostatic interaction is repulsive and even stronger than the standard Yukawa potential. We showed that the measured electric field acting on the grain is highly consistent with a model electrostatic potential around a single isolated grain that takes into account a correction due to the orbital motion limited theory. Our result is qualitatively consistent with the counterargument suggested by Markes and Williams [Phys. Lett. A 278, 152-158 (2000)], indicating the absence of the ODS attractive force.
First-principles prediction of disordering tendencies in pyrochlore oxides
NASA Astrophysics Data System (ADS)
Jiang, Chao; Stanek, C. R.; Sickafus, K. E.; Uberuaga, B. P.
2009-03-01
Using first-principles calculations, we systematically predict the order-disorder energetics of series of zirconate (A2Zr2O7) , hafnate (A2Hf2O7) , titanate (A2Ti2O7) , and stannate (A2Sn2O7) pyrochlores. The disordered defect-fluorite structure is modeled using an 88-atom two-sublattice special quasirandom structure (SQS) that closely reproduces the most relevant near-neighbor intrasublattice and intersublattice pair-correlation functions of the random mixture. The order-disorder transition temperatures of these pyrochlores estimated from our SQS calculations show overall good agreement with existing experiments. We confirm previous studies suggesting that the bonding in pyrochlores is not purely ionic and thus electronic effects also play a role in determining their disordering tendencies. Our results have important consequences for numerous applications, including nuclear waste forms and fast ion conductors.
Simulated doping of Si from first principles using pseudoatoms
NASA Astrophysics Data System (ADS)
Sinai, Ofer; Kronik, Leeor
2013-06-01
Semiconductor doping is a process of fundamental importance to semiconductor physics and solid-state electronics, but cannot be explicitly simulated from first principles due to the huge system size needed for most doping scenarios. We examine the efficacy of the simulation of doping in silicon by the inclusion of “pseudoatoms” with fractional nuclear charge, introduced via specially constructed pseudopotentials. These provide a net charge carrier concentration matching an arbitrarily chosen doping level, at no increase of the computational cost. By extending this approach to consider minute deviations from the integer charge, we demonstrate that the electron Fermi level can be set to any value within the forbidden gap, at minimal perturbation of the electronic structure. Beyond the bulk scenario, we successfully simulate the development of the space-charge region in a heavily doped p-n junction and the doping dependence of the work function of the hydrogen-passivated (semiconducting) Si(111) surface.
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 ?{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.
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.
Auger recombination in sodium-iodide scintillators from first principles
NASA Astrophysics Data System (ADS)
McAllister, Andrew; Åberg, Daniel; Schleife, André; Kioupakis, Emmanouil
2015-04-01
Scintillator radiation detectors suffer from low energy resolution that has been attributed to non-linear light yield response to the energy of the incident gamma rays. Auger recombination is a key non-radiative recombination channel that scales with the third power of the excitation density and may play a role in the non-proportionality problem of scintillators. In this work, we study direct and phonon-assisted Auger recombination in NaI using first-principles calculations. Our results show that phonon-assisted Auger recombination, mediated primarily by short-range phonon scattering, dominates at room temperature. We discuss our findings in light of the much larger values obtained by numerical fits to z-scan experiments.
First-principles Calculation of Polarons in Poly-thiophene
NASA Astrophysics Data System (ADS)
Brocks, Geert; Tol, Arie
1996-03-01
First principles calculations within the local density functional formalism are used to characterize charged states of poly- and oligo-thiophenes. We study polaron formation in a series of thiophene (th) oligomers (th)_n, n=4,..,16. The relaxation of the geometry upon removing electrons is calculated using the Car-Parrinello scheme of simultaneous electronic and geometrical optimization. For the smaller oligomers, removing electrons leads to strong geometry distortions. Such distortions become increasingly smaller and more delocalized the longer the oligomer and there is no sign of polaron self-localization even for the longest oligomer ? 60 Å . Intra-chain disorder in the polymer is necesarry to create substantial polaronic effects, even in absence of inter-chain interactions. In all cases we find that the bipolaron is unstable with respect to two separate polarons.
Vibrational and Thermophysical Properties of PETN from First Principles
NASA Astrophysics Data System (ADS)
Gonzalez, Joseph; Landerville, Aaron; Oleynik, Ivan
2015-06-01
Thermophysical properties are urgently sought as input for meso- and continuum-scale modeling of energetic materials (EMs). However, empirical data in this regard are often limited to specific pressures and temperatures. Such modeling of EMs can be greatly improved by inclusion of thermophysical properties over a wide range of pressures and temperatures, provided such data could be reliably obtained from theory. We demonstrate such a capability by calculating the equation of state, heat capacities, coefficients of thermal expansion, and Gruneissen parameters for pentaerythritol tetranitrate (PETN) using first-principles density functional theory, which includes proper description of van der Waals interactions and zero-point and thermal free energy contributions to pressure, the latter being calculated using the quasi-harmonic approximation. Further, we investigate the evolution of the vibration spectrum of PETN as a function of pressure.
Hydrogen storage in LiH: A first principle study
NASA Astrophysics Data System (ADS)
Banger, Suman; Nayak, Vikas; Verma, U. P.
2014-04-01
First principles calculations have been performed on the Lithium hydride (LiH) using the full potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory. We have extended our calculations for LiH+2H and LiH+6H in NaCl structure. The structural stability of three compounds have been studied. It is found that LiH with 6 added Hydrogen atoms is most stable. The obtained results for LiH are in good agreement with reported experimental data. Electronic structures of three compounds are also studied. Out of three the energy band gap in LiH is ˜3.0 eV and LiH+2H and LiH+6H are metallic.
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.
Thermodynamics of Magnetic Systems from First Principles: WL-LSMS
Eisenbach, Markus [ORNL; Zhou, Chenggang [ORNL; Nicholson, Don M [ORNL; Brown, Greg [ORNL; Larkin, Jeffrey M [ORNL; Schulthess, Thomas C [ORNL
2010-01-01
Density Functional calculations have proven to be a powerful tool to study the ground state of many materials. For finite temperatures the situation is less ideal and one is often forced to rely on models with parameters either fitted to zero temperature first principles calculations or experimental results. This approach is especially unsatisfacory in inhomogeneous systems, nano particles, or other systems where the model parameters could vary significantly from one site to another. Here we describe a possible solution to this problem by combining classical Monte Carlo calculations the Wang-Landau method in this case with a firs principles electronic structure calculation, specifically our locally selfconsistent multiple scallering code (LSMS). The combined code shows superb scaling behavior on massively parallel computers. The code sustained 1.836 Petaflop/s on 223232 cores of the Cray XT5 jaguar system at Oak Ridge.
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.
Hydrogen storage in LiH: A first principle study
Banger, Suman, E-mail: sumanphy28@gmail.com; Nayak, Vikas, E-mail: sumanphy28@gmail.com; Verma, U. P., E-mail: sumanphy28@gmail.com [School of Studies in Physics, Jiwaji University, Gwalior-474011 (India)
2014-04-24
First principles calculations have been performed on the Lithium hydride (LiH) using the full potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory. We have extended our calculations for LiH+2H and LiH+6H in NaCl structure. The structural stability of three compounds have been studied. It is found that LiH with 6 added Hydrogen atoms is most stable. The obtained results for LiH are in good agreement with reported experimental data. Electronic structures of three compounds are also studied. Out of three the energy band gap in LiH is ?3.0 eV and LiH+2H and LiH+6H are metallic.
Integration (Multi-Variable Included) From First Principles
NSDL National Science Digital Library
Spunde, Walter
Integration has traditionally been so closely linked to the interpretation as an area and to the techniques of anti-differentiation as to appear inseparable from them. While largely a consequence of the fact that, in pre-Personal-Computer times, anti-differentiation was the key to effective integration and that line and surface integrals were generally intractable using that technique, the advent of computer algebra systems and easy large scale numerical computation seems not to have had much effect on the way integration is presented in standard texts. On the one hand, it is not clear what sorts of skills are required for a novice to handle computer algebra effectively and on the other hand most available software does not provide the data structures and tools for dealing conveniently with numerical integration from first principles in the general case. We focus on the latter. In this article we examine an approach to the principles of integration based on computer manipulation of multi-dimensional arrays for the coordinate grids, referring to the area interpretation as only one among several possibilities and presenting integration as the solution to non-trivial anti-differentiation problems. Underlying the implementation of the approach is the mathematical notation of Iversons J, an array-processing, functional, computer language. We suggest that the mathematical foundations of the topic existence of, and convergence to, the limit should be postponed till after students can effectively compute and manipulate the approximations that are used to define integrals from first principles. Target Audience: 2-4 Year College Faculty/Administrators, Engineers
Theoretical analysis of the photoassociative spectroscopy of ultracold strontium atoms
Philippe Pellegrini; Thomas Killian
2005-01-01
Photoassociation (PA) of two colliding ultracold atoms in their ground state occurs when a laser field excites resonantly a ro-vibrational bound molecular level of an excited electronic state. By driving the frequency of the PA laser, high-precision spectroscopy of excited molecular states at large internuclear distances can be performed. The accuracy of the PA spectra allows a precise determination of
Gavrilenko, A V; Bonner, C E; Sun, S -S; Zhang, C; Gavrilenko, V I
2008-01-01
Optical absorption spectra of poly(thiophene vinylene) (PTV) conjugated polymers have been studied at room temperature in the spectral range of 450 to 800 nm. A dominant peak located at 577 nm and a prominent shoulder at 619 nm are observed. Another shoulder located at 685 nm is observed at high concentration and after additional treatment (heat, sonification) only. Equilibrium atomic geometries and optical absorption of PTV conjugated polymers have also been studied by first principles density functional theory (DFT). For PTV in solvent, the theoretical calculations predict two equilibrium geometries with different interchain distances. By comparative analysis of the experimental and theoretical data, it is demonstrated that the new measured long-wavelength optical absorption shoulder is consistent with new optical absorption peak predicted for most energetically favorable PTV phase in the solvent. This shoulder is interpreted as a direct indication of increased interchain interaction in the solvent which ha...
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).
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 codes for computational crystallography in the Quantum-ESPRESSO package
Giannozzi, Paolo
; Laasonen et al., 1993; Giannozzi, de Angelis, Car, 2004); FPMD (First-Principles Molecular DynamicsFirst-principles codes for computational crystallography in the Quantum-ESPRESSO package Sandro of codes for first-principles electronic structure calculations have been merged into a single multi
Electronic stopping of slow H and He atoms in gold from first principles
NASA Astrophysics Data System (ADS)
Ahsan Zeb, M.; Kohanoff, Jorge; Sanchez-Portal, Daniel; Arnau, Andres; Juaristi, J. I.; Artacho, Emilio
2012-02-01
In spite of a long history, the quantitative understanding of non-adiabatic processes in condensed matter and our ability to perform predictive theoretical simulations of processes coupling many adiabatic energy surfaces is very much behind what accomplished for adiabatic situations, for which first-principles calculations provide predictions of varied properties within a few percent accuracy. We will present here high-accuracy results for the electronic stopping power of H and He moving through gold, using time-evolving density-functional theory, thereby conveying usual first-principles accuracies to strongly coupled, continuum non-adiabatic processes in condensed matter. The two key unexplained features of what observed experimentally have been reproduced and understood: (i) The non-linear behavior of stopping power versus velocity is a gradual crossover as excitations tail into the d-electron spectrum; and (ii) the higher stopping for He than for H at low velocities is explained by the substantial involvement of the d electrons in the screening of the projectile even at the lowest velocities where the energy loss is generated by s-like electron-hole pair formation only.
Phase transition of Sr on Si (0 0 1): First principles prediction and experiment
NASA Astrophysics Data System (ADS)
Garrity, Kevin F.; Padmore, Myrtle-Rose; Segal, Yaron; Reiner, J. W.; Walker, F. J.; Ahn, C. H.; Ismail-Beigi, S.
2010-05-01
The ability to understand and predict the phase diagrams of surface phases from first principles can be valuable for developing processes for growth of epitaxial structures. In the growth of epitaxial oxides on Si (0 0 1), a submonolayer phase of Sr plays a key role. The physical structure for this phase, which has 2 × 3 symmetry and occurs at 1/6 monolayer Sr coverage, was recently elucidated using both first principles theory and diffraction experiments [J.W. Reiner, K.F. Garrity, F.J. Walker, S. Ismail-Beigi, C.H. Ahn, Role of strontium in oxide epitaxy on silicon (0 0 1), Phys. Rev. Lett. 101 (10) (2008) 105503.]. Our approach to understanding the broader Sr/Si phase diagram combines density functional theory with a thermodynamic analysis of the phase equilibrium between a Sr lattice gas and the 2 × 3 structure. We use reflection high energy electron diffraction (RHEED) to experimentally determine the phase diagram, finding good agreement with theoretical predictions.
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).
First-Principles Study of the (100) Interfacial Boundary in Ni-Ni_3Al
NASA Astrophysics Data System (ADS)
Woodward, Christopher; van de Walle, Axel; Asta, Mark
2004-03-01
Alloy designers are increasingly looking towards micro-structural evolution models to determine the processing regime that will produce the best materials properties for a given component. Micro-structural evolution models (e.g. Precipicalc and the Phase Field model) use a variety of materials parameters to produce a reliable assessment of growth and coarsening for a specific material system. For example, the width of the (100) Ni/Ni_3Al interfacial boundary is a fundamental scaling parameter in the Phase Field model: it determines the number of sampling points needed to simulate a volume of material. Also, some of these parameters have been difficult to determine experimentally. Experimental estimates for the Ni-Ni3Al interfacial energy range from 0.9 to 80 mJ/m^2. We have used first-principles methods to investigate the magnitude and possible range in energy and structure of this planar defect. The free energies and composition profiles of these defects have been determined using a cluster expansion and Monte-Carlo calculations. This work is based on recent extensions to the Alloy Theoretic Automated Toolkit (ATAT).ATAT incorporates a suite of well-established computational methods that enable calculations of thermodynamic properties from first-principles.
NASA Astrophysics Data System (ADS)
Atta-Fynn, Raymond; Biswas, Parthapratim
2009-07-01
Localized basis ab initio molecular dynamics simulation within the density functional framework has been used to generate realistic configurations of amorphous silicon carbide (a-SiC). Our approach consists of constructing a set of smart initial configurations that conform to essential geometrical and structural aspects of the materials obtained from experimental data, which is subsequently driven via a first-principles force field to obtain the best solution in a reduced solution space. A combination of a priori information (primarily structural and topological) along with the ab initio optimization of the total energy makes it possible to model a large system size (1000 atoms) without compromising the quantum mechanical accuracy of the force field to describe the complex bonding chemistry of Si and C. The structural, electronic and vibrational properties of the models have been studied and compared to existing theoretical models and available data from experiments. We demonstrate that the approach is capable of producing large, realistic configurations of a-SiC from first-principles simulation that display its excellent structural and electronic properties. Our study reveals the presence of predominant short range order in the material originating from heteronuclear Si-C bonds with a coordination defect concentration as small as 5% and a chemical disorder parameter of about 8%.
Niu, J. G.; Zhan, Q.; Geng, W. T.
2014-06-15
Despite well documented first-principles theoretical determination of the low migration energy (0.06 eV) of a single He in tungsten, fully quantum mechanical calculations on the migration of a He pair still present a challenge due to the complexity of its trajectory. By identifying the six most stable configurations of the He pair in W and decomposing its motion into rotational, translational, and rotational-translational routines, we are able to determine its migration barrier and trajectory. Our density functional theory calculations demonstrate a He pair has three modes of motion: a close or open circular two-dimensional motion in (100) plane with an energy barrier of 0.30 eV, a snaking motion along [001] direction with a barrier of 0.30 eV, and a twisted-ladder motion along [010] direction with the two He swinging in the plane (100) and a barrier of 0.31 eV. The graceful associative movements of a He pair are related to the chemical-bonding-like He-He interaction being much stronger than its migration barrier in W. The excellent agreement with available experimental measurements (0.24–0.32 eV) on He migration makes our first-principles result a solid input to obtain accurate He-W interatomic potentials in molecular dynamics simulations.
First-principles method to study defect properties in semiconductor nanostructures
NASA Astrophysics Data System (ADS)
Partoens, Bart; Amini, Mozhgan; Schoeters, Bob; Saniz, Rolando; Lamoen, Dirk; CMT-EMAT Collaboration
2014-03-01
The standard theoretical approach to examine the deep or shallow nature of defects in bulk crystals is through first-principles calculations of their (neutral and charged) formation energies. The character of a defect in a nanostructure might differ from its character in the bulk material and may vary with its position in the nanostructure. However, the standard method cannot be transferred directly to nanostructures. In calculations for a charged defect, a uniform background charge is considered. While this is well-defined for bulk calculations, the total energy of a charged nanostructure depends on the vacuum width. Therefore, total energies of charged nanostructures cannot be used to calculate defect formation energies. Here we propose a solution to this problem and present a first-principles method to determine formation energies for defects in different charge states in a nanostructure, together with the transition levels. As example, we focus on VO in ZnO slabs and SiGa in GaAs slabs. Their preferential position as function of the distance to the surface is determined, together with the evolution of their optical and thermal ionization energies. This new method allows to study the character of a wide range of intrinsic and extrinsic defects in nanostructures.
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.
X-ray magnetic dichroism in (Zn,Mn)O diluted magnetic semiconductors: First-principles calculations
NASA Astrophysics Data System (ADS)
Antonov, V. N.; Bekenov, L. V.; Mazur, D. V.; Germash, L. P.
2012-06-01
The electronic structure of (Zn,Mn)O diluted magnetic semiconductors was investigated theoretically from first principles by using the fully-relativistic Dirac linear muffin-tin orbital band structure method with the local spin-density approximation (LSDA) and the LSDA+ U approach. The X-ray magnetic circular dichroism (XMCD) spectra at the Mn, Zn, and O K and Mn L 2,3 edges were investigated theoretically from first principles. The origin of the XMCD spectra in these compounds was examined. The effect of oxygen vacancy atoms was found to be crucial for the X-ray magnetic dichroism at the Mn L 2,3 edges. The calculated results are compared with available experimental data.
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.
Alexander A. Demkov; Xiaodong Zhang; D. A. Drabold
2001-01-01
We describe a theoretical approach to transport and a potentially valuable scheme for screening gate dielectric materials. Realistic structural models of the Si-dielectric interface are employed for Si-SiO2-Si model metal-oxide-semiconductor (MOS) structures. The leakage current for a 1.02-nm MOS structure is calculated from first principles using Landauer's ballistic transport approach and ab inito molecular-dynamic simulation. The calculated leakage currents agree
Safeguards First Principles Initiative at the Nevada Test Site
Geneva Johnson
2007-07-08
The Material Control and Accountability (MC&A) program at the Nevada Test Site (NTS) was selected as a test bed for the Safeguards First Principles Initiative (SFPI). The implementation of the SFPI is evaluated using the system effectiveness model and the program is managed under an approved MC&A Plan. The effectiveness model consists of an evaluation of the critical elements necessary to detect, deter, and/or prevent the theft or diversion of Special Nuclear Material (SNM). The modeled results indicate that the MC&A program established under this variance is still effective, without creating unacceptable risk. Extensive performance testing is conducted through the duration of the pilot to ensure the protection system is effective and no material is at an unacceptable risk. The pilot was conducted from January 1, 2007, through May 30, 2007. This paper will discuss the following activities in association with SFPI: 1. Development of Timeline 2. Crosswalk of DOE Order and SFPI 3. Peer Review 4. Deviation 5. MC&A Plan and Procedure changes 6. Changes implemented at NTS 7. Training 8. Performance Test
Vacancy vacancy interactions in graphene using first principles calculations
NASA Astrophysics Data System (ADS)
Francis, Priya; Majumder, Chiranjib; Ghaisas, S. V.
2014-03-01
We employ first principles calculations to study the defect induced magnetism on monolayer graphene sheet. Removal of single, double and triple carbon atoms from a 6 × 6 monolayer graphene sheet gives magnetic moment (MM) comparable with those reported in the literature. For single vacancy defect, MM value lies in between 1.4 and 1.6 ?B, depending on the type of lattice atoms (? or ?). For triple vacancy, MM is 1.04 ?B. Detailed study of double vacancy in graphene layer with a removal of similar and different type of lattice atoms gives interesting results in terms of magnetic moment as well as in spin density distributions. For double vacancy our study gives magnetic moment values 0 and 3 ?B, depending on the position and lattice type of the removed atoms. Our study reveals that removal of near by same lattice type atoms (? type) also leads to 0 magnetic moment, same as that of the removal of different lattice type atoms (? and ?, irrespective of the distances). Here the distance between the removed two ? type atoms is 2.46 Å. The authors thank C-DAC for providing computing facilities. PF would like to thank Govt. of India for providing UGC-S.R.F fellowship.
First-principles prediction of disordering tendencies in complex oxides
Jiang, Chao [Los Alamos National Laboratory; Stanek, Christopher R [Los Alamos National Laboratory; Sickafus, Kurt E [Los Alamos National Laboratory; Uberuaga, Blas P [Los Alamos National Laboratory
2008-01-01
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 structure and ionicity of actinide oxides from first principles
Petit, L.; Svane, A.; Szotek, Z.; Temmerman, W. M.; Stocks, G. Malcolm
2010-01-07
The ground-state electronic structures of the actinide oxides AO , A{sub 2} O{sub 3} , and AO{sub 2} (A=U , Np, Pu, Am, Cm, Bk, and Cf) are determined from first-principles calculations, using the self-interaction corrected local spin-density approximation. Emphasis is put on the degree of f -electron localization, which for AO{sub 2} and A{sub 2} O{sub 3} is found to follow the stoichiometry, namely, corresponding to A{sup 4+} ions in the dioxide and A{sup 3+} ions in the sesquioxides. In contrast, the A{sup 2+} ionic configuration is not favorable in the monoxides, which therefore become metallic. The energetics of the oxidation and reduction in the actinide dioxides is discussed, and it is found that the dioxide is the most stable oxide for the actinides from Np onward. Our study reveals a strong link between preferred oxidation number and degree of localization which is confirmed by comparing to the ground-state configurations of the corresponding lanthanide oxides. The ionic nature of the actinide oxides emerges from the fact that only those compounds will form where the calculated ground-state valency agrees with the nominal valency expected from a simple charge counting.
Coarse graining approach to First principles modeling of structural materials
Odbadrakh, Khorgolkhuu; Nicholson, Don M; Rusanu, Aurelian; Samolyuk, German D; Wang, Yang; Stoller, Roger E; Zhang, X.-G.; Stocks, George Malcolm
2013-01-01
Classical Molecular Dynamic (MD) simulations characterizing extended defects typically require millions of atoms. First principles calculations employed to understand these defect systems at an electronic level cannot, and should not deal with such large numbers of atoms. We present an e cient coarse graining (CG) approach to calculate local electronic properties of large MD-generated structures from the rst principles. We used the Locally Self-consistent Multiple Scattering (LSMS) method for two types of iron defect structures 1) screw-dislocation dipoles and 2) radiation cascades. The multiple scattering equations are solved at fewer sites using the CG. The atomic positions were determined by MD with an embedded atom force eld. The local moments in the neighborhood of the defect cores are calculated with rst-principles based on full local structure information, while atoms in the rest of the system are modeled by representative atoms with approximated properties. This CG approach reduces computational costs signi cantly and makes large-scale structures amenable to rst principles study. Work is sponsored by the USDoE, O ce of Basic Energy Sciences, Center for Defect Physics, an Energy Frontier Research Center. This research used resources of the Oak Ridge Leadership Computing Facility at the ORNL, which is supported by the O ce of Science of the USDoE under Contract No. DE-AC05-00OR22725.
First-principles study of liquid and amorphous metals
NASA Astrophysics Data System (ADS)
Ganesh, Panchapakesan
Computer simulations using state of the art First-Principles ab-initio methods enable us to probe the structural features of novel materials like liquid metals and metallic glass forming alloys, both in their supercooled liquid state as well as in their quenched amorphous forms where available. The ab-initio nature of the calculations enable us to capture the chemical identity realistically at the atomistic level without any free parameters. The results show that even though elemental liquid metals like face-centered cubic (FCC) Cu and body-centered cubic (BCC) Fe (and W) have similar atomic structure at high temperature, which is also similar to jammed packing of hard-spheres, they differ quite appreciably even with slight supercooling. This difference enables us to further supercool Fe and W to a much greater degree than Cu. The origin of this difference between elemental metals with different crystalline ground states can be understood based on concepts of geometric frustration. Further, the role played by atoms of different sizes in controlling the geometric frustration in glass forming alloys has been investigated. Studies of Silicon in its supercooled regime have been made to investigate the existence of a possible structural transition. Attempts to clarify if the structural transition could be a thermodynamic phase transition have been made and changes in electronic properties accompanying this structural change have been studied.
Thin film bulk acoustic wave resonators tuning from first principles
NASA Astrophysics Data System (ADS)
Kvasov, Alexander; Tagantsev, Alexander K.
2013-05-01
Being important for the antiresonance frequency tuning of tunable thin Film Bulk Acoustic wave Resonators (FBARs), the non-linear electrostrictive coefficient was for the first time calculated for BaTiO3 and SrTiO3 using ab initio methods. Further, taking into account the small difference of obtained values for BaTiO3 and SrTiO3 these results were linearly interpolated to the BaxSr1-xTiO3 (BST) compositions. The obtained values are consistent with previously made order-of-magnitude estimates. Using parameters obtained with first principles calculations, we simulated the resonance parameters of BST based tunable FBARs. Resulting antiresonance tuning was smaller than expected due to the compensation of two competing terms conditioned by linear and non-linear electrostrictions. Our calculations confirm that, for tunable FBAR modeling, it is important to use a polarization-based Landau free energy expansion taking into account both non-linear electrostriction and background permittivity.
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.
Stress dependent defect energetics in Tungsten from first-principles
NASA Astrophysics Data System (ADS)
Hossain, Md.; Marian, Jaime
2013-03-01
Tungsten (W) is an important material for high temperature applications due to its refractory nature. However, like all transition metals from the VI-A group, W suffers from low-temperature brittleness and lack of ductility, which poses serious questions for its use as a structural material. Tungsten's mechanical properties can be enhanced by alloying with elements with d-electrons, such as Re, which has resulted in successful commercial alloys. In this work, we obtain the formation and migration energetics of Re solute atoms in terms of their interaction with vacancies and dislocations. To explore the influence of external stresses on Re transport properties, we examine the role of hydrostatic and shear deformation on the vacancy formation energy (VFE) and migration energy barrier (Em) in BCC W from first-principles calculations by developing a pseudopotential with 6s2, 6p0, 5d4, and 5f0 electronic states for the valence electrons. We find that under hydrostatic deformation, increase or decrease of vacancy formation energy depends on the type of deformation - tensile or compressive, while for shear deformation it decreases irrespective of the magnitude of applied deformation. On the other hand, migration energy barrier always decreases under hydrostatic deformation, but shows path-length dependent behavior under shear deformation. This talk will discuss the underlying principles and possible routes for enhancing mechanical strength from a physics perspective.
Electronic Structure and Ionicity of Actinide Oxides from First Principles
Petit, Leon; Svane, Axel; Szotek, Zdzislawa; Temmerman, Walter M; Stocks, George Malcolm
2010-01-01
The ground-state electronic structures of the actinide oxides AO, A{sub 2}O{sub 3}, and AO{sub 2} (A=U, Np, Pu, Am, Cm, Bk, and Cf) are determined from first-principles calculations, using the self-interaction corrected local spin-density approximation. Emphasis is put on the degree of f-electron localization, which for AO{sub 2} and A{sub 2}O{sub 3} is found to follow the stoichiometry, namely, corresponding to A{sup 4+} ions in the dioxide and A{sup 3+} ions in the sesquioxides. In contrast, the A{sup 2+} ionic configuration is not favorable in the monoxides, which therefore become metallic. The energetics of the oxidation and reduction in the actinide dioxides is discussed, and it is found that the dioxide is the most stable oxide for the actinides from Np onward. Our study reveals a strong link between preferred oxidation number and degree of localization which is confirmed by comparing to the ground-state configurations of the corresponding lanthanide oxides. The ionic nature of the actinide oxides emerges from the fact that only those compounds will form where the calculated ground-state valency agrees with the nominal valency expected from a simple charge counting.
Elastic properties of VO2 from first-principles calculation
NASA Astrophysics Data System (ADS)
Dong, Huafeng; Liu, Hongfei
2013-08-01
We used first-principles methods to calculate the elastic properties of rutile (R) structure and monoclinic (M1: space group P21/c, M2: space group C2/m) structure VO2, including single-crystal elastic constants cij's, polycrystalline bulk modulus, shear modulus, Young's modulus and elastic anisotropy ratio. We found that the energy difference among the R, M1 and M2 phases is small, indicating that it is easy to transit among them under a perturbation. Furthermore, from the pressure dependence of cij's, we also found that the structural instability (or phase transition) will occur when the volumes of the three phases are slightly smaller than their equilibrium volumes. Additionally, the R and M2 phases are predicted to be harder than the M1 phase, indicated by their larger bulk moduli and shear moduli. The elastic anisotropy of the M2 phase is larger than the M1 and R phases. The presently predicted elastic properties of VO2 provide helpful guidance for the strain energy estimation and stress analysis in nano-electronic devices.
Transport Properties of Nanoscale Materials by First-principles Calculations
NASA Astrophysics Data System (ADS)
Mizuseki, Hiroshi; Belosludov, Rodion V.; Lee, S.-U.; Kawazoe, Yoshiyuki
2009-03-01
Molecular devices are potential candidates for the next step towards nanoelectronic technology. Our group has covered a wide range of nanoscale wires, which have potential application in molecular electronics using first-principles calculations and nonequilibrium Green's function formalism [1]. Our target materials are supramolecular enamel wires (covered wires) [2], connection between organic molecules and metal electrodes, self-assembled nanowires on silicon surface [3], porphyrin [4], phthalocyanine, metallocene [5], fused-ring thiophene molecules, length dependence of conductance in alkanedithiols and so on. Namely, we have investigated a relationship of the energy levels of delocalized frontier orbitals (HOMO and LUMO) and Fermi level of metal electrodes and estimate the electronic transport properties through atomic and molecular wires using Green's function approach. References [1] http://www-lab.imr.edu/˜mizuseki/nanowire.html [2] R. V. Belosludov, A. A. Farajian, H. Baba, H. Mizuseki, and Y. Kawazoe, Jpn. J. Appl. Phys., 44, 2823 (2005). [3] R. V. Belosludov, A. A. Farajian, H. Mizuseki, K. Miki, and Y. Kawazoe, Phys. Rev. B, 75, 113411 (2007). [4] S.-U. Lee, R. V. Belosludov, H. Mizuseki, and Y. Kawazoe, Small 4 (2008) 962. [5] S.-U Lee, R. V. Belosludov, H. Mizuseki, and Y. Kawazoe, J. Phys. Chem. C. 111 (2007) 15397.
First-principles Simulations and the Criticality of Calving Glaciers
NASA Astrophysics Data System (ADS)
Vallot, D.; Åström, J. A.; Schäfer, M.; Welty, E.; O'Neel, S.; Bartholomaus, T. C.; Liu, Y.; Riikilä, T.; Zwinger, T.; Timonen, J.; Moore, J.
2014-12-01
The algoritm of a first principles calving-simulation computer-code is outlined and demonstrated. The code is particle-based and uses Newtonian dynamics to simulate ice-fracture, motion and calving. The code can simulate real-size glacier but is only able to simualte individual calving events within a few tens of minutes in duration. The code couples to the Elmer/Ice ice flow-simulation code: Elmer is employed to produce various glacier geomteries, which are then tested for stability using the particle code. In this way it is possible to pin-point the location of calving fronts. The particle simulation code and field observations are engaged to investigate the criticality of calving glaciers. The calving mass and inter-event waiting times both have power-law distributions with the same critical exponents as found for Abelian sand-pile models. This indicate that calving glaciers share characteristics with Self-Organized Critical systems (SOC). This would explain why many glacier found in nature may become unstable as a result of even minor changes in their environment. An SOC calving glacier at the critical point will display so large fluctuations in calving rate that it will render the concept 'average calving rate' more or less useless. I.e. 'average calving rate' will depend on measurement time and always have fluctuaions in the range of 100% more or less independent of the averaging time.
First Principles Investigation of Hydrogen Physical Adsorption on Graphynes' layers
Bartolomei, Massimiliano; Giorgi, Giacomo
2015-01-01
Graphynes are 2D porous structures deriving from graphene featuring triangular and regularly distributed subnanometer pores, which may be exploited to host small gaseous species. First principles adsorption energies of molecular hydrogen (H2) on graphene, graphdiyne and graphtriyne molecular prototypes are obtained at the MP2C level of theory. First, a single layer is investigated and it is found that graphynes are more suited than graphene for H2 physical adsorption since they provide larger binding energies at equilibrium distances much closer to the 2D plane. In particular, for graphtriyne a flat minimum located right in the geometric center of the pore is identified. A novel graphite composed of graphtriyne stacked sheets is then proposed and an estimation of its 3D arrangement is obtained at the DFT level of theory. In contrast to pristine graphite this new carbon material allow both H2 intercalation and out-of-plane diffusion by exploiting the larger volume provided by its nanopores. Related H2 binding ...
First-principles study of hydrogen storage materials
NASA Astrophysics Data System (ADS)
Ma, Zhu
2008-10-01
In this thesis, we use first-principles calculations to study the structural, electronic, and thermal properties of several complex hydrides. We investigate structural and electronic properties of Na-Li alanates. Although Na alanate can reversibly store H with Ti catalyst, its weight capacity needs to be improved. This can be accomplished by partial replacement of Na with lighter elements. We explore the structures of possible Na-Li alloy alanates, and study their phase stability. We also study the structural and thermal properties of Li/Mg/Li-Mg Amides/Imides. Current experimental results give a disordered model about the structure of Li-Mg Imide, in which the positions of Li and Mg are not specified. In addition the model gives a controversial composition stoichiometry. We try to resolve this controversy by searching for low-energy ordered phases. In the last part, we study the structural, energetic, and electronic properties of the La-Mg-Pd-H system. This quaternary system is another example of hydrogenation-induced metal-nonmetal transition without major reconstruction of metal host structure, and it is also with partial reversible H capacity. Experiment gives partially disordered H occupancy on two Wyckoff positions. Our calculation explains the structural and bonding characteristics observed in experiment.
First-principles study of water on Cu (110) surface
NASA Astrophysics Data System (ADS)
Ren, Jun; Meng, Sheng
2009-03-01
The persistent demand for cheaper and high efficient catalysts in industrial chemical synthesis, such as ammonia, and in novel energy applications, hydrogen generation and purification in fuel cells motivated us to study the fundamental interaction involved in water-Cu system, with an intension to examine Cu as a possible competitive candidate for cheaper catalysts. Water structure and dissociation kinetics on a model open metal surface: Cu (110), have been investigated in detail based on first-principles electronic structure calculations. We revealed that in both monomer and overlayer forms, water adsorbs molecularly, with a high tendency for diffusion and/or desorption rather than dissociation on clean surfaces at low temperature. With the increase of the water coverage on the Cu (110) surface, the H-bond pattern lowers the dissociation barrier efficiently. More importantly, if the water molecule is dissociated, the hydrogen atoms can diffuse freely along the [110] direction, which is very useful in the hydrogen collection. In addition, we extended to study water on other noble metal (110) surfaces. The result confirms that Cu (110) is the borderline between intact and dissociative adsorption, differing in energy by only 0.08 eV. This may lead to promising applications in hydrogen generation and fuel cells.
First principles study of the graphene/Ru(0001) interface
Jiang, Deen [ORNL; Du, Mao-Hua [ORNL; Dai, Sheng [ORNL
2009-01-01
Annealing the Ru metal that typically contains residual carbon impurities offers a facile way to grow graphene on Ru(0001) at the macroscopic scale. Two superstructures of the graphene/Ru(0001) interface with periodicities of 3.0 and 2.7 nm, respectively, were previously observed by scanning tunneling microscopy. Using first principles density functional theory, we optimized the observed superstructures and found interfacial C-Ru bonding of C atoms atop Ru atoms for both superstructures, which causes the graphene sheet to buckle and form periodic humps of {approx}1.7 {angstrom} in height within the graphene sheet. The flat region of the graphene sheet, which is 2.2-2.3 {angstrom} above the top Ru layer and has more C atoms occupying the atop sites, interacts more strongly with the substrate than does the hump region. We found that interfacial adhesion is much stronger for the 3.0 nm superstructure than for the 2.7 nm superstructure, suggesting that the former is the thermodynamically more stable phase. We explained the 3.0 nm superstructure's stability in terms of the interplay between C-Ru bonding and lattice matching.
First-principles pressure-temperature phase diagrams in metals
Moriarty, J.A.
1993-07-01
Using interatomic potentials derived from first-principles generalized pseudopotential theory, finite-temperature phase transitions in both simple and transition metals can be studied through a combination of analytic statistical methods and molecular-dynamics simulation. In the prototype simple metal-Mg, where volume and pair forces adequately describe the energetics, a complete and accurate phase diagram has thereby been obtained to 60 GPa. A rapidly temperature-dependent hcp-bcc phase line is predicted which ends in a triple point on the melting curve near 4 GPa. In central transition metals such as Mo or Fe, on the other hand, the energetics are complicated by d-state interactions which give rise to both many-body angular forces and enhanced electron-thermal contributions. We have made a detailed study of these phenomena and their impact on melting in the prototype case of Mo and a full melting curve to 2 Mbar has been obtained. In the case of Fe, we are examining the high-pressure phase diagram and the question of whether or not there exists a high-pressure, high-temperature solid bcc phase, as has been speculated. To date, we have shown that the bcc structure is both thermodynamically and mechanically unstable at high pressure and zero temperature, with a large and increasing bcc-hcp energy difference under compression.
Oxygen transport in ceria: a first-principles study
NASA Astrophysics Data System (ADS)
Sergei, Simak
2012-02-01
Ceria (CeO2) is an important material for environmentally benign applications, ranging from solid-oxide fuel cells (SOFC) to oxygen storage [1-2]. The key characteristic needed to be improved is the mobility of oxygen ions. Optimization of ionic transport in ceria has been the topic of many studies. In particular, it has been discovered how the ionic conductivity in ceria might be improved by choosing the proper kind and concentration of dopants [3]. In this presentation we will approach the problem from a different direction by adjusting structural parameters of ceria via the change of external conditions. A systematic first-principles study of the energy landscape and kinetics of reduced ceria as a function of external parameters reveals a physically transparent way to improve oxygen transport in ceria. [4pt] [1] N. Skorodumova, S. Simak, B. Lundqvist, I. Abrikosov, and B. Johansson, Physical Review Letters 89, 14 (2002). [0pt] [2] A. Trovarelli, in Catalysis by Ceria and related materials (Imperial College Press, London, 2002). [0pt] [3] D. A. Andersson, S. I. Simak, N. V. Skorodumova, I. A.Abrikosov, and B. Johansson, Proceedings of the National Academy of Sciences of the United States of America 103, 3518 (2006).
Incorporation of water in pyrope: a first principles study
NASA Astrophysics Data System (ADS)
Manga, V. R.; Mookherjee, M.; Muralidharan, K.
2014-12-01
Pyrope (Mg3Al2Si3O12) rich garnet is the most important secondary mineral phase with volume fractions ranging between 20 % in the shallow upper mantle to 40 % in the lower part of upper mantle. The volume fractions of garnet in subducted oceanic crusts are as high as 80 %. However, our understanding of the incorporation of water in garnet as proton defect is rather limited. Experimental studies conducted at pressures and temperatures relevant to the deep lower mantle have resulted in wide range of water contents ranging between 0 wt % to ~ 0.8 wt %. In a pyriolyte composition representative of the upper mantle, unlike olivine (Mg2SiO4), which remains largely iso-chemical upon compression, garnet undergoes solid solution with pyroxene (MgSiO3) and as a result there is a continuous evolution of the chemistry of garnet as a function of pressure. This complicates the analysis of proton defects using conventional thermodynamics expressing water solubility as a function of water fugacity, oxide activity, and activation volume. To circumvent this issue, we use first principles simulations to explore the relative energetics of the formation of protons in Mg, Al, and Si sites. Preliminary results at ambient conditions indicate positive enthalpy changes for the proton defects in all the sites, with silicon site being the most favorable. We intend to explore the effect of pressure and temperature on the defect formation energies. Acknowledgement- MM is supported by the US National Science Foundation grant (EAR-1250477).
First-principles prediction of disordering tendencies in pyrochlore oxides
Jiang Chao; Stanek, C. R.; Sickafus, K. E.; Uberuaga, B. P.
2009-03-01
Using first-principles calculations, we systematically predict the order-disorder energetics of series of zirconate (A{sub 2}Zr{sub 2}O{sub 7}), hafnate (A{sub 2}Hf{sub 2}O{sub 7}), titanate (A{sub 2}Ti{sub 2}O{sub 7}), and stannate (A{sub 2}Sn{sub 2}O{sub 7}) pyrochlores. The disordered defect-fluorite structure is modeled using an 88-atom two-sublattice special quasirandom structure (SQS) that closely reproduces the most relevant near-neighbor intrasublattice and intersublattice pair-correlation functions of the random mixture. The order-disorder transition temperatures of these pyrochlores estimated from our SQS calculations show overall good agreement with existing experiments. We confirm previous studies suggesting that the bonding in pyrochlores is not purely ionic and thus electronic effects also play a role in determining their disordering tendencies. Our results have important consequences for numerous applications, including nuclear waste forms and fast ion conductors.
Thermal conductivity of silicene from first-principles
Xie, Han; Bao, Hua, E-mail: hum@ghi.rwth-aachen.de, E-mail: hua.bao@sjtu.edu.cn [University of Michigan–Shanghai Jiao Tong University Joint Institute, Shanghai Jiao Tong University, Shanghai 200240 (China); Hu, Ming, E-mail: hum@ghi.rwth-aachen.de, E-mail: hua.bao@sjtu.edu.cn [Institute of Mineral Engineering, Division of Materials Science and Engineering, Faculty of Georesources and Materials Engineering, RWTH Aachen University, Aachen 52064 (Germany); Aachen Institute for Advanced Study in Computational Engineering Science (AICES), RWTH Aachen University, Aachen 52062 (Germany)
2014-03-31
Silicene, as a graphene-like two-dimensional material, now receives exceptional attention of a wide community of scientists and engineers beyond graphene. Despite extensive study on its electric property, little research has been done to accurately calculate the phonon transport of silicene so far. In this paper, thermal conductivity of monolayer silicene is predicted from first-principles method. At 300?K, the thermal conductivity of monolayer silicene is found to be 9.4?W/mK and much smaller than bulk silicon. The contributions from in-plane and out-of-plane vibrations to thermal conductivity are quantified, and the out-of-plane vibration contributes less than 10% of the overall thermal conductivity, which is different from the results of the similar studies on graphene. The difference is explained by the presence of small buckling, which breaks the reflectional symmetry of the structure. The flexural modes are thus not purely out-of-plane vibration and have strong scattering with other modes.
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.
NASA Astrophysics Data System (ADS)
Desnavi, Sameerah; Chakraborty, Brahmananda; Ramaniah, Lavanya M.
2014-04-01
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 ?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.
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
Experimental and first principle studies on electronic structure of BaTiO3
NASA Astrophysics Data System (ADS)
Sagdeo, Archna; Ghosh, Haranath; Chakrabarti, Aparna; Kamal, C.; Ganguli, Tapas; Phase, D. M.; Deb, S. K.
2014-04-01
We have carried out photoemission experiments to obtain valence band spectra of various crystallographic symmetries of BaTiO3 system which arise as a function of temperature. We also present results of a detailed first principle study of these symmetries of BaTiO3 using generalized gradient approximation for the exchange-correlation potential. Here we present theoretical results of density of states obtained from DFT based simulations to compare with the experimental valence band spectra. Further, we also perform calculations using post density functional approaches like GGA + U method as well as non-local hybrid exchange-correlation potentials like PBE0, B3LYP, HSE in order to understand the extent of effect of correlation on band gaps of different available crystallographic symmetries (5 in number) of BaTiO3.
Structural instabilities and mechanical properties of U2Mo from first principles calculations.
Liu, Ben-Qiong; Duan, Xiao-Xi; Sun, Guang-Ai; Yang, Jin-Wen; Gao, Tao
2015-02-14
We perform detailed first principles calculations of the structural parameters at zero pressure and high pressure, the elastic properties, phonon dispersion relation, and ideal strengths of U2Mo with the C11b structure. In contrast to previous theoretical studies, we show that the I4/mmm structure is indeed a mechanically and dynamically unstable phase, which is confirmed by the negative elastic constant C66 as well as the imaginary phonon modes observed along the ?1-N-P line. The calculations of ideal strengths for U2Mo are performed along the [100], [001], and [110] directions for tension and on (001)[010] and (010)[100] slip systems for shear load. The ideal shear strength is about 8.1 GPa, much smaller than a tension of 18-28 GPa, which indicates that the ductile U2Mo alloy will fail by shear rather than by tension. PMID:25560204
NASA Astrophysics Data System (ADS)
Tang, Cui-Ming; Chen, Xiao-Xu; Wang, Jun; Hu, Yan-Fei; Wang, Hong-Yan
2013-06-01
In the frame of density functional theory, first-principles calculations have been carried out to investigate the structures, elastic constants, structural phase transition between B1 and B2 phases and thermodynamic properties of the zirconium nitride (ZrN) by means of the generalized gradient approximation. The equilibrium lattice parameter we obtained for ZrN in B1 phase is closer to the experiment results than previous theoretical results. In addition, the calculations of the elastic constants show that ZrN is a brittle material. What is more, based on third-order natural strain equation of state, the phase transition pressure 338 GPa for ZrN is predicted for B1-B2 transition. According to the quasi-harmonic Debye model, the thermodynamic parameters of ZrN have been investigated systematically.
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.
Supercapacitors composed of graphene and boron nitride layers: A first-principles study
Özçelik, V Ongun; Ciraci, S
2012-01-01
We propose a model for nanoscale supercapacitor consisting of two-dimensional insulating BN layers placed between two commensurate and metallic graphene layers. First-principles Density Functional calculations of structure optimized total energy and self-consistent field potential performed on these nanoscale capacitors for different levels of charging and different number of BN layers mark the values of capacitance per unit mass, which are larger than those measured values for the supercapacitors made from other carbon based materials. Our theoretical study also compares results obtained for the present nanoscale capacitor with those of classical Helmholtz model and reveals crucial differences. Our model allows the fabrication of series/parallel mixed combinations consisting of epitaxially grown, sequential and multiple graphene/BN sheets.
First-Principles Study of Back Contact Effects on CdTe Thin Film Solar Cells
Du, Mao-Hua
2009-01-01
Forming a chemically stable low-resistance back contact for CdTe thin-film solar cells is critically important to the cell performance. This paper reports theoretical study of the effects of the back-contact material, Sb{sub 2}Te{sub 3}, on the performance of the CdTe solar cells. First-principles calculations show that Sb impurities in p-type CdTe are donors and can diffuse with low diffusion barrier. There properties are clearly detrimental to the solar-cell performance. The Sb segregation into the grain boundaries may be required to explain the good efficiencies for the CdTe solar cells with Sb{sub 2}Te{sub 3} back contacts.
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 (? orbitals), from the electronic point of view.
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.
Thermoelectric properties of AgSbTe? from first-principles calculations
Rezaei, Nafiseh; Akbarzadeh, Hadi [Department of Physics, Isfahan University of Technology, 84156-83111 Isfahan (Iran, Islamic Republic of); Hashemifar, S. Javad, E-mail: hashemifar@cc.iut.ac.ir [Department of Physics, Isfahan University of Technology, 84156-83111 Isfahan (Iran, Islamic Republic of); Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, 84156-83111 Isfahan (Iran, Islamic Republic of)
2014-09-14
The structural, electronic, and transport properties of AgSbTe? are studied by using full-relativistic first-principles electronic structure calculation and semiclassical description of transport parameters. The results indicate that, within various exchange-correlation functionals, the cubic Fd3?m and trigonal R3?m structures of AgSbTe? are more stable than two other considered structures. The computed Seebeck coefficients at different values of the band gap and carrier concentration are accurately compared with the available experimental data to speculate a band gap of about 0.1–0.35 eV for AgSbTe? 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 AgSbTe? as a function of temperature and carrier concentration.
Structural, electronic, and dynamical properties of Pca21-TiO2 by first principles
NASA Astrophysics Data System (ADS)
Abbasnejad, M.; Mohammadizadeh, M. R.; Maezono, R.
2012-03-01
First-principles calculations of the structural, electronic, and mechanical properties of the modified fluorite structure of TiO2 with Pca21 symmetry are obtained using the plane-wave pseudopotential density functional theory. The results indicate that Pca21-TiO2 is a semiconductor with an indirect band gap. The calculated static dielectric constants are larger than those of anatase and brookite, but they are much smaller than those of rutile. The calculated bulk modulus using the equation of state is in good agreement with that calculated from elastic constants. The calculated bulk modulus is in agreement with a recent theoretical and experimental report, which confirms that the experimentally claimed structure (cubic fluorite phase) can be Pca21-TiO2.
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.
Formation and annealing behaviors of qubit centers in 4H-SiC from first principles
Wang, Xiaopeng; Zhao, Mingwen Bu, Hongxia; He, Xiujie; Wang, Aizhu; Zhang, Hongyu
2013-11-21
Inspired by finding that the nitrogen-vacancy center in diamond is a qubit candidate, similar defects in silicon carbide (SiC) have drawn considerable interest. However, the generation and annealing behaviors of these defects remain unclear. Using first-principles calculations, we describe the equilibrium concentrations and annealing mechanisms based on the diffusion of silicon vacancies. The formation energies and energy barriers along different migration paths, which are responsible for the formation rates, stability, and concentrations of these defects, are investigated. The effects on these processes of charge states, annealing temperature, and crystal orientation are also discussed. These theoretical results are expected to be useful in achieving controllable generation of these defects in experiments.
Thermal conductivity of Si nanowires: A first-principles analysis of the role of defects
NASA Astrophysics Data System (ADS)
Kang, By.; Estreicher, S. K.
2014-04-01
The theoretical laser-flash method is used to calculate the thermal conductivity of the Si200X32 (X =H, D, or OH) and Si296X112 (X =H or D) nanowires. The main emphasis is on the role of defects, which are described using first-principles methods. The defects considered are the surface of the nanowire, random distributions of substitutional C or Ge impurities, and monoatomic ? layers of C or Ge. The localized vibrational modes of these defects are explicitly included in the calculations and no empirical defect-related parameter is introduced. We find that the surface Si-H wag modes couple resonantly to each other much faster than they decay into bulk modes, which leads to distinct surface and bulk contributions to the thermal conductivity. The spatially-localized vibrational modes associated with the Ge or C impurities as well as the ? layers trap thermal phonons thus reducing the thermal conductivity.
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.
A digitally reconstructed radiograph algorithm calculated from first principles
Staub, David; Murphy, Martin J.
2013-01-01
Purpose: To develop an algorithm for computing realistic digitally reconstructed radiographs (DRRs) that match real cone-beam CT (CBCT) projections with no artificial adjustments. Methods: The authors used measured attenuation data from cone-beam CT projection radiographs of different materials to obtain a function to convert CT number to linear attenuation coefficient (LAC). The effects of scatter, beam hardening, and veiling glare were first removed from the attenuation data. Using this conversion function the authors calculated the line integral of LAC through a CT along rays connecting the radiation source and detector pixels with a ray-tracing algorithm, producing raw DRRs. The effects of scatter, beam hardening, and veiling glare were then included in the DRRs through postprocessing. Results: The authors compared actual CBCT projections to DRRs produced with all corrections (scatter, beam hardening, and veiling glare) and to uncorrected DRRs. Algorithm accuracy was assessed through visual comparison of projections and DRRs, pixel intensity comparisons, intensity histogram comparisons, and correlation plots of DRR-to-projection pixel intensities. In general, the fully corrected algorithm provided a small but nontrivial improvement in accuracy over the uncorrected algorithm. The authors also investigated both measurement- and computation-based methods for determining the beam hardening correction, and found the computation-based method to be superior, as it accounted for nonuniform bowtie filter thickness. The authors benchmarked the algorithm for speed and found that it produced DRRs in about 0.35 s for full detector and CT resolution at a ray step-size of 0.5 mm. Conclusions: The authors have demonstrated a DRR algorithm calculated from first principles that accounts for scatter, beam hardening, and veiling glare in order to produce accurate DRRs. The algorithm is computationally efficient, making it a good candidate for iterative CT reconstruction techniques that require a data fidelity term based on the matching of DRRs and projections. PMID:23298093
A digitally reconstructed radiograph algorithm calculated from first principles
Staub, David; Murphy, Martin J. [Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298 (United States)
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 explanation of the positive Seebeck coefficient of lithium.
Xu, Bin; Verstraete, Matthieu J
2014-05-16
Lithium is one of the simplest metals, with negative charge carriers and a close reproduction of free-electron dispersion. Experimentally, however, Li is one of a handful of elemental solids (along with Cu, Ag, and Au) where the sign of the Seebeck coefficient (S) is opposite to that of the carrier. This counterintuitive behavior still lacks a satisfactory interpretation. We calculate S fully from first principles, within the framework of Allen's formulation of Boltzmann transport theory. Here it is crucial to avoid the constant relaxation time approximation, which gives a sign for S which is necessarily that of the carriers. Our calculated S are in excellent agreement with experimental data, up to the melting point. In comparison with another alkali metal, Na, we demonstrate that within the simplest nontrivial model for the energy dependency of the electron lifetimes, the rapidly increasing density of states (DOS) across the Fermi energy is related to the sign of S in Li. The exceptional energy dependence of the DOS is beyond the free-electron model, as the dispersion is distorted by the Brillouin zone edge; this has a stronger effect in Li than other alkali metals. The electron lifetime dependency on energy is central, but the details of the electron-phonon interaction are found to be less important, contrary to what has been believed for several decades. Band engineering combined with the mechanism exposed here may open the door to new "ambipolar" thermoelectric materials, with a tunable sign for the thermopower even if either n- or p-type doping is impossible. PMID:24877957
Liquid Water from First Principles: Validation of Different Sampling Approaches
Mundy, C J; Kuo, W; Siepmann, J; McGrath, M J; Vondevondele, J; Sprik, M; Hutter, J; Parrinello, M; Mohamed, F; Krack, M; Chen, B; Klein, M
2004-05-20
A series of first principles molecular dynamics and Monte Carlo simulations were carried out for liquid water to assess the validity and reproducibility of different sampling approaches. These simulations include Car-Parrinello molecular dynamics simulations using the program CPMD with different values of the fictitious electron mass in the microcanonical and canonical ensembles, Born-Oppenheimer molecular dynamics using the programs CPMD and CP2K in the microcanonical ensemble, and Metropolis Monte Carlo using CP2K in the canonical ensemble. With the exception of one simulation for 128 water molecules, all other simulations were carried out for systems consisting of 64 molecules. It is found that the structural and thermodynamic properties of these simulations are in excellent agreement with each other as long as adiabatic sampling is maintained in the Car-Parrinello molecular dynamics simulations either by choosing a sufficiently small fictitious mass in the microcanonical ensemble or by Nos{acute e}-Hoover thermostats in the canonical ensemble. Using the Becke-Lee-Yang-Parr exchange and correlation energy functionals and norm-conserving Troullier-Martins or Goedecker-Teter-Hutter pseudopotentials, simulations at a fixed density of 1.0 g/cm{sup 3} and a temperature close to 315 K yield a height of the first peak in the oxygen-oxygen radial distribution function of about 3.0, a classical constant-volume heat capacity of about 70 J K{sup -1} mol{sup -1}, and a self-diffusion constant of about 0.1 Angstroms{sup 2}/ps.
First-principles 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.
Risk reduction and the privatization option: First principles
Bjornstad, D.J.; Jones, D.W.; Russell, M. [Joint Inst. for Energy and Environment, Knoxville, TN (United States); Cummings, R.C.; Valdez, G. [Georgia State Univ., Atlanta, GA (United States); Duemmer, C.L. [Hull, Duemmer and Garland (United States)
1997-06-25
The Department of Energy`s Office of Environmental Restoration and Waste Management (EM) faces a challenging mission. To increase efficiency, EM is undertaking a number of highly innovative initiatives--two of which are of particular importance to the present study. One is the 2006 Plan, a planning and budgeting process that seeks to convert the clean-up program from a temporally and fiscally open-ended endeavor to a strictly bounded one, with firm commitments over a decade-long horizon. The second is a major overhauling of the management and contracting practices that define the relationship between the Department and the private sector, aimed at cost reduction by increasing firms` responsibilities and profit opportunities and reducing DOE`s direct participation in management practices and decisions. The goal of this paper is to provide an independent perspective on how EM should create new management practices to deal with private sector partners that are motivated by financial incentives. It seeks to ground this perspective in real world concerns--the background of the clean-up effort, the very difficult technical challenges it faces, the very real threats to environment, health and safety that have now been juxtaposed with financial drivers, and the constraints imposed by government`s unique business practices and public responsibilities. The approach is to raise issues through application of first principles. The paper is targeted at the EM policy officer who must implement the joint visions of the 2006 plan and privatization within the context of the tradeoff between terminal risk reduction and interim risk management.
Fikri, Mustapha; Makeich, Alexander; Rollmann, Georg; Schulz, Christof; Entel, Peter
2008-07-17
The thermal decomposition of Ga(CH3)3 has been studied both experimentally in shock-heated gases and theoretically within an ab-initio framework. Experiments for pressures ranging from 0.3 to 4 bar were performed in a shock tube equipped with atomic resonance absorption spectroscopy (ARAS) for Ga atoms at 403.3 nm. Time-resolved measurements of Ga atom concentrations were conducted behind incident waves as well as behind reflected shock waves at temperatures between 1210 and 1630 K. The temporal variation in Ga-atom concentration was described by a reaction mechanism involving the successive abstraction of methyl radicals from Ga(CH3)3 (R1), Ga(CH3)2 (R2), and GaCH3 (R3), respectively, where the last reaction is the rate-limiting step leading to Ga-atom formation. The rate constant of this reaction (R3) was deduced from a simulation of the measured Ga-atom concentration profiles using thermochemical data from ab-initio calculations for the reactions R1 and R2 as input. The Rice-Ramsperger-Kassel-Marcus (RRKM) method including variational transition state theory was applied for reaction R3 assuming a loose transition state. Structural parameters and vibrational frequencies of the reactant and transition state required for the RRKM calculations were obtained from first-principles simulations. The energy barrier E3(0) of reaction R3, which is the most sensitive parameter in the calculation, was adjusted until the RRKM rate constant matched the experimental one and was found to be E(0) = 288 kJ/mol. This value is in a good agreement with the corresponding ab-initio value of 266 kJ/mol. The rate constant of reaction R3 was found to be k 3/(cm(3) mol(-1)s(-1)) = 2.34 x 10(11) exp[-23330(K/ T)]. PMID:18578466
Theoretical spectroscopy of acetylene dication and its deuterated species.
Palaudoux, J; Jutier, L; Hochlaf, M
2010-05-21
We mapped the six-dimensional potential energy surface of the electronic ground state of HCCH(++)(X (3)Sigma(g) (-)) dication using the coupled cluster approach. This potential energy surface is incorporated later into perturbative and full variational treatments to solve the nuclear motions. We derived a set of spectroscopic data for HCCH(++), HCCD(++), and DCCD(++). Our calculations reveal the presence of anharmonic resonances even at low energies, which complicates their assignment by vibrational quantum numbers. In light of our theoretical vibrational spectra, we propose an assignment of the experimental vibrationally resolved valence double ionization spectra of HCCH, HCCD, and DCCD. These spectra are viewed to be mostly composed by a pure vibrational progression involving the CC stretching mode together with a second progression involving both the CC stretching and the bendings. PMID:20499957
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
Gonzálvez, Alicia G; González Ureña, Ángel
2012-10-01
A laser spectroscopic technique is described that combines transmission and resonance-enhanced Raman inelastic scattering together with low laser power (< 30 mW) and good spatial resolution (< 200 ?m) as operational features. The monitoring of the transmitted inelastic scattering provides an increased signal-to-noise ratio because the low fluorescence background and, on the other hand, the resonant character of the laser excitation, leads to enhanced analytical sensitivity. The spectroscopic technique was applied to investigate the carotenoid content (specifically the ?-carotene concentration) of distinct samples that included fruits, reaching a detection limit of the order of hundreds of picograms in solid samples, which is below the level needed for typical food control analysis. Additional features of the present development are direct sampling, noninvasive character, and fast analysis that is not time consuming. From a theoretical point of view, a model for the Raman signal dependence on the sample thickness is also presented. Essentially, the model considers the sample to be homogeneous and describes the underlying physics using only three parameters: the Raman cross-section, the laser-radiation attenuation cross-section, and the Raman signal attenuation cross-section. The model was applied successfully to describe the sample-size dependence of the Raman signal in both ?-carotene standards and carrot roots. The present technique could be useful for direct, fast, and nondestructive investigations in food quality control and analytical or physiological studies of animal and human tissues. PMID:23031699
Electron Exchange and Conduction in Nontronite from First-Principles
Alexandrov, Vitali Y.; Neumann, Anke; Scherer, Michelle; Rosso, Kevin M.
2013-01-11
Fe-bearing clay minerals serve as an important source and sink for electrons in redox reactions in various subsurface geochemical environments, and electron transfer (ET) properties of the Fe2+/Fe3+ redox couple play a decisive role in a variety of physicochemical processes involving clays. Here, we apply first-principles calculations using both periodic GGA+U planewave and Hartree-Fock molecular-cluster frameworks in conjuction with small polaron hopping approach and Marcus electron transfer theory to examine electron exchange mobilities in an Fe-rich smectite, taking nontronite as a case study. GGA+U calculations of the activation barrier for small-polaron migration provide rates of electron hopping that agree very well with values deduced from variable temperature Mössbauer data (M. V. Schaefer, et. al., Environ. Sci. Technol. 45, 540, (2011)), indicating a surprisingly fast electron mobility at room temperature. Based on molecular cluster calculations, we show that the state with tetrahedral Fe2+ ion in the nontronite lattice is about 0.9 eV higher than the one with octahedral Fe2+. Also, evaluation of the ET rates for the Fe2+/Fe3+ electron hopping in tetrahedral (TS) and octahedral sheets (OS), as well as across the sheets (TS–OS) shows that the dominant contribution to the bulk electronic conductivity should come from the ET within the OS. Deprotonation of structural OH groups mediating ET between the Fe ions in the OS is found to decrease the internal reorganization energy and to increase the magnitude of the electronic coupling matrix element, whereas protonation (to OH2 groups) has the opposite effect. Overall, our calculations suggest that the major factors affecting ET rates are the nature and structure of the nearest-neighbor local environment and the degree of covalency of the bonds between Fe and ligands mediating electron hops. The generally higher reorganization energy and weaker electronic coupling found in Fe-bearing clay minerals leads to electron mobilities much lower than in iron oxides.
Diffusion in Post-Perovskite from First Principles
NASA Astrophysics Data System (ADS)
Ammann, M. W.; Brodholt, J. P.; Dobson, D.
2009-12-01
Diffusion controls many physical and chemical processes in the solid Earth. While experiments are limited to conditions of the shallow lower mantle, ab initio calculations allow us to probe conditions at any pressure and temperature. By applying harmonic transition state theory (Vineyard-theory [1]), we have calculated self-diffusion coefficients of MgO and MgSiO3 perovskite and post-perovskite at conditions of Earth's lower mantle from first principles. Our analysis shows that magnesium and oxygen in MgSiO3 perovskite migrate via simple single jumps to nearest neighbour sites. For silicon we propose diffusion via a six-jump cycle on the silicon-magnesium sublattice. We compared our absolute diffusion rates with experimental data of magnesium and oxygen diffusion in MgO at temperatures between 1873 K and 2273 K and at pressures ranging from 7 GPa to 35 GPa. Our calculated diffusion rates of magnesium, silicon and oxygen in MgSiO3 perovskite have also been compared with experiments at 25 GPa with temperatures ranging from 1259 K to 2273K. All our diffusion rates are in excellent agreement with all the available experimental data. The six-jump cycle mechanism works also in MgSiO3 post-perovskite allowing both cations to cross the SiO6-octahedra-layers. Nevertheless, we find that cation diffusion in MgSiO3 post-perovskite is very anisotropic. This is not due to slow diffusion across the SiO6-octahedra-layers (which is similar to diffusion in perovskite), but because of very fast diffusion along the a-axis. Therefore, if deformation takes place via diffusion-controlled mechanism, we would expect post-perovskite to be weaker than perovskite. There is strong evidence that lower mantle deforms in the diffusion-creep regime. However, how a material with strongly anisotopic diffusion deforms under low stresses is not clear. Although it is generally believed that diffusion creep does not produce a lattice preferred orientation (LPO), this may not be true for a diffusionally very anisotropic material. If so, we would expect a significant history-dependent rheology in D’’, with the development of fast diffusion creep as grains become increasingly oriented. This may lead to localised shear-zones that separate large regions undergoing little deformation. REFERENCES: [1] G. H. Vineyard (1957), J. Phys. Chem. Solids 3 121-127
First-principles study of semiconductor and metal surfaces
NASA Astrophysics Data System (ADS)
Kim, Sungho
In this dissertation, we study the electronic and geometric structure of semiconductors and metal surfaces based on quantum mechanical first-principles calculations. We determine the geometry of vacancy defects of hydrogen adsorbed on a Pd(111) surface by treating the motion of a hydrogen atom, in addition to electrons, quantum mechanically. The calculated ground state wave function has high probability density in the hcp site located at the center of the vacancy instead of the fcc sites where the potential is minimum and hydrogen atoms on a Pd(111) surface normally adsorb. The geometry of quantum mechanically determined divacancy provides a simple and clear explanation for the scanning tunneling microscopy (STM) images of these defects that appear as three-lobed objects as observed in recent experiments [Mitsui, et al, Nature 422, 705 (2003)]. We employ the same principle to successfully elucidate the STM images of larger size vacancy defects. Our model also provides a compelling argument to explain the unusual recent experimental result that aggregates of three or more hydrogen vacancies are much more active in adsorption of hydrogen molecules while two-vacancy defects are never inactive. The InAs (110) surfaces appear lower than GaSb in STM images. This height difference is caused primarily by differences in the electronic structure of the two materials according to our calculations in a good agreement with measurements. In contrast, local variations in the apparent height of (110) surface atoms at InSb- or GaAs-like interfaces arise primarily from geometric distortions associated with local differences in bond length. The arsenic atoms adsorb preferably at the bridge sites between the dimerized Sb atoms on Sb-terminating (001) surfaces. Indium atoms, on the other hand, have somewhat equal probabilities at a few different sites on Ga-terminating (001) surfaces. Our calculated energies for atomic intermixing indicate that anion exchanges are exothermic for As atoms on Ga-terminating (001) interfaces but endothermic for In atoms on Sb-terminating (001) interfaces. This difference may explain why GaAs interfaces are typically more disordered than InSb interfaces in these heterostructures.
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.
Richmond, Geraldine L.
Vibrational Sum-Frequency Spectroscopy of Alkane/Water Interfaces: Experiment and Theoretical, 2002 The vibrational spectra of interfacial water at a series of alkane/water interfaces have been to characterize the water-water and alkane-water interactions present at these hydrophobic/aqueous interfaces
Dielectric spectroscopy in a micromachined flow cytometer: theoretical and practical considerations
Cheung, Karen C.
Dielectric spectroscopy in a micromachined flow cytometer: theoretical and practical considerations the influence of different cell properties, such as size, membrane capacitance and cytoplasm conductivity, on the impedance spectrum as measured in a microfabricated cytometer. A dielectric sphere of equivalent complex
First principle optoelectronic studies of visible light sensitive CZT
NASA Astrophysics Data System (ADS)
Khan, Imad; Aliabad, H. A. Rahnamaye; Ahmad, Waqar; Ali, Zahid; Ahmad, Iftikhar
2013-11-01
The most attractive optical material in solar cells applications, CdZnTe (CZT), has been extensively studied both, experimentally and theoretically. The results obtained by various theoretical models are way too far from the experiments and hence, they fail to demonstrate the true theoretical picture of the compound. In the present study the electronic band gap, band gap bowing and optical properties of Cd1-xZnxTe (0 ? x ? 1) are revisited with an effective theoretical model to provide accurate theoretical foundations to the experimental results. The calculations are performed with the full potential linearized augmented plane wave (FPLAPW) method with the generalized gradient approximation + modified Becke-Johnson (GGAmBJ) potential. The calculated results are in agreement with the experimental results. The band gap increases non-lineally with the concentration. The optical properties of the compound like absorption coefficient, refractive index, reflectivity, energy loss function and oscillator strength are also calculated. The number of effective electrons in optical excitations decreases with the increase in concentration of Zn.
First-principles theory, coarse-grained models, and simulations of ferroelectrics.
Waghmare, Umesh V
2014-11-18
CONSPECTUS: A ferroelectric crystal exhibits macroscopic electric dipole or polarization arising from spontaneous ordering of its atomic-scale dipoles that breaks inversion symmetry. Changes in applied pressure or electric field generate changes in electric polarization in a ferroelectric, defining its piezoelectric and dielectric properties, respectively, which make it useful as an electromechanical sensor and actuator in a number of applications. In addition, a characteristic of a ferroelectric is the presence of domains or states with different symmetry equivalent orientations of spontaneous polarization that are switchable with large enough applied electric field, a nonlinear property that makes it useful for applications in nonvolatile memory devices. Central to these properties of a ferroelectric are the phase transitions it undergoes as a function of temperature that involve lowering of the symmetry of its high temperature centrosymmetric paraelectric phase. Ferroelectricity arises from a delicate balance between short and long-range interatomic interactions, and hence the resulting properties are quite sensitive to chemistry, strains, and electric charges associated with its interface with substrate and electrodes. First-principles density functional theoretical (DFT) calculations have been very effective in capturing this and predicting material and environment specific properties of ferroelectrics, leading to fundamental insights into origins of ferroelectricity in oxides and chalcogenides uncovering a precise picture of electronic hybridization, topology, and mechanisms. However, use of DFT in molecular dynamics for detailed prediction of ferroelectric phase transitions and associated temperature dependent properties has been limited due to large length and time scales of the processes involved. To this end, it is quite appealing to start with input from DFT calculations and construct material-specific models that are realistic yet simple for use in large-scale simulations while capturing the relevant microscopic interactions quantitatively. In this Account, we first summarize the insights obtained into chemical mechanisms of ferroelectricity using first-principles DFT calculations. We then discuss the principles of construction of first-principles model Hamiltonians for ferroelectric phase transitions in perovskite oxides, which involve coarse-graining in time domain by integrating out high frequency phonons. Molecular dynamics simulations of the resulting model are shown to give quantitative predictions of material-specific ferroelectric transition behavior in bulk as well as nanoscale ferroelectric structures. A free energy landscape obtained through coarse-graining in real-space provides deeper understanding of ferroelectric transitions, domains, and states with inhomogeneous order and points out the key role of microscopic coupling between phonons and strain. We conclude with a discussion of the multiscale modeling strategy elucidated here and its application to other materials such as shape memory alloys. PMID:25361389
First-Principles Investigations of Defects in Minerals
NASA Astrophysics Data System (ADS)
Verma, Ashok K.
2011-07-01
The ideal crystal has an infinite 3-dimensional repetition of identical units which may be atoms or molecules. But real crystals are limited in size and they have disorder in stacking which as called defects. Basically three types of defects exist in solids: 1) point defects, 2) line defects, and 3) surface defects. Common point defects are vacant lattice sites, interstitial atoms and impurities and these are known to influence strongly many solid-state transport properties such as diffusion, electrical conduction, creep, etc. In thermal equilibrium point defects concentrations are determined by their formation enthalpies and their movement by their migration barriers. Line and surface defects are though absent from the ideal crystal in thermal equilibrium due to higher energy costs but they are invariably present in all real crystals. Line defects include edge-, screw- and mixed-dislocations and their presence is essential in explaining the mechanical strength and deformation of real crystals. Surface defects may arise at the boundary between two grains, or small crystals, within a larger crystal. A wide variety of grain boundaries can form in a polycrystal depending on factors such growth conditions and thermal treatment. In this talk we will present our first-principles density functional theory based defect studies of SiO2 polymorphs (stishovite, CaCl2-, ?-PbO2-, and pyrite-type), Mg2SiO4 polymorphs (forsterite, wadsleyite and ringwoodite) and MgO [1-3]. Briefly, several native point defects including vacancies, interstitials, and their complexes were studied in silica polymorphs upto 200 GPa. Their values increase by a factor of 2 over the entire pressure range studied with large differences in some cases between different phases. The Schottky defects are energetically most favorable at zero pressure whereas O-Frenkel pairs become systematically more favorable at pressures higher than 20 GPa. The geometric and electronic structures of defects and migrating ions vary largely among different types of defects. In particular, the O-defects introduce localized electronic states. For Mg2SiO4 polymorphs native and protonic point defects were investigated upto 30 GPa. The Mg2+-Frenkel defects in forsterite and MgO pseudo-Schottky defects in wadsleyite and ringwoodite are energetically most favorable. Mg migration is easiest in forsterite and ringwoodite whereas Si migration is easiest in wadsleyite. Protons show substantially effect on structural transition pressures and PV equations-of-states. In our work on MgO, we showed that the point defect formation is easier in grain boundary interfacial regions than in bulk and pressure increasingly stabilizes interfacial vacancies relative to bulk thereby causing as enhancement in the vacancy concentrations. Symmetric tilt grain boundaries show structural phase transitions to asymmetric tilt grain boundaries under pressure.
First-principles 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.
NASA Astrophysics Data System (ADS)
Khazaei, Mohammad; Farajian, Amir A.; Kawazoe, Yoshiyuki
2005-10-01
A general approach is introduced to calculate field emission properties of any kind of nanostructure based on the first-principles local density of states (LDOS) and effective potentials. The experimental field emission spectroscopy images are explained as LDOS at the structure-vacuum barrier, weighted by the probability of electron tunneling. The method excellently reproduces the experimental field emission patterns of pristine capped carbon nanotubes. We show that cesium adsorbates even with a low doping ratio of one dopant per nanotube increase the emission current around 2.5 times, due to a generated dipole field.
First-principles study of the electronic structure of NiS, NiS2, and Ni3S2
NASA Astrophysics Data System (ADS)
Noyola, Joaquin; Zhang, Qiming
2012-02-01
First-principles calculations of the electronic structure of NiS, NiS2, and Ni3S2 are performed. The HSE hybrid functionals are used as the primary exchange-correlation scheme. The resulting electronic structures at various phases are analyzed. And the results are compared with previous experimental and theoretical work.
First-principles study of the electronic structure of NiS, NiS2, and Ni3S2
NASA Astrophysics Data System (ADS)
Noyola, Joaquin; Zhang, Qiming
2012-03-01
First-principles study of the electronic structure of NiS, NiS2, and Ni3S2 are performed. DFT+U and HSE hybrid functional are used as the primary exchange-correlation schemes. The resulting electronic structures at various phases are analyzed, and the results compared with previous experimental and theoretical work.
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 simulations of Li ion migration in materials related to LiPON electrolytes
Holzwarth, Natalie
First principles simulations of Li ion migration in materials related to LiPON electrolytes Y. A electrolytes, we have carried out first principles calculations of several related crystalline materials. Sim state film electrolyte LiPON, developed at Oak Ridge National Laboratory,[1, 2] is the most widely used
Mechanisms of Li+ diffusion in crystalline -and -Li3PO4 electrolytes from first principles
Holzwarth, Natalie
Mechanisms of Li+ diffusion in crystalline - and -Li3PO4 electrolytes from first principles Yaojun electrolytes for use in batteries and related technologies. We have used first-principles modeling techniques diffusion mechanisms in idealized crystals of the electrolyte material Li3PO4 in both the and crystalline
Length dependence of electron transport through molecular wires--a first principles perspective.
Khoo, Khoong Hong; Chen, Yifeng; Li, Suchun; Quek, Su Ying
2015-01-01
One-dimensional wires constitute a fundamental building block in nanoscale electronics. However, truly one-dimensional metallic wires do not exist due to Peierls distortion. Molecular wires come close to being stable one-dimensional wires, but are typically semiconductors, with charge transport occurring via tunneling or thermally-activated hopping. In this review, we discuss electron transport through molecular wires, from a theoretical, quantum mechanical perspective based on first principles. We focus specifically on the off-resonant tunneling regime, applicable to shorter molecular wires (first principles theory are discussed, starting with the computational workhorse - density functional theory (DFT), and moving on to many-electron GW methods as well as GW-inspired DFT + Sigma calculations. These different levels of theory are applied in two major computational frameworks - complex band structure (CBS) calculations to estimate the tunneling decay constant, beta, and Landauer-Buttiker transport calculations that consider explicitly the effects of contact geometry, and compute the transmission spectra directly. In general, for the same level of theory, the Landauer-Buttiker calculations give more quantitative values of beta than the CBS calculations. However, the CBS calculations have a long history and are particularly useful for quick estimates of beta. Comparing different levels of theory, it is clear that GW and DFT + Sigma calculations give significantly improved agreement with experiment compared to DFT, especially for the conductance values. Quantitative agreement can also be obtained for the Seebeck coefficient - another independent probe of electron transport. This excellent agreement provides confirmative evidence of off-resonant tunneling in the systems under investigation. Calculations show that the tunneling decay constant beta is a robust quantity that does not depend on details of the contact geometry, provided that the same contact geometry is used for all molecular lengths considered. However, because conductance is sensitive to contact geometry, values of beta obtained by considering conductance values where the contact geometry is changing with the molecular junction length can be quite different. Experimentally measured values of beta in general compare well with beta obtained using DFT + Sigma and GW transport calculations, while discrepancies can be attributed to changes in the experimental contact geometries with molecular length. This review also summarizes experimental and theoretical efforts towards finding perfect molecular wires with high conductance and small beta values. PMID:25407785
First-Principles Investigation of Water Properties at Functionalized Silicon surface
NASA Astrophysics Data System (ADS)
Lee, Donghwa; Schwegler, Eric; Kanai, Yosuke
2013-03-01
Numerous experimental and theoretical investigations have been made to understand the behavior of water molecules under various conditions. Interfacial water behavior at semiconductor interfaces is one of the most important areas of investigation for diverse industrial applications such as crystal growth, lubrication, catalysis, electrochemistry and sensors. Although the terms, hydrophobic and/or hydrophilic, are often used to describe the properties of water in this context at macroscopic level, the effect of hydrophobicity on water behavior at nano-scale interfaces is still not well understood. Computational simulations could offer atomistic basis to build a better foundation for understanding this important dynamics. In this study, first principles molecular dynamics is employed to investigate the water behavior at silicon surfaces that are functionalized with several different molecules. In particular, various analysis methods are used to elucidate the effect of surface polarity on structural and dynamical properties of interfacial water. Our studies show that properties of interfacial water are not always governed by surface polarity alone but also by other atomistic factors. Numerous experimental and theoretical investigations have been made to understand the behavior of water molecules under various conditions. Interfacial water behavior at semiconductor interfaces is one of the most important areas of investigation for diverse industrial applications such as crystal growth, lubrication, catalysis, electrochemistry and sensors. Although the terms, hydrophobic and/or hydrophilic, are often used to describe the properties of water in this context at macroscopic level, the effect of hydrophobicity on water behavior at nano-scale interfaces is still not well understood. Computational simulations could offer atomistic basis to build a better foundation for understanding this important dynamics. In this study, first principles molecular dynamics is employed to investigate the water behavior at silicon surfaces that are functionalized with several different molecules. In particular, various analysis methods are used to elucidate the effect of surface polarity on structural and dynamical properties of interfacial water. Our studies show that properties of interfacial water are not always governed by surface polarity alone but also by other atomistic factors. This work is Prepared by LLNL under Contract DE-AC52-07NA27344.
First-Principles Study of Casimir Repulsion in Metamaterials
Yannopapas, Vassilios [Department of Materials Science, University of Patras, GR-26504 Patras (Greece); Department of Physics, Sofia University, James Bourchier 5 Boulevard, 1164 Sofia (Bulgaria); Vitanov, Nikolay V. [Department of Physics, Sofia University, James Bourchier 5 Boulevard, 1164 Sofia (Bulgaria); Institute of Solid State Physics, Bulgarian Academy of Sciences, Tsarigradsko chaussee 72, 1784 Sofia (Bulgaria)
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 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
Defect complexes in GaAs: First-principles calculations
NASA Astrophysics Data System (ADS)
Janotti, A.; Fazzio, A.; Piquini, P.; Mota, R.
1997-11-01
The electronic and structural properties of selected defect complexes in GaAs, created during electron or ion irradiation, are studied. An ab initio calculation based on pseudopotential density-functional theory is used. A supercell with 128 atoms is adopted in Car-Parrinello scheme. For the antistructure pair (AsGa+GaAs), from the total-energy calculations, first donor, first acceptor, and second acceptor levels are observed, and a comparison is made with earlier, both theoretical and experimental, results. Two other possible defect complexes (VAs+AsGa+Gai and VGa+GaAs+Asi), are discussed. It is shown that the first one presents a metastable configuration, and the second one is unstable presenting a spontaneous recombination. For all defect complexes the formation energies and charge densities are discussed.
First-principles calculations of zero-field splitting parameters.
Ganyushin, Dmitry; Neese, Frank
2006-07-14
In this work, an implementation of an approach to calculate the zero-field splitting (ZFS) constants in the framework of ab initio methods such as complete active space self-consistent field, multireference configuration interaction, or spectroscopy oriented configuration interaction is reported. The spin-orbit coupling (SOC) contribution to ZFSs is computed using an accurate multicenter mean-field approximation for the Breit-Pauli Hamiltonian. The SOC parts of ZFS constants are obtained directly after diagonalization of the SOC operator in the basis of a preselected number of roots of the spin-free Hamiltonian. This corresponds to an infinite order treatment of the SOC in terms of perturbation theory. The spin-spin (SS) part is presently estimated in a mean-field fashion and appears to yield results close to the more complete treatments available in the literature. Test calculations for the first- and second-row atoms as well as first-row transition metal atoms and a set of diatomic molecules show accurate results for the SOC part of ZFSs. SS contributions have been found to be relatively small but not negligible (exceeding 1 cm(-1) for oxygen molecule). At least for the systems studied in this work, it is demonstrated that the presented method provides much more accurate estimations for the SOC part of ZFS constants than the emerging density functional theory approaches. PMID:16848573
NASA Astrophysics Data System (ADS)
Mukhopadhyay, S.; Gutmann, M. J.; Jura, M.; Jochym, D. B.; Jimenez-Ruiz, M.; Sturniolo, S.; Refson, K.; Fernandez-Alonso, F.
2013-12-01
A combination of neutron-scattering experiments and first-principles calculations using density-functional theory have been performed to explore the structural and dynamical properties of the single-component organic ferroelectric croconic acid. Neutron diffraction and spectroscopy have been used to determine the location and underlying vibrational motions of the hydrogen ions within the crystalline lattice, respectively. On the computational front we find that dispersion corrections within the generalised-gradient approximation are essential to obtain a satisfactory crystal structure for this organic solid. Two distinct types of hydrogen ions in the crystal also have been identified, located at the ‘hinge’ and ‘terrace’ positions of a pleated, accordion-like structure. Phonon calculations and simulated neutron spectra show that the prominent doublet observed at ca. 1000 cm-1 arises from out-of-plane motions associated with these two types of hydrogen ions. Calculated Born-effective-charge tensors yield an anomalously high dynamic charge centered on the hydrogen ions at the hinges, a finding which serves to identify the primary motif underpinning ferroelectric behaviour in this novel material.
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).
First-principle study of the physics properties of DO3-Mg3Nd compound under high pressure
NASA Astrophysics Data System (ADS)
Zhang, Xudong; Ying, Caihong; Jiang, Wei; Shi, Haifeng
2014-09-01
A theoretical investigation on the structural, phonon, elastic and thermodynamic properties of the DO3-Mg3Nd compound has been conducted through the first-principles calculations within the frame of the density functional theory (DFT) and the density functional perturbation theory (DFPT) in the VASP and PHONONPY code. We calculate the phonon spectrum and phonon density of states under different pressures and find that the DO3-Mg3Nd compound keeps dynamically stable up to 100 GPa. The elastic constants and thermodynamic quantities as a function of pressures and temperatures are also reported and discussed.
Kanagaprabha, S. [Department of Physics, Kamaraj College, Tuticorin, Tamil nadu-628003 (India); Rajeswarapalanichamy, R., E-mail: rrpalanichamy@gmail.com; Sudhapriyanga, G., E-mail: rrpalanichamy@gmail.com; Murugan, A., E-mail: rrpalanichamy@gmail.com; Santhosh, M., E-mail: rrpalanichamy@gmail.com [Department of Physics, N.M.S.S.V.N College, Madurai, Tamilnadu-625019 (India); Iyakutti, K. [Department of Physics and Nanotechnology, SRM University, Chennai, Tamilnadu-603203 (India)
2014-04-24
The electronic, structural and mechanical properties of ZrH and ZrH{sub 2} are investigated by means of first principles calculation based on density functional theory as implemented in VASP code with generalized gradient approximation. The calculated ground state properties are in good agreement with previous experimental and other theoretical results. Among the six crystallographic structures considered for ZrH, ZB phase is found to be the most stable phase, whereas ZrH{sub 2} is energetically stable in tetragonal structure at ambient condition. A structural phase transition from ZB?NaCl at a pressure 10 GPa is predicted for ZrH.
NASA Astrophysics Data System (ADS)
Mary, J. Arul; Vijaya, J. Judith; Bououdina, M.; Kennedy, L. John; Dai, J. H.; Song, Y.
2015-02-01
Ce, Cu co-doped ZnO (Zn1-2xCexCuxO: x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) nanocrystals were synthesized by a microwave combustion method. These nanocrystals were investigated by using X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The stability and magnetic properties of Ce and Cu co-doped ZnO were probed by first principle calculations. XRD results revealed that all the compositions are single crystalline. hexagonal wurtzite structure. The optical band gap of pure ZnO was found to be 3.22 eV, and it decreased from 3.15 to 3.10 eV with an increase in the concentration of Cu and Ce content. The morphologies of Ce and Cu co-doped ZnO samples confirmed the formation of nanocrystals with an average grain size ranging from 70 to 150 nm. The magnetization measurement results affirmed the antiferro and ferromagnetic state for Ce and Cu co-doped ZnO samples and this is in agreement with the first principles theoretical calculations.
First-principle Simulations of Heavy Fermion Materials
NASA Astrophysics Data System (ADS)
Dong, Ruanchen
Heavy fermion materials, one of the most challenging topics in condensed matter physics, pose a variety of interesting properties and have attracted extensive studies for decades. Although there has been great success in explaining many ground- state properties of solids, the well-known theoretical calculations based on density functional theory (DFT) in its popular local density approximation (LDA) fail to describe heavy fermion materials due to improper treatment of many-body correlation effects. Here with the implementations of dynamical mean-field theory (DMFT) and the Gutzwiller variational method, the computational simulation of the heavy fermion materials is explored further and better compared with experimental data. In this dissertation, first, the theoretical background of DMFT and LDA+G methods is described in detail. The rest is the application of these techniques and is basically divided into two parts. First, the continuous-time quantum Monte Carlo (CT-QMC) method combined with DMFT is used to calculate and compare both the periodic Anderson model (PAM) and the Kondo lattice model (KLM). Different parameter sets of both models are connected by the Schrieffer-Wolff transformation. For spin and orbital degeneracy N = 2 case, a special particle-hole symmetric case of PAM at half-filling which always fixes one electron per impurity site is compared with the results of the KLM. We find a good mapping between PAM and KLM in the limit of large on-site Hubbard interaction U for different properties like self-energy, quasiparticle residue and susceptibility. This allows us to extract quasiparticle mass renormalizations for the f-electrons directly from KLM. The method is further applied to higher degenerate cases and to the realistic heavy fermion system CeRhIn5 in which the estimate of the Sommerfeld coefficient is proven to be close to the experimental value. Second, a series of Cerium based heavy fermion materials is studied using a combination of local density functional theory and many-body Gutzwiller approximation. Computed orbital dependent electronic mass enhancement parameters are compared with available data extracted from measured values of the Sommerfeld coefficient. The Gutzwiller density functional theory is shown to follow the trends across a variety of Ce compounds remarkably well, and to give important insights on orbital selective mass renormalizations that allows a better understanding of a wide spread of data.
High-pressure elastic properties of major materials of Earth's mantle from first principles
NASA Astrophysics Data System (ADS)
Karki, Bijaya B.; Stixrude, Lars; Wentzcovitch, Renata M.
2001-11-01
The elasticity of materials is important for our understanding of processes ranging from brittle failure, to flexure, to the propagation of elastic waves. Seismologically revealed structure of the Earth's mantle, including the radial (one-dimensional) profile, lateral heterogeneity, and anisotropy are determined largely by the elasticity of the materials that make up this region. Despite its importance to geophysics, our knowledge of the elasticity of potentially relevant mineral phases at conditions typical of the Earth's mantle is still limited: Measuring the elastic constants at elevated pressure-temperature conditions in the laboratory remains a major challenge. Over the past several years, another approach has been developed based on first-principles quantum mechanical theory. First-principles calculations provide the ideal complement to the laboratory approach because they require no input from experiment; that is, there are no free parameters in the theory. Such calculations have true predictive power and can supply critical information including that which is difficult to measure experimentally. A review of high-pressure theoretical studies of major mantle phases shows a wide diversity of elastic behavior among important tetrahedrally and octahedrally coordinated Mg and Ca silicates and Mg, Ca, Al, and Si oxides. This is particularly apparent in the acoustic anisotropy, which is essential for understanding the relationship between seismically observed anisotropy and mantle flow. The acoustic anisotropy of the phases studied varies from zero to more than 50% and is found to depend on pressure strongly, and in some cases nonmonotonically. For example, the anisotropy in MgO decreases with pressure up to 15 GPa before increasing upon further compression, reaching 50% at a pressure of 130 GPa. Compression also has a strong effect on the elasticity through pressure-induced phase transitions in several systems. For example, the transition from stishovite to CaCl2 structure in silica is accompanied by a discontinuous change in the shear (S) wave velocity that is so large (60%) that it may be observable seismologically. Unifying patterns emerge as well: Eulerian finite strain theory is found to provide a good description of the pressure dependence of the elastic constants for most phases. This is in contrast to an evaluation of Birch's law, which shows that this systematic accounts only roughly for the effect of pressure, composition, and structure on the longitudinal (P) wave velocity. The growing body of theoretical work now allows a detailed comparison with seismological observations. The athermal elastic wave velocities of most important mantle phases are found to be higher than the seismic wave velocities of the mantle by amounts that are consistent with the anticipated effects of temperature and iron content on the P and S wave velocities of the phases studied. An examination of future directions focuses on strategies for extending first-principles studies to more challenging but geophysically relevant situations such as solid solutions, high-temperature conditions, and mineral composites.
First principles study of NH3 adsorption on carbon nanowires
NASA Astrophysics Data System (ADS)
Tapia, Jorge-Alejandro; Sanchez, Alvaro-Daniel; Acosta, Cesar; Canto, Gabriel
2009-03-01
Recently has been reported a new type of one-dimensional carbon structures. Carbon nanowires formed by a linear carbon-atom chain inside an armchair (5,5) carbon nanotube has been observed using high-resolution transmission electron microscopy. Theoretical and experimental studies of the NH3 adsorption in the carbon nanotubes report changes in the electronic properties of the carbon nanotubes. In the present work we have studied the electronic and structure properties of carbon nanowires (chain@SWCNT) when NH3 atoms are adsorbed. We used the Density Functional Theory and the calculations where performed by the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We have analyzed the changes in the atomic structure and density of states (DOS). We found that the electronic character of the carbon chain of the chain@SWCNT system, can be modulate by NH3 adsorption. This research was supported by SEP under Grant No. PROMEP/103.5/07/2595 and the Consejo Nacional de Ciencia y Tecnolog'ia (Conacyt) under Grants No. 82497 and 60534.
First Principles 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.
First-principles studies of electronic, optical, and vibrational properties of LaVO4 polymorph
NASA Astrophysics Data System (ADS)
Sun, Liming; Zhao, Xian; Li, Yanlu; Li, Pan; Sun, Honggang; Cheng, Xiufeng; Fan, Weiliu
2010-11-01
First-principles calculations of electronic, optical, and vibrational properties of LaVO4 polymorph were performed with the density functional theory plane-wave pseudopotential method. The results of the electronic structure reveal that the different coordinated structure for monoclinic LaVO4 leads to an indirect band gap, while tetragonal LaVO4 has a direct band gap. Besides, the analysis of the electronic structure shows ionic nature in La-O bonds and covalent nature in V-O bonds. From further study in chemical bonding behavior, we find that the V-O covalent bonds have four types: ? bonding, ? bonding, ?? antibonding, and ?? antibonding states. Various optical properties, including the dielectric function, reflectivity, absorption coefficient, refractive index, and the energy-loss spectrum as functions of the photon energy were calculated. Our calculations indicate that monoclinic LaVO4 has excellent dielectric properties along [0 0 1] direction. In the optical-frequency (? ??) contributed from electrons the optical properties of tetragonal LaVO4 show the isotropy, while the diagonal components of static dielectric tensors ?(0) of tetragonal LaVO4 have the ?xx=?yy??zz relation by adding the lattice vibration contribution (? ?0) to the electronic dielectric tensor. The vibrational spectra of LaVO4 polymorph have also been calculated from first principles by the linear response method. The calculated frequencies are in good agreement with the experimental data available for these crystals obtained by the methods of infrared and Raman spectroscopies. The vibrational spectra of monoclinic and tetragonal LaVO4 crystal exhibit three groups of frequencies: the low-frequency (<240 cm-1), middle-frequency (270-450 cm-1), and high-frequency region (850-970 cm-1), according to the vibration dominated by translation of La atoms, the bending vibration of O-V-O bonds, and stretching vibration of O-V-O bonds, respectively. Our studies report on microstructure of LaVO4 polymorph, and provide useful information for the potential application of this material.
NASA Astrophysics Data System (ADS)
Guo, W.; Granger, J.; Sigman, D. M.
2010-12-01
Coupled fractionations of N and O isotopes during biological nitrate reduction provide important constraints on the marine nitrogen cycle at present and in the geologic past. Recent laboratory experiments with mono-cultures of nitrate-assimilative algae and plankton, and denitrifying bacteria demonstrate that N and O isotopic compositions of the residual nitrate co-vary linearly with a constant ratio (i.e., ??18O: ??15N) of ~1 or ~0.6 [1]. These systematic variations have been inferred to derive from the kinetic isotope fractionations associated with nitrate reductases. The isotope fractionation mechanisms at the enzymatic level, however, remain elusive. Here we present models of isotope fractionations accompanying the nitrate reduction (NO3-?NO2-) by three functional types of nitrate reductases, using techniques from ab initio, transition state and statistical thermodynamic theory. We consider three types of nitrate reductases: eukNR (eukaryotic assimilatory nitrate reductase), NAR (prokaryotic respiratory nitrate reductase) and Nap (prokaryotic periplasmic nitrate reductase). All are penta- or hexa-coordinated molybdo-enzymes, but bear considerable differences in protein geometry among functional types. Our models, based on the simplified structures of their active sites, predict N and O isotope effects (15? and 18?) ranging from 32.7 to 36.6‰ and from 33.5 to 34.8‰, respectively, at 300K with 18?:15? ratios of 0.9-1.1. The predicted amplitudes of N and O isotope fractionations are in the range measured for eukNR in vitro (~27‰, Karsh et al. in prep), and also correspond to the upper amplitudes observed for denitrifiers in vivo (~25‰, [1]). Moreover, the computed 18?:15? ratios corroborate the consistent relationships of ~1 observed experimentally for eukNR and the respiratory NAR. These findings indicate the enzymatic reduction is likely the rate-limiting step in most biological nitrate reductions. In addition, the predicted similarity of 18?:15? ratios among different nitrate reductases suggests that the nitrate isotope fractionations by nitrate reductases are governed by the kinetics of the O-N bond cleavage, which incurs negligible differences from variations in surrounding moieties at the active sites. However, our model similarly predicts a 15? of 36.6‰ and 18?:15? of 0.9 for the auxiliary Nap, although it exhibits a 15? of ~15‰ and 18?:15? of ~0.6 in vivo [1]. This discrepancy is suspected to arise from slower binding and release of NO3- from Nap, which could be partially rate-determining in this enzymatic catalysis, or from the assumptions of our modeled enzyme structures. By extending our above models to include the multiply-substituted (clumped) isotopologues, we predict that isotope fractionations during biological nitrate reduction decrease the proportion of 15N-18O bonds in the residual nitrate relative to their expected equilibrium abundances (~0.02‰ decrease for every 1‰ kinetic enrichment in nitrate ?15N). Future quantification of 15N-18O clumped isotope anomalies in natural nitrate may provide additional constraints on the nitrogen cycle in the ocean. Reference: [1] Granger et al. (2010) GCA, 74: 1030-1040.
NASA Astrophysics Data System (ADS)
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 proc
Adsorption of carbon on Pd clusters of nanometer size: A first-principles theoretical study
NASA Astrophysics Data System (ADS)
Neyman, Konstantin M.; Inntam, Chan; Gordienko, Alexei B.; Yudanov, Ilya V.; Rösch, Notker
2005-05-01
Adsorbed atomic C species can be formed in the course of surface reactions and commonly decorate metal catalysts. We studied computationally C adsorption on Pd nanoclusters using an all-electron scalar relativistic density functional method. The metal particles under investigation, Pd55, Pd79, Pd85, Pd116, Pd140, and Pd146, were chosen as fragments of bulk Pd in the form of three-dimensional octahedral or cuboctahedral crystallites, exposing (111) and (100) facets as well as edge sites. These cluster models are shown to yield size-converged adsorption energies. We examined which surface sites of these clusters are preferentially occupied by adsorbed C. According to calculations, surface C atoms form strongly adsorbed carbide species (with adsorption energies of more than 600kJmol-1) bearing a significant negative charge. Surface sites allowing high, fourfold coordination of carbon are overall favored. To avoid effects of adsorbate-adsorbate interaction in the cluster models for carbon species in the vicinity of cluster edges, we reduced the local symmetry of selected adsorption complexes on the nanoclusters by lowering the global symmetry of the nanocluster models from point group Oh to D4h. On (111) facets, threefold hollow sites in the center are energetically preferred; adsorbed C is calculated to be slightly less stable when displaced to the facet borders.
Goddard III, William A.
Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights for oxygen- and carbon-isotope analysis of carbonate minerals since 1950, and was recently established of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results
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.
Lattice structures and electronic properties of CIGS/CdS interface: First-principles calculations
NASA Astrophysics Data System (ADS)
Tang, Fu-Ling; Liu, Ran; Xue, Hong-Tao; Lu, Wen-Jiang; Feng, Yu-Dong; Rui, Zhi-Yuan; Huang, Min
2014-07-01
Using first-principles calculations within density functional theory, we study the atomic structures and electronic properties of the perfect and defective (2VCu+InCu) CuInGaSe2/CdS interfaces theoretically, especially the interface states. We find that the local lattice structure of (2VCu+InCu) interface is somewhat disorganized. By analyzing the local density of states projected on several atomic layers of the two interfaces models, we find that for the (2VCu+InCu) interface the interface states near the Fermi level in CuInGaSe2 and CdS band gap regions are mainly composed of interfacial Se-4p, Cu-3d and S-3p orbitals, while for the perfect interface there are no clear interface states in the CuInGaSe2 region but only some interface states which are mainly composed of S-3p orbitals in the valance band of CdS region.
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.
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
NASA Astrophysics Data System (ADS)
Bondi, Robert J.; Desjarlais, Michael P.; Thompson, Aidan P.; Brennecka, Geoff L.; Marinella, Matthew J.
2013-11-01
We apply first-principles density-functional theory (DFT) calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula to predict electrical conductivity in Ta2Ox (0 ? x ? 5) as a function of composition, phase, and temperature, where additional focus is given to various oxidation states of the O monovacancy (VOn; n = 0,1+,2+). In the crystalline phase, our DFT calculations suggest that VO0 prefers equatorial O sites, while VO1+ and VO2+ are energetically preferred in the O cap sites of TaO7 polyhedra. Our calculations of DC conductivity at 300 K agree well with experimental measurements taken on Ta2Ox thin films (0.18 ? x ? 4.72) and bulk Ta2O5 powder-sintered pellets, although simulation accuracy can be improved for the most insulating, stoichiometric compositions. Our conductivity calculations and further interrogation of the O-deficient Ta2O5 electronic structure provide further theoretical basis to substantiate VO0 as a donor dopant in Ta2O5. Furthermore, this dopant-like behavior is specific to the neutral case and not observed in either the 1+ or 2+ oxidation states, which suggests that reduction and oxidation reactions may effectively act as donor activation and deactivation mechanisms, respectively, for VOn in Ta2O5.
Thermoelectric properties of AgSbTe2 from first-principles calculations
NASA Astrophysics Data System (ADS)
Rezaei, Nafiseh; Hashemifar, S. Javad; Akbarzadeh, Hadi
2014-09-01
The structural, electronic, and transport properties of AgSbTe2 are studied by using full-relativistic first-principles electronic structure calculation and semiclassical description of transport parameters. The results indicate that, within various exchange-correlation functionals, the cubic F d 3 ¯ m and trigonal R 3 ¯ m structures of AgSbTe2 are more stable than two other considered structures. The computed Seebeck coefficients at different values of the band gap and carrier concentration are accurately compared with the available experimental data to speculate a band gap of about 0.1-0.35 eV for AgSbTe2 compound, in agreement with our calculated electronic structure within the hybrid HSE (Heyd-Scuseria-Ernzerhof) functional. By calculating the semiclassical Seebeck coefficient, electrical conductivity, and electronic part of thermal conductivity, we present the theoretical upper limit of the thermoelectric figure of merit of AgSbTe2 as a function of temperature and carrier concentration.
First-principles calculations of CsMX3(M = Sn, Pb; X = Cl, Br, I)
NASA Astrophysics Data System (ADS)
Yuan, Ye; Xu, Run; Xu, Haitao; Wang, Linjun
2013-12-01
CsMX3(M = Sn, Pb; X = Cl, Br, I) are strong candidates for the fast high energy irradiation detectors, ionic conductors, and optoelectronic devices. There are many experimental and theoretical investigations devoted to the study of perovskites ABX3 (A is a cation with different valence, B is a transition metal and X is oxides, halides or chlorides). But there is no systematic study of CsMX3 using HSE approximation particularly. In this paper, the band structures, density of states and optical properties of CsMX3(M = Sn, Pb; X = Cl, Br, I) have been studied by first-principles calculations using both the hybrid functional (HSE) approximation and the PBE-GGA approximation. The results of both approximations are compared with the experimental values. The results of HSE are closer to the experimental values. The changes of properties have been founded by comparing the band structures, density of states and optical properties of this series of thin film materials respectively. The trend of impact on these properties by replace elements has also been found. Our results provide a basis for the design of specific performance thin film materials.
Review of high pressure phases of calcium by first-principles calculations
NASA Astrophysics Data System (ADS)
Ishikawa, T.; Nagara, H.; Suzuki, N.; Tsuchiya, J.; Tsuchiya, T.
2010-03-01
We review high pressure phases of calcium which have obtained by recent experimental and first-principles studies. In this study, we investigated the face-centered cubic (fcc) structure, the body-centered cubic (bcc) structure, the simple cubic (sc) structure, a tetragonal P43212 [Ishikawa T et al. 2008 Phys. Rev. B 77 020101(R)], an orthorhombic Cmca [Ishikawa T et al. 2008 Phys. Rev. B 77 020101(R)], an orthorhombic Cmcm [Teweldeberhan A M and Bonev S A 2008 Phys. Rev. B 78 140101(R)], an orthorhombic Pnma [Yao Y et al. 2008 Phys. Rev. B 78 054506] and a tetragonal I4/mcm(00) [Arapan S et al. 2008 Proc. Natl. Acad. Sci. USA 105 20627]. We compared the enthalpies among the structures up to 200 GPa and theoretically determined the phase diagram of calcium. The sequence of the structural transitions is fcc (0- 3.5 GPa) ? bcc (3.5 - 35.7 GPa) ? Cmcm (35.7- 52GPa) ? P43212 (52-109 GPa) ? Cmca (109-117.4GPa) ? Pnma (117.4-134.6GPa) ? I4/mcm(00) (134.6 GPa -). The sc phase is experimentally observed in the pressure range from 32 to 113 GPa but, in our calculation, there is no pressure region where the sc phase is the most stable. In addition, we found that the enthalpy of the hexagonal close-packed (hcp) structure is lower than that of I4/mcm(00) above 495 GPa.
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)
First-principles investigation of solute-hydrogen interaction in a ?-Ti solid solution
NASA Astrophysics Data System (ADS)
Hu, Q. M.; Xu, D. S.; Yang, R.; Li, D.; Wu, W. T.
2002-08-01
In this paper, a first-principles method is used to calculate the interaction energy between substitutional solute atoms and hydrogen in ?-Ti. The results show that simple metal (SM) solute atoms are repulsive to H and therefore are detraps for H, whereas transition metal (TM) solute atoms, with smaller sizes than that of the host atoms, attract H and provide traps for H. The relationship between the interaction energy and lattice distortion as well as the electronic structure is investigated. The SM-H and TM-H interactions are dominated by different factors. The repulsive interaction between SM atoms and H is mainly due to the hybridization between the electrons of SM atoms and H when they are close to each other. The interaction between the TM solutes and H is attributable to the atomic size effect, and can be described satisfactorily by Matsumoto's strain field relaxation model. From the solute-H interaction energy and available measured terminal solubility of hydrogen (TSH), the relationship between the solute trapping of hydrogen and TSH in ?-Ti is discussed. No coherent relationship is found between the theoretical hydrogen trapping effect and the experimental TSH in ?-Ti alloys.
First principles DFT study of dye-sensitized CdS quantum dots
Jain, Kalpna; Singh, Kh. S.; Kishor, Shyam; Josefesson, Ida; Odelius, Michael; Ramaniah, Lavanya M.
2014-04-24
Dye-sensitized quantum dots (QDs) are considered promising candidates for dye-sensitized solar cells. In order to maximize their efficiency, detailed theoretical studies are important. Here, we report a first principles density functional theory (DFT) investigation of experimentally realized dye - sensitized QD / ligand systems, viz., Cd{sub 16}S{sub 16}, capped with acetate molecules and a coumarin dye. The hybrid B3LYP functional and a 6?311+G(d,p)/LANL2dz basis set are used to study the geometric, energetic and electronic properties of these clusters. There is significant structural rearrangement in all the clusters studied - on the surface for the bare QD, and in the positions of the acetate / dye ligands for the ligated QDs. The density of states (DOS) of the bare QD shows states in the band gap, which disappear on surface passivation with the acetate molecules. Interestingly, in the dye-sensitised QD, the HOMO is found to be localized mainly on the dye molecule, while the LUMO is on the QD, as required for photo-induced electron injection from the dye to the QD.
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.
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 ??Al + ??C, proving Al2CO particles can exist stably in Mg-Al alloys melt and become effective nucleation substrate for ?-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.
First-principles calculation on ?-SiC(111)/?-WC(0001) interface
Jin, Na; Yang, Yanqing, E-mail: yqyang@nwpu.edu.cn, E-mail: jinna319@163.com; Li, Jian; Luo, Xian; Huang, Bin; Sun, Qing; Guo, Pengfei [State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072 (China)
2014-06-14
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.
NASA Astrophysics Data System (ADS)
Chang, Jing; Zhao, Guo-Ping; Zhou, Xiao-Lin; Liu, Ke; Lu, Lai-Yu
2012-10-01
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 (? and ?) and a cubic ? phase. The obtained equilibrium structure parameters and ground state mechanical properties are in excellent agreement with the experimental and other theoretical results. A full elastic tensor and crystal anisotropy of the ultra-incompressible TaN in three stable phases are determined in the wide pressure range. Results indicated that the elastic properties of TaN in three phases are strongly pressure dependent. And the hexagonal ?-TaN is the most ultraincompressible among the consider phases, which suggests that the ? phase of TaN is a potential candidate structure to be one of the ultraincompressible and hard materials. By the elastic stability criteria, it is predicted that ?-TaN is not stable above 53.9 GPa. In addition, the calculated B/G ratio indicated that the ? and ? phases possess brittle nature in the range of pressure from 0 to 100 GPa. While ? phase is brittleness at low pressure (below 8.2 GPa) and is strongly prone to ductility at high pressure (above 8.2 GPa). The calculated elastic anisotropic factors for three phases of TaN suggest that they are elastically highly anisotropic and strongly dependent on the propagation direction.
First-principles prediction of superconductivity in LiBSi1-xAlx
NASA Astrophysics Data System (ADS)
Miao, Rende; Yang, Jun; Bai, Zhong; Can, Dan; Zhang, Xi; Jiang, Min; Liu, Cuicui; Wu, Fangping; Ma, Shuyun
2015-02-01
Electronic structure, lattice dynamics and superconducting properties for theoretically devised superconductor LiBSi1-x Alx are obtained by first-principles calculations. We assume that Lithium Boron Silicon (LiBSi) has the same crystal structure as that of Lithium borocarbide (LiBC). The pristine LiBSi is predicted to be a zero-gap semiconductor. Hole doping of LiBSi through partial substitution of Si by SiAl atoms can produce a semiconductor-metal transition and develop superconductivity. To assess the thermodynamic stability of LiBSi1-xAlx, the formation energies are calculated using the supercell method. For LiBSi0.75 Al0.25 and LiBSi0.875Al0.125, the obtained formation energies are -5.9 and -6.1 eV, respectively, indicating that LiBSi1-xAlx is energetically favorable at least in the range of 0 ? x ? 0.25. Phonon spectra and superconducting properties are obtained within the virtual-crystal approximation (VCA) treatment. The results show that LiBSi1-xAlx is dynamically stable approximately in the range of 0 ? x ? 0.35. For LiBSi0.8Al0.2, the obtained electron-phonon coupling constant ? is 0.86 and superconducting transition temperature TC is predicted to be in the range of 11-13 K (0.14 ? ?* ? 0.1).
NASA Astrophysics Data System (ADS)
Jang, Woosun; Yoo, Su-Hyun; Soon, Aloysius
2015-03-01
Owing to its unique and exotic physical and chemical properties, there has been a lot of effort undertaken to explore and study ultrathin low-dimensional nanostructures (e.g. graphene and MoS2). Of late, two-dimensional (2D) nanomembranes of silicon - a well-known prototypical bulk semiconductor - have attracted much attention, and has found its potential in niche nanodevice applications e.g. field effect transistors (FET) and secondary battery anodes. In this work, after considering various nanomembranes of Si with varying thicknesses, we study geometric and electronic structures using first-principles density-functional theory calculations (and beyond). Here, we consider both bulk-terminated pristine Si nanomembranes as well as surface-reconstructed ones, as motivated by available experimental and theoretical reports. To understand the influence of growth conditions on these Si nanomembranes, we have also studied the role of surface-passivation (e.g. with O, H, and OH) on their electronic and optical properties. Namely, we carefully investigate their thickness-dependent electronic band structure (i.e. both their fundamental and optical band gap energies), so as to elucidate their intrinsic structure-property relations for designing future technologically important nanodevices.
A semiconductive superhard FeB? phase from first-principles calculations.
Wang, Qianqian; Zhang, Qian; Hu, Meng; Ma, Mengdong; Xu, Bo; He, Julong
2014-10-28
An oP10-FeB4 phase [H. Gou, et al., Phys. Rev. Lett., 2013, 111, 157002] was recently synthesized based on previous theoretical predictions. In this study, a high-pressure phase of FeB4 (tP10-FeB4) was proposed through first-principles calculations. The tP10-FeB4 structure, which contains two formula units per unit cell, belongs to tetragonal symmetry with the space group P42/nmc. The boron atoms in tP10-FeB4 are present as tetrahedron configurations. Enthalpies as a function of pressure indicate that this new phase is probable to achieve through a phase transition from the oP10-FeB4 phase above ?65.9 GPa. The softening of acoustic phonon at T points in the Brillouin zone may be the driving force behind the phase transition. Further analyses reveal that the tP10-FeB4 phase is a potential superhard semiconductor. PMID:25204967
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.
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 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.
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
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.
NASA Astrophysics Data System (ADS)
Hu, Anguang; Zhang, Fan
2012-02-01
High pressure as a thermodynamic parameter provides a strong structural constraint to lead chemical transformations with selective ways. Thus, chemical transformations under pressure can create novel materials which may not be accessible by covalent synthesis. However, bonding evolution toward high pressure chemical transformations can be a complex process and may happen over widely different pressures. To understand bonding evolution pathways of high pressure chemical transformations, first-principles simulations were performed following hydrostatic compression enthalpy minimization paths to obtain experimentally and theoretically established phase transitions of carbon. The results showed that the chemical transformations from hydrostatic compression carbon to single-bonded phases were characterized by a sudden decrease in principal stress components, indicating the onset of chemical transformation. On this basis, a number of hydrostatic compression chemical transformations from molecular precursors to novel materials were predicted, such as hydrocarbon graphane, a hydrogenated carbon nitride sheet, and carbon nitrides. All predicted hydrostatic compression transformations are featured as a sudden change in principal stress components, representing chemical bonding destruction and formation reactions with a cell volume collapse.
Phase Transition and Thermodynamic Properties of Magnesium Fluoride by First Principles
NASA Astrophysics Data System (ADS)
Zhang, Tian; Cheng, Yan; Lv, Zhen-Long; Ji, Guang-Fu; Gong, Min
2014-12-01
The structural stabilities, phase transitions and thermodynamic properties of MgF2 under high pressure and temperature are investigated by first-principles calculations based on plane-wave pseudopotential density functional theory method within the local density approximation. The calculated lattice parameters of MgF2 in all four phases under zero pressure and zero temperature are in good agreement with the existing experimental data and other theoretical results. Our results demonstrate that MgF2 undergoes a series of structural phase transitions from rutile (P42/mnm)?CaCl2-type (Pnnm)?modified fluorite (Pa-3)?cotunnite (Pnam) under high pressure and the obtained transition pressures are in fairly good agreement with the experimental results. The temperature-dependent volume and thermodynamic properties of MgF2 in the rutile phase at 0 GPa are presented and the thermodynamic properties of MgF2 in the rutile, CaCl2-type, modified fluorite and cotunnite phases at 300 K are also predicted using the quasi-harmonic approximation model (QHA) and the quasi-harmonic Debye model (QHD), respectively. Moreover, the partial density of states and the electronic density of the four phases under the phase transition are also investigated.
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 DFT study of dye-sensitized CdS quantum dots
NASA Astrophysics Data System (ADS)
Jain, Kalpna; Kishor, Shyam; Singh, Kh. S.; Josefesson, Ida; Odelius, Michael; Ramaniah, Lavanya M.
2014-04-01
Dye-sensitized quantum dots (QDs) are considered promising candidates for dye-sensitized solar cells. In order to maximize their efficiency, detailed theoretical studies are important. Here, we report a first principles density functional theory (DFT) investigation of experimentally realized dye - sensitized QD / ligand systems, viz., Cd16S16, capped with acetate molecules and a coumarin dye. The hybrid B3LYP functional and a 6-311+G(d,p)/LANL2dz basis set are used to study the geometric, energetic and electronic properties of these clusters. There is significant structural rearrangement in all the clusters studied - on the surface for the bare QD, and in the positions of the acetate / dye ligands for the ligated QDs. The density of states (DOS) of the bare QD shows states in the band gap, which disappear on surface passivation with the acetate molecules. Interestingly, in the dye-sensitised QD, the HOMO is found to be localized mainly on the dye molecule, while the LUMO is on the QD, as required for photo-induced electron injection from the dye to the QD.
First-principles studies of hydrogen interaction with ultrathin Mg and Mg-based alloy films
NASA Astrophysics Data System (ADS)
Yoon, Mina; Weitering, Hanno H.; Zhang, Zhenyu
2011-01-01
The search for technologically and economically viable storage solutions for hydrogen fuel would benefit greatly from research strategies that involve systematic property tuning of potential storage materials via atomic-level modification. Here, we use first-principles density-functional theory to investigate theoretically the structural and electronic properties of ultrathin Mg films and Mg-based alloy films and their interaction with atomic hydrogen. Additional delocalized charges are distributed over the Mg films upon alloying them with 11.1% of Al or Na atoms. These extra charges contribute to enhance the hydrogen binding strength to the films. We calculated the chemical potential of hydrogen in Mg films for different dopant species and film thickness, and we included the vibrational degrees of freedom. By comparing the chemical potential with that of free hydrogen gas at finite temperature (T) and pressure (P), we construct a hydrogenation phase diagram and identify the conditions for hydrogen absorption or desorption. The formation enthalpies of metal hydrides are greatly increased in thin films, and in stark contrast to its bulk phase, the hydride state can only be stabilized at high P and T (where the chemical potential of free H2 is very high). Metal doping increases the thermodynamic stabilities of the hydride films and thus significantly helps to reduce the required pressure condition for hydrogen absorption from H2 gas. In particular, with Na alloying, hydrogen can be absorbed and/or desorbed at experimentally accessible T and P conditions.
First-principle study on the surface atomic relaxation properties of sphalerite
NASA Astrophysics Data System (ADS)
Liu, Jian; Wen, Shu-ming; Xian, Yong-jun; Bai, Shao-jun; Chen, Xiu-min
2012-09-01
The surface properties of sphalerite (ZnS) were theoretically investigated using first principle calculations based on the density functional theory (DFT). DFT results indicate that both the (110) and the (220) surfaces of sphalerite undergo surface atom relaxation after geometry optimization, which results in a considerable distortion of the surface region. In the normal direction, i.e., perpendicular to the surface, S atoms in the first surface layer move outward from the bulk ( d 1), whereas Zn atoms move toward the bulk ( d 2), forming an S-enriched surface. The values of these displacements are 0.003 nm for d 1 and 0.021 nm for d 2 on the (110) surface, and 0.002 nm for d 1 and 0.011 nm for d 2 on the (220) surface. Such a relaxation process is visually interpreted through the qualitative analysis of molecular mechanics. X-ray photoelectron spectroscopic (XPS) analysis provides the evidence for the S-enriched surface. A polysulphide (S{/n 2-}) surface layer with a binding energy of 163.21 eV is formed on the surface of sphalerite after its grinding under ambient atmosphere. This S-enriched surface and the S{/n 2-} surface layer have important influence on the flotation properties of sphalerite.
Thermodynamic ground state of MgB6 predicted from first principles structure search methods.
Wang, Hui; LeBlanc, K A; Gao, Bo; Yao, Yansun
2014-01-28
Crystalline structures of magnesium hexaboride, MgB6, 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 MgB6. The energy of the Cmcm structure is significantly lower than the theoretical MgB6 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 B6 octahedra and extended B? 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 MgB6 maintains a semiconducting state with permanent dipole moments. MgB6 is estimated to have much weaker electron-phonon coupling compared with that of MgB2, and therefore it is not expected to be able to sustain superconductivity at high temperatures. PMID:25669570
Lithium Intercalation in Graphene/MoS2 Composites: First-Principles Insights
Shao, Xiji; Pang, Rui; Shi, Xingqiang
2015-01-01
As a storage material for Li-ion batteries, graphene/molybdenum disulfide (Gr/MoS2) composites have been intensively studied in experiments. But the relevant theoretical works from first-principles are lacking. In the current work, van-der-Waals-corrected density functional theory calculations are performed to investigate the interaction of Li in Gr/MoS2 composites. Three interesting features are revealed for the intercalated Gr/Li(n)/MoS2 composites (n = 1 to 9). One is the reason for large Li storage capacity of Gr/MoS2: due to the binding energies per Li atom increase with the increasing number of intercalated Li atoms. Secondly, the band gap opening of Gr is found, and the band gap is enlarged with the increasing number of intercalated Li atoms, up to 160 meV with nine Li; hence these results suggest an efficient way to tune the band gap of graphene. Thirdly, the Dirac cone of Gr always preserve for different number of ionic bonded Li atoms.
First-principles calculation of transport property in nano-devices under an external magnetic field
NASA Astrophysics Data System (ADS)
Chen, Jing-Zhe; Zhang, Jin; Han, Ru-Shan
2008-06-01
The mesoscopic quantum interference phenomenon (QIP) can be observed and behaves as the oscillation of conductance in nano-devices when the external magnetic field changes. Excluding the factor of impurities or defects, specific QIP is determined by the sample geometry. We have improved a first-principles method based on the matrix Green's function and the density functional theory to simulate the transport behaviour of such systems under a magnetic field. We have studied two kinds of QIP: universal conductance fluctuation (UCF) and Aharonov-Bohm effect (A-B effect). We find that the amplitude of UCF is much smaller than the previous theoretical prediction. We have discussed the origin of difference and concluded that due to the failure of ergodic hypothesis, the ensemble statistics is not applicable, and the conductance fluctuation is determined by the flux-dependent density of states (DOSs). We have also studied the relation between the UCF and the structure of sample. For a specific structure, an atomic circle, the A-B effect is observed and the origin of the oscillation is also discussed.
Thermodynamic ground state of MgB{sub 6} predicted from first principles structure search methods
Wang, Hui [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China) [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China); Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada); LeBlanc, K. A. [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada)] [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada); Gao, Bo [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China)] [State Key Lab of Superhard Materials, Jilin University, Changchun 130012 (China); Yao, Yansun, E-mail: yansun.yao@usask.ca [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada) [Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2 (Canada); Canadian Light Source, Saskatoon, Saskatchewan S7N 0X4 (Canada)
2014-01-28
Crystalline structures of magnesium hexaboride, MgB{sub 6}, were investigated using unbiased structure searching methods combined with first principles density functional calculations. An orthorhombic Cmcm structure was predicted as the thermodynamic ground state of MgB{sub 6}. The energy of the Cmcm structure is significantly lower than the theoretical MgB{sub 6} models previously considered based on a primitive cubic arrangement of boron octahedra. The Cmcm structure is stable against the decomposition to elemental magnesium and boron solids at atmospheric pressure and high pressures up to 18.3 GPa. A unique feature of the predicted Cmcm structure is that the boron atoms are clustered into two forms: localized B{sub 6} octahedra and extended B{sub ?} ribbons. Within the boron ribbons, the electrons are delocalized and this leads to a metallic ground state with vanished electric dipoles. The present prediction is in contrast to the previous proposal that the crystalline MgB{sub 6} maintains a semiconducting state with permanent dipole moments. MgB{sub 6} is estimated to have much weaker electron-phonon coupling compared with that of MgB{sub 2}, and therefore it is not expected to be able to sustain superconductivity at high temperatures.
First-principles study of lattice thermal conductivity of MgSiO3 perovskite
NASA Astrophysics Data System (ADS)
Dong, J.; Tang, X.; Ntam, M. C.; Kavner, A.
2011-12-01
The Earth's lower mantle is composed of mostly magnesium silicate (MgSiO3) with significant amounts of magnesium oxide (MgO), and with about 8-10% Fe2+ and/or Fe3+ distributed in these phases. Though the thermal conductivity of MgO crystals at high temperature and pressure conditions has been studied both experimentally and theoretically, only one experimental study has measured the thermal conductivity of MgSiO3 perovskite, and only at ambient pressure and temperature. The large uncertainty in current estimates of lower mantle thermal conductivity is a direct consequence of a long extrapolation in both pressure and temperature of measured ? of lower mantle minerals at relatively low temperatures and pressures. Ab initio computational methods provide a complimentary approach to determining the phonon thermal conductivity of lower mantle materials as a function of temperature, pressure, and composition, and can enhance physical understanding of lattice phonon contribution to thermal conductivity. In this talk, we will present our recent first-principles calculation of lattice thermal conductivity of MgSiO3 perovskite. Our methods combine first-principles calculations of lattice dynamics, quantum phonon-phonon scattering theory, and the kinetic Peierls-Boltzmann phonon transport theory. This method has been well-established for the B1-structured MgO, with good agreement with independent experimental work. However, it is significantly more challenging to accurately compute all the phonon-phonon scattering rates in the 20-atom orthorhombic unit-cell of MgSiO3 perovskite crystals than in the 2-atom cubic unit-cell of MgO crystals. To adopt this method to the study of perovskite, we further improved the numerical efficiency of our algorithm and used the TeraGrid Ranger supercomputer to directly calculate the lattice thermal conductivity at 36 pressure-temperature (p-T) conditions. Using the calculated results of single crystal MgSiO3 perovskite as the starting point, we then quantitatively accounted for the phonon scattering rates due to Mg/Fe substitution to evaluate the thermal conductivity of Fe-bearing (Mg,Fe)SiO3 solid solutions within the vibrational virtual crystal approximation.
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.
First principles Raman study of boron and nitrogen doped planar T-graphene clusters
NASA Astrophysics Data System (ADS)
Bandyopadhyay, Arka; Pal, Parthasarathi; Chowdhury, Suman; Jana, Debnarayan
2015-09-01
Tetragonal graphene (TG) is one of the theoretically proposed dynamically stable graphene allotropes. In this study, the Raman spectra, IR spectra and some electronic properties of pristine and doped (single boron (B) and nitrogen (N)) TG have been investigated by first-principles based density functional theory (DFT) at the B3LYP/6-31G(d) level. Formation energy computation indicates that for the pristine structures, stability increases with increasing cluster size. In addition, for a particular cluster size, single B doping introduces some distortion in the system while single N doping increases the stability of it. The Raman spectrum of the N doped system is dominated by a single peak but for the B doped system several intense lines are found. For all the structures low intensity similar breathing-like modes have been observed. Besides, relatively low (high) intensity Raman lines are found for single B (N) doping compared to those of the pristine one. The vibrational study also reveals the existence of a prominent phonon Raman line for pristine clusters which hardly changes its position and nature of vibration with varying cluster size. So this mode can be used for identification of pristine TG structures. Unlike pristine TG, the doped structures possess non-zero finite dipole moments due to asymmetry in charge distribution. Large values of the HOMO–LUMO gap as well as the absence of DOS at the Fermi level lead to the semiconducting nature of all the structures. All these theoretical predictions from DFT calculations may shed light on experimental observations involving TG systems.
Phase diagram and electrochemical properties of mixed olivines from first-principles calculations
Malik, Rahul
Using first-principles calculations, we study the effect of cation substitution on the transition-metal sublattice in phospho-olivines, with special attention given to the Li[subscript x](Fe[subscript 1?y]Mn[subscript ...
First-principles investigation of Li intercalation kinetics in phospho-olivines
Malik, Rahul
2013-01-01
This thesis focuses broadly on characterizing and understanding the Li intercalation mechanism in phospho-olivines, namely LiFePO? and Li(Fe,Mn)PO?, using first-principles calculations. Currently Li-ion battery technology ...
Luo, Tengfei
In this paper, thermal conductivity of crystalline GaAs is calculated using first-principles lattice dynamics. The harmonic and cubic force constants are obtained by fitting them to the force-displacement data from density ...
Prediction of semiconductor band edge positions in aqueous environments from first principles
Wu, Yabi
The ability to predict a semiconductor's band edge positions in solution is important for the design of water-splitting photocatalyst materials. In this paper, we introduce a first-principles method to compute the ...
Aqueous systems from first-principles : structure, dynamics and electron-transfer reactions
Sit, Patrick Hoi Land
2006-01-01
In this thesis, we show for the first time how it is possible to calculated fully from first-principles the diabatic free-energy surfaces of electron-transfer reactions. The excitation energy corresponding to the transfer ...
First-principles modeling of the amyloid-forming peptide GNNQQNY
Li, Chen, S.B. Massachusetts Institute of Technology
2007-01-01
This thesis presents an ab initio study of biological molecules using first-principles molecular dynamics. Density functional theory and Car-Parrinello molecular dynamics are used in the computational modeling of the water ...
Lin, Xi, 1973-
2003-01-01
In this thesis, the chemistry of sulfur oxides on transition metals is studied extensively via first-principles density functional theory (DFT) computations, focusing on the chemical reactivity and selectivity in sulfur ...
First principles high throughput screening of oxynitrides for water-splitting photocatalysts
Wu, Yabi
2013-01-01
In this paper, we present a first principles high throughput screening system to search for new water-splitting photocatalysts. We use the approach to screen through nitrides and oxynitrides. Most of the known photocatalytic ...
Thermal conductivity from first-principles in bulk, disordered, and nanostructured materials
Garg, Jivtesh
2011-01-01
Thermal conductivity is an important transport property that plays a vital role in applications such as high efficiency thermoelectric devices as well as in thermal management of electronics. We present a first-principles ...
Bonnet, Nicephore
A first-principles model of the electrochemical double layer is applied to study surface energies and surface coverage under realistic electrochemical conditions and to determine the equilibrium shape of metal nanoparticles ...
Kim, Sejoong
We present a first-principles approach for inelastic quantum transport calculations based on maximally localized Wannier functions. Electronic-structure properties are obtained from density-functional theory in a plane-wave ...
cond-mat* First-principles prediction of a decagonal quasicrystal containing Boron
Widom, Michael
First-principles prediction of a decagonal quasicrystal containing Boron: May 20, 2004) We interpret experimentally known B-Mg-Ru crystals as quasicrystal a-fold rotational symmetry. We created a se- ries of hypothetical quasicrystal approximant structures
cond-mat* First-principles prediction of a decagonal quasicrystal containing Boron
Widom, Michael
First-principles prediction of a decagonal quasicrystal containing Boron: July 2, 2004) We interpret experimentally known B-Mg-Ru crystals as quasicrystal a-fold rotational symmetry. We created a se- ries of hypothetical quasicrystal approximant structures
First-principles XANES simulations of spinel zinc ferrite with a disordered cation distribution
Nakashima, Seisuke; Fujita, Koji; Tanaka, Katsuhisa; Hirao, Kazuyuki; Yamamoto, Tomoyuki; Tanaka, Isao
2007-05-01
Theoretical calculations of Zn K and Fe K x-ray absorption near-edge structures (XANES) using a first-principles method have been performed to evaluate the degree of cation disordering in spinel zinc ferrite (ZnFe{sub 2}O{sub 4}) thin film prepared by a sputtering method, ZnFe{sub 2}O{sub 4} thin films annealed at elevated temperatures, and ZnFe{sub 2}O{sub 4} bulk specimen prepared by a solid-state reaction. Using the full-potential linearized augmented plane-wave + local orbitals method, a theoretical spectrum is generated for the tetrahedral and octahedral environments for each of the two cations. The experimental XANES spectrum of the thin film annealed at 800 deg. C as well as that of bulk specimen is successfully reproduced by using either the theoretical spectrum for Zn{sup 2+} on the tetrahedral site (A site) or that for Fe{sup 3+} on the octahedral site (B site), which is indicative of the normal spinel structure. For the as-deposited film, on the other hand, excellent agreement between theoretical and experimental spectra is obtained by considering the presence of either ion in both the A and B sites. The degree of cation disordering, x, defined as [Zn{sub 1-x}{sup 2+}Fe{sub x}{sup 3+}]{sub A}[Zn{sub x}{sup 2+}Fe{sub 2-x}{sup 3+}]{sub B}O{sub 4}, is estimated to be approximately 0.6 in the as-deposited film, which is consistent with the analysis of the extended x-ray absorption fine structure on the Zn K edge. Curious magnetic properties as we previously observed for the as-deposited thin film--i.e., ferrimagnetic behaviors accompanied by large magnetization at room temperature and cluster spin-glass-like behavior--are discussed in connection with disordering of Zn{sup 2+} and Fe{sup 3+} ions in the spinel-type structure.
Sun, Shou-Tian; Jiang, Ling; Liu, J W; Heine, Nadja; Yacovitch, Tara I; Wende, Torsten; Asmis, Knut R; Neumark, Daniel M; Liu, Zhi-Feng
2015-09-30
We report infrared multiple photon dissociation (IRMPD) spectra of cryogenically-cooled H2PO4(-)(H2O)n anions (n = 2-12) in the spectral range of the stretching and bending modes of the solute anion (600-1800 cm(-1)). The spectra cannot be fully understood using the standard technique of comparison to harmonic spectra of minimum-energy structures; a satisfactory assignment requires considering anharmonic effects as well as entropy-driven hydrogen bond network fluctuations. Aided by finite temperature ab initio molecular dynamics simulations, the observed changes in the position, width and intensity of the IRMPD bands with cluster size are related to the sequence of microsolvation. Due to stronger hydrogen bonding to the two terminal P[double bond, length as m-dash]O groups, these are hydrated before the two P-OH groups. By n = 6, all four end groups are involved in the hydrogen bond network and by n = 12, the cluster spectra show similarities to the condensed phase spectrum of H2PO4(-)(aq). Our results reveal some of the microscopic details concerning the formation of the aqueous solvation environment around H2PO4(-), provide ample testing grounds for the design of model solvation potentials for this biologically relevant anion, and support a new paradigm for the interpretation of IRMPD spectra of microhydrated ions. PMID:26105043
Tetrapeptide unfolding dynamics followed by core-level spectroscopy: a first-principles approach.
Taioli, Simone; Simonucci, Stefano; A Beccara, Silvio; Garavelli, Marco
2015-05-01
In this work we demonstrate that core level analysis is a powerful tool for disentangling the dynamics of a model polypeptide undergoing conformational changes in solution and disulphide bond formation. In particular, we present computer simulations within both initial and final state approximations of 1s sulphur core level shifts (S1s CLS) of the CYFC (cysteine-phenylalanine-tyrosine-cysteine) tetrapeptide for different folding configurations. Using increasing levels of accuracy, from Hartree-Fock and density functional theory to configuration interaction via a multiscale algorithm capable of reducing drastically the computational cost of electronic structure calculations, we find that distinct peptide arrangements present S1s CLS sizeably different (in excess of 0.5 eV) with respect to the reference disulfide bridge state. This approach, leading to experimentally detectable signals, may represent an alternative to other established spectroscopic techniques. PMID:25839064
Goldman, N; Leforestier, C; Saykally, R J
2004-05-25
We present results of gas phase cluster and liquid water simulations from the recently determined VRT(ASP-W)III water dimer potential energy surface. VRT(ASP-W)III is shown to not only be a model of high ''spectroscopic'' accuracy for the water dimer, but also makes accurate predictions of vibrational ground-state properties for clusters up through the hexamer. Results of ambient liquid water simulations from VRT(ASP-W)III are compared to those from ab initio Molecular Dynamics, other potentials of ''spectroscopic'' accuracy, and to experiment. The results herein represent the first time that a ''spectroscopic'' potential surface is able to correctly model condensed phase properties of water.
Bidimensional electronic spectroscopy on indole in gas phase and in water from first principles
Mukamel, Shaul
Received in revised form 27 March 2014 Accepted 27 March 2014 Available online 4 April 2014 Keywords. In order to obtain accurate energies of high lying excited states and reliable 2DES spectra, extravalence because they visualize characteristic transition bands to higher lying states and correlations between
Sun, Shou -Tian; Jiang, Ling; Liu, J. W.; Heine, Nadja; Yacovitch, Tara I.; Wende, Torsten; Asmis, Knut R.; Neumark, Daniel M.; Liu, Zhi -Feng
2015-06-05
We report infrared multiple photon dissociation (IRMPD) spectra of cryogenically-cooled H2PO4-(H2O)n anions (n = 2–12) in the spectral range of the stretching and bending modes of the solute anion (600–1800 cm-1). The spectra cannot be fully understood using the standard technique of comparison to harmonic spectra of minimum-energy structures; a satisfactory assignment requires considering anharmonic effects as well as entropy-driven hydrogen bond network fluctuations. Aided by finite temperature ab initio molecular dynamics simulations, the observed changes in the position, width and intensity of the IRMPD bands with cluster size are related to the sequence of microsolvation. Due to stronger hydrogenmore »bonding to the two terminal P=O groups, these are hydrated before the two P–OH groups. By n = 6, all four end groups are involved in the hydrogen bond network and by n = 12, the cluster spectra show similarities to the condensed phase spectrum of H2PO4-(aq). Our results reveal some of the microscopic details concerning the formation of the aqueous solvation environment around H2PO4-, provide ample testing grounds for the design of model solvation potentials for this biologically relevant anion, and support a new paradigm for the interpretation of IRMPD spectra of microhydrated ions.« less
Mukamel, Shaul
Dipeptide Zhenyu Li and Shaul Mukamel* Department of Chemistry, UniVersity of California, IrVine, California and circular dichroism (CD) of a cyclic dipeptide, cyclo(L-Pro-D- Tyr), in the 185-300 nm region and structural rigidity, cyclic dipeptides, also known as diketopip- erazines (DKP) or 2,5-piperazinediones, have
Combined Raman spectroscopy and first-principles calculation for essential oil of Lemongrass
NASA Astrophysics Data System (ADS)
Faria, Rozilaine A. P. G.; Picanço, Nágela F. M.; Campo, Gladís S. D. L.; Faria, Jorge L. B.; Instituto de Física/UFMT Collaboration; Instituto Federal de Mato Grosso/IFMT Team
2014-03-01
The essential oils have increased food's industry interest by the presence of antioxidant and antimicrobial. Many of them have antimicrobial and antioxidant, antibacterial and antifungal activities. But, due to the concentrations required to be added in the food matrix, the sensory quality of the food is changed. The production and composition of essential oil extracted from plants depend on the plant-environment interactions, the harvest season, phenophase and physiological state of the vegetal. Cymbopogom citratus (Lemongrass) has a good yield in essential oil with neral (citral A), geranial (citral B) and myrcene, reaching 90% of the oil composition. In our experimental work, the essential oil of lemongrass was obtained by hydrodistillation in Clevenger apparatus for 4 hours. The compound was further analyzed by Raman scattering in a spectrometer HR 800, with excitation at 633nm, in the range 80-3400 cm-1. The spectrum obtained was compared with DFT calculations of molecules of the oil components. Our results show the vibrational signatures of the main functional groups and suggest a simple, but very useful, methodology to quantify the proportions of these components in the oil composition, showing good agreement with Raman data. CNPq/Capes/Fapemat.
Prediction of new high pressure structural sequence in thorium carbide: A first principles study
NASA Astrophysics Data System (ADS)
Sahoo, B. D.; Joshi, K. D.; Gupta, Satish C.
2015-05-01
In the present work, we report the detailed electronic band structure calculations on thorium monocarbide. The comparison of enthalpies, derived for various phases using evolutionary structure search method in conjunction with first principles total energy calculations at several hydrostatic compressions, yielded a high pressure structural sequence of NaCl type (B1) ? Pnma ? Cmcm ? CsCl type (B2) at hydrostatic pressures of ˜19 GPa, 36 GPa, and 200 GPa, respectively. However, the two high pressure experimental studies by Gerward et al. [J. Appl. Crystallogr. 19, 308 (1986); J. Less-Common Met. 161, L11 (1990)] one up to 36 GPa and other up to 50 GPa, on substoichiometric thorium carbide samples with carbon deficiency of ˜20%, do not report any structural transition. The discrepancy between theory and experiment could be due to the non-stoichiometry of thorium carbide samples used in the experiment. Further, in order to substantiate the results of our static lattice calculations, we have determined the phonon dispersion relations for these structures from lattice dynamic calculations. The theoretically calculated phonon spectrum reveal that the B1 phase fails dynamically at ˜33.8 GPa whereas the Pnma phase appears as dynamically stable structure around the B1 to Pnma transition pressure. Similarly, the Cmcm structure also displays dynamic stability in the regime of its structural stability. The B2 phase becomes dynamically stable much below the Cmcm to B2 transition pressure. Additionally, we have derived various thermophysical properties such as zero pressure equilibrium volume, bulk modulus, its pressure derivative, Debye temperature, thermal expansion coefficient and Gruneisen parameter at 300 K and compared these with available experimental data. Further, the behavior of zero pressure bulk modulus, heat capacity and Helmholtz free energy has been examined as a function temperature and compared with the experimental data of Danan [J. Nucl. Mater. 57, 280 (1975)].
Bondi, Robert J. Desjarlais, Michael P.; Thompson, Aidan P.; Brennecka, Geoff L.; Marinella, Matthew J.
2013-11-28
We apply first-principles density-functional theory (DFT) calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula to predict electrical conductivity in Ta{sub 2}O{sub x} (0???x???5) as a function of composition, phase, and temperature, where additional focus is given to various oxidation states of the O monovacancy (V{sub O}{sup n}; n?=?0,1+,2+). In the crystalline phase, our DFT calculations suggest that V{sub O}{sup 0} prefers equatorial O sites, while V{sub O}{sup 1+} and V{sub O}{sup 2+} are energetically preferred in the O cap sites of TaO{sub 7} polyhedra. Our calculations of DC conductivity at 300?K agree well with experimental measurements taken on Ta{sub 2}O{sub x} thin films (0.18???x???4.72) and bulk Ta{sub 2}O{sub 5} powder-sintered pellets, although simulation accuracy can be improved for the most insulating, stoichiometric compositions. Our conductivity calculations and further interrogation of the O-deficient Ta{sub 2}O{sub 5} electronic structure provide further theoretical basis to substantiate V{sub O}{sup 0} as a donor dopant in Ta{sub 2}O{sub 5}. Furthermore, this dopant-like behavior is specific to the neutral case and not observed in either the 1+ or 2+ oxidation states, which suggests that reduction and oxidation reactions may effectively act as donor activation and deactivation mechanisms, respectively, for V{sub O}{sup n} in Ta{sub 2}O{sub 5}.
NASA Astrophysics Data System (ADS)
Sonis, M.
Socio-ecological dynamics emerged from the field of Mathematical SocialSciences and opened up avenues for re-examination of classical problems of collective behavior in Social and Spatial sciences. The ``engine" of this collective behavior is the subjective mental evaluation of level of utilities in the future, presenting sets of composite socio-economic-temporal-locational advantages. These dynamics present new laws of collective multi-population behavior which are the meso-level counterparts of the utility optimization individual behavior. The central core of the socio-ecological choice dynamics includes the following first principle of the collective choice behavior of ``Homo Socialis" based on the existence of ``collective consciousness": the choice behavior of ``Homo Socialis" is a collective meso-level choice behavior such that the relative changes in choice frequencies depend on the distribution of innovation alternatives between adopters of innovations. The mathematical basis of the Socio-Ecological Dynamics includes two complementary analytical approaches both based on the use of computer modeling as a theoretical and simulation tool. First approach is the ``continuous approach" --- the systems of ordinary and partial differential equations reflecting the continuous time Volterra ecological formalism in a form of antagonistic and/or cooperative collective hyper-games between different sub-sets of choice alternatives. Second approach is the ``discrete approach" --- systems of difference equations presenting a new branch of the non-linear discrete dynamics --- the Discrete Relative m-population/n-innovations Socio-Spatial Dynamics (Dendrinos and Sonis, 1990). The generalization of the Volterra formalism leads further to the meso-level variational principle of collective choice behavior determining the balance between the resulting cumulative social spatio-temporal interactions among the population of adopters susceptible to the choice alternatives and the cumulative equalization of the power of elites supporting different choice alternatives. This balance governs the dynamic innovation choice process and constitutes the dynamic meso-level counterpart of the micro-economic individual utility maximization principle.
Jaramillo-Botero, Andres; Nielsen, Robert; Abrol, Ravi; Su, Julius; Pascal, Tod; Mueller, Jonathan; Goddard, William A
2012-01-01
We expect that systematic and seamless computational upscaling and downscaling for modeling, predicting, or optimizing material and system properties and behavior with atomistic resolution will eventually be sufficiently accurate and practical that it will transform the mode of development in the materials, chemical, catalysis, and Pharma industries. However, despite truly dramatic progress in methods, software, and hardware, this goal remains elusive, particularly for systems that exhibit inherently complex chemistry under normal or extreme conditions of temperature, pressure, radiation, and others. We describe here some of the significant progress towards solving these problems via a general multiscale, multiparadigm strategy based on first-principles quantum mechanics (QM), and the development of breakthrough methods for treating reaction processes, excited electronic states, and weak bonding effects on the conformational dynamics of large-scale molecular systems. These methods have resulted directly from filling in the physical and chemical gaps in existing theoretical and computational models, within the multiscale, multiparadigm strategy. To illustrate the procedure we demonstrate the application and transferability of such methods on an ample set of challenging problems that span multiple fields, system length- and timescales, and that lay beyond the realm of existing computational or, in some case, experimental approaches, including understanding the solvation effects on the reactivity of organic and organometallic structures, predicting transmembrane protein structures, understanding carbon nanotube nucleation and growth, understanding the effects of electronic excitations in materials subjected to extreme conditions of temperature and pressure, following the dynamics and energetics of long-term conformational evolution of DNA macromolecules, and predicting the long-term mechanisms involved in enhancing the mechanical response of polymer-based hydrogels. PMID:21243466
First principles viscosity and derived models for MgO-SiO2 melt system at high temperature
Stixrude, Lars
First principles viscosity and derived models for MgO-SiO2 melt system at high temperature Bijaya B stages. First-principles molecular dynamics simulations of seven liquids across the MgO-SiO2 binary show a first principles study of the melt viscosity of the entire MgO-SiO2 binary system. The viscosity
Experimental and theoretical investigation on the vibrational spectroscopy of L-theanine
NASA Astrophysics Data System (ADS)
Chen, Yongjian; Xi, Gangqin; Chen, Rong; Li, Yongzeng; Feng, Shangyuan; Lei, Jinping; Lin, Hongxing
2011-12-01
In this work, experimental and theoretical investigations on vibrational spectroscopy of L-theanine were presented. FT-IR and Raman spectra of L-theanine powder sample were recorded and corresponding theoretical calculations were performed based on Density Functional Theory (DFT) at B3LYP level using 6-31++G(d,p) and 6-311++G(d,p) basis sets combined with the Polarized Continuum Model (PCM) with water as the solvent. The experimental vibrational bands were assigned based on the basis of calculations while the predicted geometric parameters were compared with those obtained in experiment, most of the bands measured were well reproduced in the calculations while the discrepancies are significant for the bands mainly related to the vibrations of protonated amino group ( NH3+) and ionized carboxyl group (COO -), which are affected by the intramolecular hydrogen bond interaction. Good agreements between the theoretical and experimental results confirm the feasibility of the DFT method combined with PCM in the study of the molecular structure and vibrational spectra of L-theanine.
NASA Astrophysics Data System (ADS)
Qiu, Bo; Tian, Zhiting; Vallabhaneni, Ajit; Liao, Bolin; Mendoza, Jonathan M.; Restrepo, Oscar D.; Ruan, Xiulin; Chen, Gang
2015-03-01
The mean free paths (MFPs) of energy carriers are of critical importance to the nano-engineering of better thermoelectric materials. Despite significant progress in the first-principles–based understanding of the spectral distribution of phonon MFPs in recent years, the spectral distribution of electron MFPs remains unclear. In this work, we compute the energy-dependent electron scatterings and MFPs in silicon from first principles. The electrical conductivity accumulation with respect to electron MFPs is compared to that of the phonon thermal conductivity accumulation to illustrate the quantitative impact of nanostructuring on electron and phonon transport. By combining all electron and phonon transport properties from first principles, we predict the thermoelectric properties of the bulk and nanostructured silicon, and find that silicon with 20 nm nanograins can result in a higher than five times enhancement in their thermoelectric figure of merit as the grain boundaries scatter phonons more significantly than that of electrons due to their disparate MFP distributions.
Buck, D.R.
2000-09-12
Theoretical simulations and ultrafast pump-probe laser spectroscopy experiments were used to study photosynthetic pigment-protein complexes and antennae found in green sulfur bacteria such as Prosthecochloris aestuarii, Chloroflexus aurantiacus, and Chlorobium tepidum. The work focused on understanding structure-function relationships in energy transfer processes in these complexes through experiments and trying to model that data as we tested our theoretical assumptions with calculations. Theoretical exciton calculations on tubular pigment aggregates yield electronic absorption spectra that are superimpositions of linear J-aggregate spectra. The electronic spectroscopy of BChl c/d/e antennae in light harvesting chlorosomes from Chloroflexus aurantiacus differs considerably from J-aggregate spectra. Strong symmetry breaking is needed if we hope to simulate the absorption spectra of the BChl c antenna. The theory for simulating absorption difference spectra in strongly coupled photosynthetic antenna is described, first for a relatively simple heterodimer, then for the general N-pigment system. The theory is applied to the Fenna-Matthews-Olson (FMO) BChl a protein trimers from Prosthecochloris aestuarii and then compared with experimental low-temperature absorption difference spectra of FMO trimers from Chlorobium tepidum. Circular dichroism spectra of the FMO trimer are unusually sensitive to diagonal energy disorder. Substantial differences occur between CD spectra in exciton simulations performed with and without realistic inhomogeneous distribution functions for the input pigment diagonal energies. Anisotropic absorption difference spectroscopy measurements are less consistent with 21-pigment trimer simulations than 7-pigment monomer simulations which assume that the laser-prepared states are localized within a subunit of the trimer. Experimental anisotropies from real samples likely arise from statistical averaging over states with diagonal energies shifted by in homogeneous broadening and as such, are quite sensitive to diagonal energy disorder. The experimental anisotropies exhibit strong oscillations with {approximately}220 fs period for certain wavelengths in one-color absorption difference experiments. The oscillations only appear when the laser pulse spectrum overlaps both of the lowest-energy groups of exciton levels clustered near 815 and 825 nm. Results suggest that the oscillations stem from quantum beating between exciton levels, rather than from coherent nuclear motion.
NASA Astrophysics Data System (ADS)
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.
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.
NASA Astrophysics Data System (ADS)
Shi, Siqi; Ouyang, Chuying; Xiong, Zhihua; Liu, Lijun; Wang, Zhaoxiang; Li, Hong; Wang, Ding-Sheng; Chen, Liquan; Huang, Xuejie
2005-04-01
We present a first-principles investigation of the structural, magnetic, and electronic properties of LiFePO4 olivine. The ground-state antiferromagnetic spin arrangement consistent with a superexchange mechanism through oxygen orbits is found to be preferred, showing an antiferromagnetic order between corner sharing octahedra along the [010] direction. This is in agreement with an oxygen-mediated superexchange mechanism for the iron-iron magnetic interaction. The theoretical Néel temperature estimated by a mean-field approximation is in the range between 33.1K and 53.5K , in acceptable agreement with the experimental susceptibility measurement, which gives TN=52K . The calculated magnetic moment of 3.72?B is close to the value of 4?B deduced from the ionic model according to the Hund’s rule.
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 180? domain walls in BaTiO3: Mixed Bloch-Néel-Ising character
NASA Astrophysics Data System (ADS)
Li, Menglei; Gu, Yijia; Wang, Yi; Chen, Long-Qing; Duan, Wenhui
2014-08-01
The 180? ferroelectric domain walls (FDWs) have long been regarded as purely Ising type in ferroelectrics, but recent theoretical works suggested that they can also have Néel- and/or Bloch-like rotations. Using a combination of first-principles calculations with phase-field simulations, we studied the 180? FDWs on different crystallographic planes in prototypical ferroelectric perovskite BaTiO3. The polarization profiles of 180? FDWs on (100) and (410) planes revealed that the (100)- and (410)-FDWs both exhibit Néel-like character besides their intrinsic Ising character, while the (410)-FDW also simultaneously shows a Bloch-like oblique of ˜6 nm, as a consequence of the deviation of polarization gradient from the high symmetry direction. Due to the existence of the Néel-like component of polarization, 180? FDWs in BaTiO3 exhibit a multilayer charge redistribution and thus may strongly trap charged defects.
NASA Astrophysics Data System (ADS)
Mayfield, Cedric; Huda, Muhammad
2015-03-01
Transition metal inclusion has enhanced photocatalytic activity of bismuth titanate (Bi2Ti2O7) up to an impurity threshold concentration. Beyond the threshold, spectral absorbance is continually red shifted but increased photocurrent is not reciprocated. We investigated, from first principles, the origin of decreased photocurrent in modified Bi2Ti2O7 (BTO) by calculating the electronic structures of a representative set of doping configurations and by performing a phase stability analysis of the doping. We report our theoretical/computational strategy of analyzing free energy space and show an explicit dependence of pure phase synthesis on changes in free energy. Also, we present a probability distribution of the doping configurations based on formation enthalpy to better understand the nature of doping in BTO. We found that transition metal substitutions are favorable at the A-sites due to unchanging coordination with O ions. This work is supported by National Science Foundation, Award No. 1133672.
NASA Astrophysics Data System (ADS)
Zhai, Hong-Cun; Li, Xiao-Feng; Du, Jun-Yi; Ji, Guang-Fu
2012-05-01
The mechanical stability, elastic, and thermodynamic properties of the anti-perovskite superconductors MNNi3 (M = Zn, Mg, Al) are investigated by means of the first-principles calculations. The calculated structural parameters and elastic properties of MNNi3 are in good agreement with the experimental and the other theoretical results. From the elastic constants under high pressure, we predict that ZnNNi3, MgNNi3, and AlNNi3 are not stable at the pressures above 61.2 GPa, 113.3 GPa, and 122.4 GPa, respectively. By employing the Debye model, the thermodynamic properties, such as the heat capacity and the thermal expansion coefficient, under pressures and at finite temperatures are also obtained successfully.
NASA Astrophysics Data System (ADS)
Hellman, Anders; Iandolo, Beniamino; Wickman, Björn; Grönbeck, 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.
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.
First-principles study of alkali-atom doping in a series of zigzag and armchair carbon nanotubes
NASA Astrophysics Data System (ADS)
Wen, Y. W.; Liu, H. J.; Tan, X. J.; Pan, L.; Shi, J.
2010-02-01
First-principles calculations are performed to study the Li doping in a series of carbon nanotubes with different diameters and chiralities. It is found that the Li-Li interaction inside or outside zigzag tubes is repulsive but strongly screened. Moreover, small diameter zigzag tubes are energetically more favorable than larger ones for Li doping. In contrast, almost all the armchair tubes have the same Li binding energy, especially for the outside doping. Our theoretical results suggest that small diameter zigzag tubes could be plausible candidates for Li-ion battery application. In addition, the doping of other alkali atoms in zigzag tubes is also investigated and the optimal binding distance between them are determined.
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.
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.
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)
Rey, M.; Nikitin, A. V.; Tyuterev, V.
2014-06-01
Knowledge of near infrared intensities of rovibrational transitions of polyatomic molecules is essential for the modeling of various planetary atmospheres, brown dwarfs and for other astrophysical applications 1,2,3. For example, to analyze exoplanets, atmospheric models have been developed, thus making the need to provide accurate spectroscopic data. Consequently, the spectral characterization of such planetary objects relies on the necessity of having adequate and reliable molecular data in extreme conditions (temperature, optical path length, pressure). On the other hand, in the modeling of astrophysical opacities, millions of lines are generally involved and the line-by-line extraction is clearly not feasible in laboratory measurements. It is thus suggested that this large amount of data could be interpreted only by reliable theoretical predictions. There exists essentially two theoretical approaches for the computation and prediction of spectra. The first one is based on empirically-fitted effective spectroscopic models. Another way for computing energies, line positions and intensities is based on global variational calculations using ab initio surfaces. They do not yet reach the spectroscopic accuracy stricto sensu but implicitly account for all intramolecular interactions including resonance couplings in a wide spectral range. The final aim of this work is to provide reliable predictions which could be quantitatively accurate with respect to the precision of available observations and as complete as possible. All this thus requires extensive first-principles quantum mechanical calculations essentially based on three necessary ingredients which are (i) accurate intramolecular potential energy surface and dipole moment surface components well-defined in a large range of vibrational displacements and (ii) efficient computational methods combined with suitable choices of coordinates to account for molecular symmetry properties and to achieve a good numerical convergence. Because high-resolution ab initio spectra predictions for systems with N>4 atoms is a very challenging task, the major issue is to minimize the cost of computations and the loss of accuracy during calculations. To this end, a truncation-reduction technique for the Hamiltonian operator as well as an extraction-compression procedure for the basis set functions will be introduced and discussed in detail. We will give a review on the recent progress in computational methods as well as on existing experimental and theoretical databases 4,5,6,7,8,9. This presentation will be focused on highly symmetric molecules such as methane and phosphine, with the corresponding applications at low-T in relation with Titan's atmosphere and at high-T with the production of theoretical line lists for astrophysical opacity calculations10. The study of isotopic H?D and 12C?13C substitutions will be also addressed and carried out by means of symmetry and coordinate transformations11. Finally we hope this work will help refining studies of currently available analyses which are not yet finalized. The modeling of non-LTE emissions accounting for contribution of many fundamental and hot bands could also be possible. Support from PNP (French CNRS national planetology program) is acknowledged.
Cesium stability in a typical mica structure in dry and wet environments from first-principles
NASA Astrophysics Data System (ADS)
Suehara, Shigeru; Yamada, Hirohisa
2013-05-01
Cesium ion stability in a typical mica structure in various environments of solid salts (XCl; X = Cs, K, and Na), metals (X) and saltwaters (XCl aqueous liquids) was investigated using first-principles density-functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE) functional as well as a van der Waals (vdW) corrected functional (vdW-DFC09x). We specifically examined interlayer ion-exchange in bulk phlogopite-type mica, which is expected to produce a well-defined benchmark in a thermodynamic equilibrium state. In general, theoretical models have well reproduced the experimental and theoretical data found in the literature from the viewpoints of structure, heat capacity, and entropy. The vdW-DFC09x lattice parameters of the mica appear to be better reproducible than the PBE parameters are. However, the vdW correction calculations of the thermodynamic properties with the harmonic approximation using the phonon frequencies showed poor results in some cases, whereas the PBE calculations yielded robust and reasonable results in terms of structure and thermodynamic properties. The isotope effect of the 137Cs atom appears to be confined in thermodynamic properties such as entropy, heat capacity, and ion-exchange energy, although the theoretical infrared spectra showed a small redshift ca. 1 cm-1 in the far-infrared region of 50-75 cm-1. The calculated RDF and the coordination number for X-O (i.e., X-H2O) for the saltwater model indicated that the Cs, K, and Na ions with respective hydrated radii of 0.323, 0.284, and 0.238 nm were surrounded, respectively, by 6.5, 4.5, and 4.0 of H2O molecules in a water solution. Ion-exchange energy values based on free-energy calculations around ambient temperatures derived using the PBE functional and a harmonic approximation suggest that the cesium ion in mica interlayer phlogopite is stable in an environment consisting of KCl, NaCl, K, and Na solids, and in NaCl saltwater as well. However, it can be exchanged competitively by potassium ion in the KCl saltwater environment. The theoretical ion-selectivity order of the mica interlayer site is Cs > K > Na (PBE, vdW-DFC09x) in the solid environment, whereas the order of K ? Cs > Na (PBE) or K > Cs ? Na (vdW-DFC09x) is suggested in the saltwater environment. This selectivity-order difference between Cs and K underscores the importance of investigating the physical and chemical states of the counterphase as well as the host materials, and suggests that catching and releasing of the Cs atom in micaceous soil is possible through solvent control from a thermodynamic perspective.
NASA Astrophysics Data System (ADS)
Pacheco-Londono, Leonardo C.; Pena, Alvaro J.; Primera-Pedrozo, Oliva M.; Hernandez-Rivera, Samuel P.; Mina, Nairmen; Garcia, Rafael; Chamberlain, R. Thomas; Lareau, Richard T.
2004-09-01
Non nitrogen containing, organic peroxides explosives Triacetone triperoxide and diacetone diperoxide have been prepared in the laboratory in order to study various aspects of their synthesis and their experimental and theoretical spectroscopic characteristics. By using different proportions of acetone/hydrogen peroxide (Ac/H2O2), sulfuric, hydrochloric and methanosulfuric acids as catalyzers, it was possible to obtain both compounds in a rapid and simple form. Raman, IR spectroscopy, and GC-MS were used in order to determine the precursors, intermediates and final analytes. Experiments and theoretical studies using density functional theory (DFT) have been used in the elucidation step of the mechanism of the synthesis of the so called "transparent" explosives. The B3LYP functional with the 6-31G** basis set was used to carry out the electronic structure calculation of the intermediates and internal rotations and vibrations of TATP. Raman spectra of solid TATP and FTIR spectra of gas TATP, were recorded in order to assign the experimental spectra. Although full agreement with experiment was not obtained, spectral features of the main TATP bands were assigned.
Bennett, Joseph W. [Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 (United States)] [Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 (United States); Rabe, Karin M., E-mail: rabe@physics.rutgers.edu [Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854 (United States)
2012-11-15
In this concept paper, the development of strategies for the integration of first-principles methods with crystallographic database mining for the discovery and design of novel ferroelectric materials is discussed, drawing on the results and experience derived from exploratory investigations on three different systems: (1) the double perovskite Sr(Sb{sub 1/2}Mn{sub 1/2})O{sub 3} as a candidate semiconducting ferroelectric; (2) polar derivatives of schafarzikite MSb{sub 2}O{sub 4}; and (3) ferroelectric semiconductors with formula M{sub 2}P{sub 2}(S,Se){sub 6}. A variety of avenues for further research and investigation are suggested, including automated structure type classification, low-symmetry improper ferroelectrics, and high-throughput first-principles searches for additional representatives of structural families with desirable functional properties. - Graphical abstract: Integration of first-principles methods with crystallographic database mining, for the discovery and design of novel ferroelectric materials, could potentially lead to new classes of multifunctional materials. Highlights: Black-Right-Pointing-Pointer Integration of first-principles methods and database mining. Black-Right-Pointing-Pointer Minor structural families with desirable functional properties. Black-Right-Pointing-Pointer Survey of polar entries in the Inorganic Crystal Structural Database.
First-Principles Calculation of the Cu-Li Phase Diagram
van de Walle, Axel
First-Principles Calculation of the Cu-Li Phase Diagram Axel van de Walle Materials Science-Li phase diagram based on the cluster expansion formalism coupled with the use of (i) bond length the existence of additional phases in the Cu-Li phase diagram that have been postulated, but not yet clearly
Li-Fe-P-O2 Phase Diagram from First Principles Calculations Shyue Ping Ong,
Ceder, Gerbrand
Li-Fe-P-O2 Phase Diagram from First Principles Calculations Shyue Ping Ong, Lei Wang, ByoungwooVised Manuscript ReceiVed NoVember 26, 2007 We present an efficient way to calculate the phase diagram phases, the phase diagram was constructed as a function of oxidation conditions, with the oxygen chemical
Pressure dependent phase stability transformations of GaS: A first principles study
Melnik, Roderick
Pressure dependent phase stability transformations of GaS: A first principles study Bin Wen a principle calculations are used to determine the pressure dependent phase stability transformations for GaS polytypes at pressures up to 1000 GPa. Our results indicate that the relative stability sequence changes
First-principles study of iron oxyfluorides and lithiation of FeOF
Chevrier, Vincent L.
First-principles studies of iron oxyfluorides in the FeF[subscript 2] rutile framework (FeO[subscript x]F[subscript 2?x], 0?x?1) are performed using density functional theory (DFT) in the general gradient approximation ...
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.
First principles simulations of Li ion migration in materials related to LiPON electrolytes a
Holzwarth, Natalie
First principles simulations of Li ion migration in materials related to LiPON electrolytes, USA · Comments on solid electrolytes · Overview of LiPON family · Computational methods · Simulations-Universit¨at Bochum, Germany 216th ECS Meeting, Vienna 1 #12;Solid vs liquid electrolytes in Li ion batteries Solid
Super hard cubic phases of period VI transition metal nitrides: First principles investigation
Khare, Sanjay V.
of the noble metals Au and Pt has been wanting till recently [5,6]. Synthesis of super hard platinum nitrideSuper hard cubic phases of period VI transition metal nitrides: First principles investigation S of mechanical and electronic properties of 32 cubic phases of nitrides of the transition metals M (M=Hf, Ta, W
A FIRST PRINCIPLES BASED METHOD FOR THE PREDICTION OF LOADING OVER FIXED AND ROTARY WING GEOMETRIES
A FIRST PRINCIPLES BASED METHOD FOR THE PREDICTION OF LOADING OVER FIXED AND ROTARY WING GEOMETRIES Lakshmi N. Sankar and Mert Berkman School of Aerospace Engineering Georgia Institute of Technology-principles based techniques for the prediction of fixed and rotary wing wake geometry are described
Approved for public release; distribution is unlimited. First Principles Used in Orbital Prediction
-all and does not truly represent its dynamical purpose. 14. SUBJECT TERMS Atmospheric Models, Orbital in Orbital Prediction and an Atmospheric Model Comparison by Brian Â£. Bowden Lieutenant , United States Navy COVERED Master's Thesis 4. TITLE AND SUBTITLE FIRST PRINCIPLES USED IN ORBITAL PREDICTION
Complete collisions approximation to the Kadanoff-Baym equation: a first-principles implementation
NASA Astrophysics Data System (ADS)
Sangalli, Davide; Marini, Andrea
2015-05-01
We show carriers dynamics on bulk Silicon in the sub pico-second time scale. The results are obtained from a first-principles implementation of the the Kadanoff-Baym equations within the generalized Baym-Kadanoff ansatz and the complete collision approximation. The resulting scattering term is similar to the scattering described within the semi-classical Boltzmann equation [1].
First-principles study of binary bcc alloys using special quasirandom structures Chao Jiang,1
Chen, Long-Qing
conditions, the calculations are fairly straightforward for perfectly-ordered stoichiometric compounds-principles calculations of bcc solid solutions, even those with significant size-mismatch or atomic relaxation. We have tested our SQS's via first-principles calculations in the MoNb, TaW and CrFe systems, in which the bcc
Hafnium binary alloys from experiments and first principles Ohad Levy a,b
Curtarolo, Stefano
]. Hafnium is used extensively as an alloying element in nickel-, niobium- and tantalum-based superalloysHafnium binary alloys from experiments and first principles Ohad Levy a,b , Gus L.W. Hart c, experimental and computational data on its binary alloys is sparse. In particular, data is scant on those
Quantitative Current-Voltage Characteristics in Molecular Junctions from First Principles
Venkataraman, Latha
-voltage characteristics, Stark effect, molecular electronics, single molecule junction There is significant interest- junction (STM-BJ) experiments5,6 of molecular junctions devices formed by trapping organic moleculesQuantitative Current-Voltage Characteristics in Molecular Junctions from First Principles Pierre
Learning Biped Locomotion from First Principles on a Simulated Humanoid Robot
Fernandez, Thomas
Learning Biped Locomotion from First Principles on a Simulated Humanoid Robot using Linear Genetic programs to the real robot and continue to evolve on the robot. The Genetic Programming system controllers for dynamic gait of their quadruped robot dog AIBO. These results show that evolutionary
First principles study of Li diffusion in I-Li_{2}NiO_{2} structure
Ceder, Gerbrand
First principles computations have been used to study Li mobility in the orthorhombic Li2NiO2 structure with the Immm space group (I-Li2NiO2). Understanding Li mobility in I-Li2NiO2 structure other than the conventional ...
The New Face of Rhodium Alloys: Revealing Ordered Structures from First Principles
Hart, Gus
The New Face of Rhodium Alloys: Revealing Ordered Structures from First Principles Ohad Levy-mail: stefano@duke.edu Abstract: The experimental and computational data on rhodium binary alloys is sparse, and platinum), rhodium has a high catalytic potential for a variety of chemical reactions.1,2 It is used
Queensland, University of
35 . An important recent development in the study of ultracold quantum gases is the direct measurement of atom-atomFirst-principles quantum simulations of dissociation of molecular condensates: Atom correlations for Quantum-Atom Optics, Department of Physics, Australian National University, Canberra ACT 0200, Australia 2
Superconducting Properties of MgB2 from First Principles A. Floris,1,2
Gross, E.K.U.
Superconducting Properties of MgB2 from First Principles A. Floris,1,2 G. Profeta,3 N. N; published 25 January 2005) Solid MgB2 has rather interesting and technologically important properties, and the specific heat of MgB2 in very good agreement with experiment. Moreover, our calculations show how
Magneto-elastic effects in compressed cobalt from first-principles Gerd Steinle-Neumann*
Steinle-Neumann, Gerd
Magneto-elastic effects in compressed cobalt from first-principles Gerd Steinle of the close-packed phases of cobalt are computed as a function of compression using the linearized augmented, a lot of effort has focused on the metal adjacent to iron in the Periodic Table: cobalt. The study
First-principles calculations of step formation energies and step interactions on TiN(001)
Ciobanu, Cristian
functional calculations; Single crystal surfaces; Surface energy; Step formation energy; Titanium nitrideFirst-principles calculations of step formation energies and step interactions on TiN(001) Cristian the formation energies and repulsive interactions of monatomic steps on the TiN(001) surface, using den- sity
First-principles investigation on the structural stability of methane and ethane clathrate hydrates
Luzhi Xu; Xu Wang; Liuxia Liu; Minghui Yang
2011-01-01
Clathrate hydrates are characterized by their hydrogen-bond linked cavities that can encapsulate guest molecules. First-principles calculations were performed in this study to compare the prototypical structures of methane and ethane hydrates. The relative stability of singly and doubly occupied structures was studied in terms of their structural variations and binding energies. The preferred structures under various gaseous components (methane, ethane,
Electron transport in SiGe alloy nanowires in the ballistic regime from first-principles.
Amato, Michele; Ossicini, Stefano; Rurali, Riccardo
2012-06-13
Silicon-germanium alloying is emerging as one of the most promising strategies to engineer heat transport at the nanoscale. Here, we perform first-principles electron transport calculations to assess at what extent such approach can be followed without worsening the electrical conduction properties of the system, providing then a path toward high-efficiency thermoelectric materials. PMID:22545577
Curtarolo, Stefano
Calculation of solubility in titanium alloys from first principles Roman V. Chepulskii, Stefano solubility in binary alloys based on the statistical-thermodynamic theory of dilute lattice gas. The model dependence determined by a ``low-solubility formation enthalpy". This quantity, directly obtainable from
Ordered magnesium-lithium alloys: First-principles predictions Richard H. Taylor
Hart, Gus
Ordered magnesium-lithium alloys: First-principles predictions Richard H. Taylor Department 2010 Magnesium-lithium Mg-Li alloys are among the lightest structural materials. Although considerable Emerging technologies increasingly depend on the pro- duction of ultralight-weight materials. Magnesium
ccsd00001055 First principles study of the solubility of Zr in Al
ccsd00001055 (version 1) : 22 Jan 2004 First principles study of the solubility of Zr in Al (Dated: January 22, 2004) The experimental solubility limit of Zr in Al is well-known. Al3Zr has a stable the solubility limit of Zr in Al for the stable as well as the metastable phase diagrams. The formation energies
cond-mat/0405298 First-principles prediction of a decagonal quasicrystal containing Boron
Widom, Michael
cond-mat/0405298 First-principles prediction of a decagonal quasicrystal containing Boron M, 2004) We interpret experimentally known B-Mg-Ru crystals as quasicrystal approximants. These ap45 forms a decagonal quasicrystal that is metastable at low temperatures and may be thermodynamically
First-Principles Prediction of a Decagonal Quasicrystal Containing Boron M. Mihalkovic* and M. Widom
Widom, Michael
First-Principles Prediction of a Decagonal Quasicrystal Containing Boron M. Mihalkovic* and M as quasicrystal approximants whose deter- ministic decoration of tiles by atoms can be extended quasiperiodically45 forms a metastable decagonal quasicrystal that may be thermodynamically stable at high
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.
On structural and lattice dynamic stability of LaF3 under high pressure: A first principle study
NASA Astrophysics Data System (ADS)
Sahoo, B. D.; Joshi, K. D.; Gupta, Satish C.
2015-06-01
Structural and lattice dynamical stability of the LaF3 has been analyzed as a function of hydrostatic compression through first principle electronic band structure calculations. The comparison of enthalpies of various plausible structures calculated at various pressures suggests a phase transition from ambient condition tysonite structure (space group P-3c1) to a primitive orthorhombic structure (space group Pmmn) at a pressure of ˜19.5 GPa, in line with the experimental value of 16 GPa. Further, it is predicted that this phase will remain stable up to 100 GPa (the maximum pressure up to which calculations have been performed in the present work). The theoretically determined equation of state displays a good agreement with experimental data. Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state and compared with the available experimental data. Our lattice dynamic calculations correctly demonstrate that at zero pressure the tysonite structure is lattice dynamically stable whereas the Pmmn structure is unstable lattice dynamically. Further, at transition pressure the theoretically calculated phonon spectra clearly show that the Pmmn phase emerges as lattice dynamically stable phase whereas the tysonite structure becomes unstable dynamically, supporting our static lattice calculations.
Solution-based thermodynamic modeling of the Ni-Al-Mo system using first-principles calculations
Zhou, S H [Ames Laboratory; Wang, Y [Pennsylvania State University; Chen, L -Q [Pennsylvania State University; Liu, Z -K [Pennsylvania State University; Napolitano, R E [Ames Laboratory
2014-09-01
A solution-based thermodynamic description of the ternary Ni–Al–Mo system is developed here, incorporating first-principles calculations and reported modeling of the binary Ni–Al, Ni–Mo and Al–Mo systems. To search for the configurations with the lowest energies of the N phase, the Alloy Theoretic Automated Toolkit (ATAT) was employed and combined with VASP. The liquid, bcc and ?-fcc phases are modeled as random atomic solutions, and the ??-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, ?-fcc and ??-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 ?-AlMo, ?-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 ?-fcc and ??-Ni3Al phases in the ternary Ni–Al–Mo system, the specific ?-fcc and ??-Ni3Al phase fields are plotted in x(Al)–x(Mo)–T space for a temperature range 1200–1800 K.
a Theoretical Study of CN Spectroscopy from the IR to the VUV
NASA Astrophysics Data System (ADS)
Schwenke, David W.
2013-06-01
We have carried out first principles calculations of the spectrum of CN. We have included Rydberg states, spin-orbit and non-adiabatic coupling between different electronic states, and an accurate treatment of the ro-vib-spin-electronic wavefunctions. Doublet, quartet and sextet spin states are included in our calculations. These wavefunctions are used with electric dipole and quadrapole and magnetic dipole transition moments to predict spectra. We consider both bound-bound, bound-free and free-bound transitions. When possible, comparisons are made with experimental results, both for energy levels and radiative lifetimes. Many previously uncharacterized bands are predicted.
REMPI spectroscopy and theoretical calculations of cis and trans 3-fluoro-N-methylaniline
NASA Astrophysics Data System (ADS)
Zhang, Lijuan; Liu, Sheng; Dong, Changwu; Cheng, Min; Du, Yikui; Zhu, Qihe; Zhang, Cunhao
2014-02-01
The ab initio and density functional theory (DFT) calculations of 3-fluoro-N-methylaniline (3FNMA) reveal that two rotamers, cis and trans 3FNMA, are stable for each of the S0, S1, and D0 states. The vibronic spectra of the two rotamers in the S1 state have been recorded by the one-color resonant two-photon ionization (R2PI) spectroscopy. The band origins of the S1 ? S0 electronic transition of cis and trans 3FNMA are found to be 33 816 ± 3 and 34 023 ± 3 cm-1. The two rotamers display similar vibrational frequencies, and the slight energy difference in some modes reflects the conformation effect due to the relative orientation of the NHCH3 group. Besides, the trans rotamer displays more vibronic features in the low-frequency region, which are active modes mainly involving the CH3 and the NHCH3 groups. By the two-color R2PI spectroscopy, the adiabatic ionization energies (IEs) of cis and trans 3FNMA are determined to be 61 742 ± 10 and 61 602 ± 10 cm-1, respectively. It is derived from the R2PI spectroscopic data that, compared with the trans rotamer, the cis rotamer is more stable by 302 ± 25 cm-1 in the S1 state, but less stable by 45 ± 25 cm-1 in the D0 state. With the aid of theoretical calculations, the substitution and conformation effects on the properties of 3FNMA, including the molecular structures, vibrational frequencies, and the relative stability of the two rotamers, were discussed in detail.
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)
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.
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+}.
Methane Oxidation over PdO(101) Revealed by First-Principles Kinetic Modeling.
Bossche, Maxime Van den; 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
Structure of the (111) surface of bismuth: LEED analysis and first-principles calculations
Moenig, H.; Wells, J.; Hofmann, Ph. [Institute for Storage Ring Facilities, University of Aarhus, 8000 Aarhus C (Denmark); Sun, J.; Pohl, K. [Department of Physics and Materials Science Program, University of New Hampshire, Durham, New Hampshire 03824 (United States); Koroteev, Yu.M. [Donostia International Physics Center (DIPC), 20018 San Sebastian, Basque Country (Spain); Institute of Strength Physics and Materials Science, Russian Academy of Sciences, 634021, Tomsk (Russian Federation); Bihlmayer, G. [Institut fuer Festkoerperforschung, Forschungszentrum Juelich, D-52425 Juelich (Germany); Chulkov, E.V. [Donostia International Physics Center (DIPC), 20018 San Sebastian, Basque Country (Spain); Departamento de Fisica de Materiales and Centro Mixto CSIC-UPV/EHU, Facultad de Ciencias Quimicas, UPV/EHU, Apdo. 1072, 20080 San Sebastian, Basque Country (Spain)
2005-08-15
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.
Zhou, Fei; Nielson, Weston; Xia, Yi; Ozoli?š, Vidvuds
2014-10-01
First-principles prediction of lattice thermal conductivity ?_{L} of strongly anharmonic crystals is a long-standing challenge in solid-state physics. Making use of recent advances in information science, we propose a systematic and rigorous approach to this problem, compressive sensing lattice dynamics. Compressive sensing is used to select the physically important terms in the lattice dynamics model and determine their values in one shot. Nonintuitively, high accuracy is achieved when the model is trained on first-principles forces in quasirandom atomic configurations. The method is demonstrated for Si, NaCl, and 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.
NASA Astrophysics Data System (ADS)
Liu, Mingjie; Artyukhov, Vasilii I.; Yakobson, Boris I.
2015-03-01
The past decade has brought great progress in fabrication and characterization of single-atom chains of carbon (carbyne). Very recently novel atomic chain compositions such as BN and CsI were reported. The extreme and unusual properties of such 1D materials motivate the search for other possible compositions with interesting behaviors. We use first-principles calculations to uncover the rich structural and mechanical properties of 1D boron. While the ground state structure of linear boron is a two-atoms-wide ribbon, tension can unravel it into a single-atom string structure. We analyze the mechanical and electronic properties of these two ``phases'' and study the thermodynamics and kinetics of transition between them using static first-principles calculations and semiempirical (DFTB) molecular dynamics. The interesting properties of 1D boron nanostructures make them an attractive system for experimental investigations.
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.
Ammonia synthesis and decomposition on a Ru-based catalyst modeled by first-principles
NASA Astrophysics Data System (ADS)
Hellman, A.; Honkala, K.; Remediakis, I. N.; Logadóttir, Á.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Nørskov, J. K.
2009-06-01
A recently published first-principles model for the ammonia synthesis on an unpromoted Ru-based catalyst is extended to also describe ammonia decomposition. In addition, further analysis concerning trends in ammonia productivity, surface conditions during the reaction, and macro-properties, such as apparent activation energies and reaction orders are provided. All observed trends in activity are captured by the model and the absolute value of ammonia synthesis/decomposition productivity is predicted to within a factor of 1-100 depending on the experimental conditions. Moreover it is shown: (i) that small changes in the relative adsorption potential energies are sufficient to get a quantitative agreement between theory and experiment ( Appendix A) and (ii) that it is possible to reproduce results from the first-principles model by a simple micro-kinetic model ( Appendix B).
Tunneling properties of ultra-thin SiO2 barriers: a first-principles study
NASA Astrophysics Data System (ADS)
Ko, Eunjung; Choi, Hyoung Joon
2008-03-01
We performed first-principles simulations of the electron tunneling through ultra-thin SiO2 barriers in Si(100)/SiO2/Si(100) structures. The atomic structures of the Si/SiO2 interfaces are generated by considering various silicon suboxide states observed in photoemission studies. For comparison, we also consider sharp Si/SiO2 interfaces with dangling bonds. For each atomic structure, the tunneling conductance is calculated by a first-principles scattering-state method based on the ab-initio pseudopotentials and the density functional theory within the local density approximation. As a result we obtained the dependence of the tunneling probabilities on the oxide thickness and on the interfacial structures. Effects of the dangling bonds on the tunneling probabilities will also be discussed. Computational resource for this work is provided by KISTI under the 8th Strategic Supercomputing Support Program.
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
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
A Massive Core in Jupiter Predicted From First-Principles Simulations
B. Militzer; W. B. Hubbard; J. Vorberger; I. Tamblyn; S. A. Bonev
2008-07-26
Hydrogen-helium mixtures at conditions of Jupiter's interior are studied with first-principles computer simulations. The resulting equation of state (EOS) implies that Jupiter possesses a central core of 14-18 Earth masses of heavier elements, a result that supports core accretion as standard model for the formation of hydrogen-rich giant planets. Our nominal model has about 2 Earth masses of planetary ices in the H-He-rich mantle, a result that is, within modeling errors, consistent with abundances measured by the 1995 Galileo Entry Probe mission (equivalent to about 5 Earth masses of planetary ices when extrapolated to the mantle), suggesting that the composition found by the probe may be representative of the entire planet. Interior models derived from this first-principles EOS do not give a match to Jupiter's gravity moment J4 unless one invokes interior differential rotation, implying that jovian interior dynamics has an observable effect on the measured gravity field.
First-principles materials applications and design of nonlinear optical crystals
NASA Astrophysics Data System (ADS)
Lin, Zheshuai; Jiang, Xingxing; Kang, Lei; Gong, Pifu; Luo, Siyang; Lee, Ming-Hsien
2014-06-01
With the development of laser technology and related scientific fields, understanding of the structure-property relationships in nonlinear optical (NLO) crystals is becoming more and more important. In this article, first-principles studies based on density functional theory, and their applications to elucidate the microscopic origins of the linear and NLO properties in NLO crystals, are reviewed. The ab initio approaches have the ability to accurately predict the optical properties in NLO crystals, and the developed analysis tools are vital to investigating their intrinsic mechanism. This microscopic understanding has further guided molecular engineering design for NLO crystals with novel structures and properties. It is anticipated that first-principle material approaches will greatly improve the search efficiency and greatly help experiments to save resources in the exploration of new NLO crystals with good performance.
First-principles study of Si(111)?{31}×?{31}-In reconstruction
NASA Astrophysics Data System (ADS)
Chukurov, E. N.; Alekseev, A. A.; Kotlyar, V. G.; Olyanich, D. A.; Zotov, A. V.; Saranin, A. A.
2012-12-01
Using first-principles total-energy calculations, structural properties of the Si(111)?{31}×?{31}-In reconstruction have been studied. New refined structural model of the reconstruction has been proposed which adopts 17 In atoms and 31 Si atoms. The model is characterized by the reasonably low surface energy and demonstrates good correspondence between simulated and experimental scanning tunneling microscopy images. Calculations reveal semiconducting nature of the model structure in agreement with experiment.
Comparison of Jupiter Interior Models Derived from First-Principles Simulations
B. Militzer; W. B. Hubbard
2008-07-27
Recently two groups used first-principles computer simulations to model Jupiter's interior. While both studies relied on the same simulation technique, density functional molecular dynamics, the groups derived very different conclusions. In particular estimates for the size of Jupiter's core and the metallicity of its hydrogen-helium mantle differed substantially. In this paper, we discuss the differences of the approaches and give an explanation for the differing conclusions.
First-principles calculations on TiO 2 doped by N, Nd, and vacancy
Y. Wang; D. J. Doren
2005-01-01
First-principles density functional theory calculations have been carried out to investigate electronic structures of anatase TiO2 with substitutional dopants of N, Nd, and vacancy, which replace O, Ti, and O, respectively. The calculation on N-doped TiO2 with the local density approximation (LDA) demonstrates that N doping introduces some states located at the valence band maximum and thus makes the original
NASA Astrophysics Data System (ADS)
Kibalchenko, Mikhail; Lee, Daniel; Shao, Limin; Payne, Mike C.; Titman, Jeremy J.; Yates, Jonathan R.
2010-10-01
First principles calculations and solid-state NMR experiments are used to distinguish between possible hydrogen bonding networks in ?- D-galactose. In contrast to 13C, the 1H chemical shift parameters show differences which are sufficient to allow the correct network to be identified by comparison with experiments which make use of modern homonuclear decoupling schemes. In addition, clear linear correlations are established between both 1H chemical shift and chemical shift anisotropy, and hydrogen bond length.
Size dependence of the bulk modulus of semiconductor nanocrystals from first-principles calculations
R. Cherian; C. Gerard; P. Mahadevan; Nguyen Thanh Cuong; Ryo Maezono
2010-01-01
The variation in the bulk modulus of semiconductor nanoparticles has been studied within first-principles electronic-structure calculations using the local density approximation (LDA) for the exchange correlation. Quantum Monte Carlo calculations carried out for a silicon nanocrystal Si87H76 provided reasonable agreement with the LDA results. An enhancement was observed in the bulk modulus as the size of the nanoparticle was decreased,
Energetics of point and planar defects in aluminium from first-principles calculations
NASA Astrophysics Data System (ADS)
Denteneer, P. J. H.; Soler, J. M.
1991-06-01
Formation energies of the vacancy and self-interstitial in Al, as well as energies of intrinsic, extrinsic, and twin-boundary stacking faults are calculated from first-principles. The electronic structure and forces on the atoms are calculated in the framework of the Augmented Plane Wave method using new algorithms proposed by Williams and Soler, enabling an ab initio approach to long-standing questions on defects in metals.
First-principles method for high-Q photonic crystal cavity mode calculations.
Mahmoodian, Sahand; Sipe, J E; Poulton, Christopher G; Dossou, Kokou B; Botten, Lindsay C; McPhedran, Ross C; de Sterke, C Martijn
2012-09-24
We present a first-principles method to compute radiation properties of ultra-high quality factor photonic crystal cavities. Our Frequency-domain Approach for Radiation (FAR) can compute the far-field radiation pattern and quality factor of cavity modes ~ 100 times more rapidly than conventional finite-difference time domain calculations. We explain how the radiation pattern depends on the perturbation used to create the cavity and on the Bloch modes of the photonic crystal. PMID:23037427
Wojciech Kami?ski; Rubén Pérez
2010-01-01
We present an extensive first-principles study of the interaction between a silicon oxide nanoasperity and a sexithiophene\\u000a monolayer in order to investigate the individual molecular processes responsible for the energy dissipation during atomic\\u000a force microscope (AFM) operation. Our approach includes not only ground-state calculations of the tip–sample interaction,\\u000a but an extensive set of molecular dynamics simulations at room temperature to
Toward the Accurate First-Principles Prediction of Ionization Equilibria in Proteins †
Jana Khandogin; Charles L. Brooks
2006-01-01
The calculation of pKa values for ionizable sites in proteins has been traditionally based on numerical solutions of the Poisson-Boltzmann equation carried out using a high-resolution protein structure. In this paper, we present a method based on continuous constant pH molecular dynamics (CPHMD) simulations, which allows the first-principles description of protein ionization equilibria. Our method utilizes an improved generalized Born
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-based modeling of geomagnetically induced currents at mid- and low- latitudes
A. Pulkkinen; N. Buzulukova; L. Rastaetter; M. Kuznetsova; A. Viljanen; R. Pirjola
2008-01-01
Recently, Pulkkinen et al. (2007, Annales Geophysicae) introduced an approach to predict geomagnetically induced current (GIC) flow in high-voltage power transmission systems based on first-principles modeling of the near-space plasma environment. Their approach that has already been implemented as an experimental real-time system providing forecasts of GIC in the North American power transmission system, however, is applicable only to high-latitude
First-principles study of lattice dynamics of LiFePO 4
Siqi Shi; Hua Zhang; Xuezhi Ke; Chuying Ouyang; Minsheng Lei; Liquan Chen
2009-01-01
Lattice dynamics of lithium iron orthophosphate (LiFePO4) isostructural with olivine have been investigated using the first-principles calculations taking into account the on-site Coulomb interaction within the GGA+U scheme. Born effective charge tensors, phonon frequencies at the Brillouin zone center and phonon dispersion curves are calculated and analyzed. The Born effective charge tensors exhibit anisotropy, which gives a convincing evidence for
Unusual compression behavior of TiO2 polymorphs from first principles
Xiang-Feng Zhou; Xiao Dong; Guang-Rui Qian; Lixin Zhang; Yonjun Tian; Hui-Tian Wang
2010-01-01
The physical mechanisms behind the reduction in the bulk modulus of a high-pressure cubic TiO2 phase are revealed by first-principles calculations. An unusual and abrupt change occurs in the dependence of energy on pressure at 43 GPa, indicating a pressure-induced phase transition from columbite TiO2 to a modified fluorite TiO2 with a Pca21 symmetry. Oxygen atom displacement in Pca21 TiO2
Unusual Compression Behavior of Columbite TiO2 via First-Principles Calculations
Xiang-Feng Zhou; Xiao Dong; Guang-Rui Qian; Lixin Zhang; Yonjun Tian; Hui-Tian Wang
2010-01-01
The physical mechanisms behind the reduction of the bulk modulus of a high-pressure cubic TiO2 phase are confirmed by first-principles calculations. An unusual and abrupt change occurs in the dependence of energy on pressure at 43 GPa, indicating a pressure-induced phase transition from columbite TiO2 to a newly-identified modified fluorite TiO2 with a Pca21 symmetry. Oxygen atom displacement in Pca21
Structural, electronic, and dynamical properties of Pca21-TiO2 by first principles
M. Abbasnejad; M. R. Mohammadizadeh; R. Maezono
2012-01-01
First-principles calculations of the structural, electronic, and mechanical properties of the modified fluorite structure of TiO2 with Pca21 symmetry are obtained using the plane-wave pseudopotential density functional theory. The results indicate that Pca21-TiO2 is a semiconductor with an indirect band gap. The calculated static dielectric constants are larger than those of anatase and brookite, but they are much smaller than
First-principles calculations of the phase stability of TiO2
Joseph Muscat; Varghese Swamy; Nicholas M. Harrison
2002-01-01
First-principles calculations of the crystal structures, bulk moduli, and relative stabilities of seven known and hypothetical TiO2 polymorphs (anatase, rutile, columbite, baddeleyite, cotunnite, pyrite, and fluorite structures) have been carried out with the all-electron linear combination of atomic orbitals (LCAO) and pseudopotential planewave (PW) methods. The anatase versus rutile relative phase stability at 0 K and zero pressure has been
First principle study of crystal growth morphology: An application to crystalline urea
M. K. Singh
2006-01-01
The growth morphology and the effects of surface reconstruction\\/relaxation of the habit faces on the crystal morphology of urea crystal are investigated from first principle using the Hartman-Perdok approach. A procedure has been developed to find out surface termination when a slab of d(hkl) thickness is created having {hkl} orientation. Slice energy of different habit faces have been computed and
NASA Astrophysics Data System (ADS)
Yang, J. Y.; Zhang, W. J.; Liu, L. H.
2015-09-01
This work aims at understanding and predicting the high-temperature anisotropic dielectric functions of (0001) sapphire over the entire infrared-visible-ultraviolet spectrum via the state-of-the-art infrared variable angle spectroscopic ellipsometry (IR-VASE) and first-principles method without empirical parameters. Upon measuring the high-symmetry orientation of c-plane surface, the IR-VASE determines the anisotropic infrared dielectric functions of (0001) sapphire for wavelengths ranging from 10 to 25 ?m and temperatures from 300 to 573 K. As ellipsometry experiments indicate, the maxima of ordinary infrared dielectric functions consistently decrease and shift to longer wavelength as temperature increases. By fitting ellipsometry data with the Lorentz oscillator model, the Lorentz parameters, e.g., oscillator strength, resonance frequency and broadening parameter, for infrared-active phonon modes are obtained to interpret the temperature effect. Moreover, the calculated vibration frequencies at varying temperatures by the first-principles method coincide with the positions of infrared absorption peaks, indicating that infrared optical absorption mainly arises from the coupling of incident photon with lattice vibration. In the visible-ultraviolet spectral range, the first-principles and lattice dynamics methods are combined to understand the temperature effect on dielectric functions of (0001) sapphire. This method reproduces the essential feature of previous room-temperature reflectivity experiments and detects the slight change of dielectric functions as temperature increases. The convincing results enable us to predict the high-temperature visible-ultraviolet dielectric functions of (0001) sapphire by the first-principles method.
Negative thermal expansion in TiF3 from the first-principles prediction
NASA Astrophysics Data System (ADS)
Wang, Lei; Yuan, Peng-Fei; Wang, Fei; Sun, Qiang; Liang, Er-Jun; Jia, Yu; Guo, Zheng-Xiao
2014-08-01
In negative thermal expansion (NTE) materials, rhombohedral TiF3 as a new member is predicted from first-principles calculation. The NTE behavior of rhombohedral TiF3 occurs at low temperatures. In our work, the NTE mechanism is elaborated in accordance with vibrational modes. It is confirmed that the rigid unit mode (RUM) of internal TiF6 octahedra in low-frequency optical range is most responsible for the NTE properties.
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.
Ferromagnetism and antiferromagnetism in hydrogenated g-C3N4: A first-principles study
NASA Astrophysics Data System (ADS)
Qiu, Huanhuan; Wang, Zhijun; Sheng, Xianlei
2013-07-01
Magnetism in light elements materials has attracted considerable attention due to its potential application in spintronics. Based on the two-dimensional nonmagnetic graphitic carbon nitride structure (g-C3N4), we have constructed some hydrogenated graphitic carbon nitride structures and studied their electronic structures and magnetic properties by first-principles calculations. Both ferromagnetism and antiferromagnetism are found in these materials and the magnetic moments are mainly from the p electrons of N and C atoms.
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 calculation of the magnetic properties of paramagnetic fcc iron
Johnson, D.D.; Gyorffy, B.L.; Pinski, F.J.; Staunton, J.; Stocks, G.M.
1985-01-01
Using the disordered local moment picture of itinerant magnetism, we present calculations of the temperature and volume dependence of the magnetic moment and spin-spin correlations for fcc Fe in the paramagnetic state. These calculations are based on the parameter-free, first principles approach of local spin density functional theory and the coherent potential approximation is used to treat the disorder associated with the random orientation of the local moments.
Cu-doped GaN: A dilute magnetic semiconductor from first-principles study
R. Q. Wu; G. W. Peng; L. Liu; Y. P. Fenga; Z. G. Huang; Q. Y. Wu
First-principles calculations based on spin density functional theory are performed to study the spin-resolved electronic properties of GaN doped with 6.25% of Cu. The Cu dopants are found spin polarized and the calculated band structures suggest a 100% polarization of the conduction carriers. The Cu-doped GaN favors ferromagnetic ground state which can be explained in terms of p-d hybridization mechanism,
Roy, S.; Gruenbaum, S. M.; Skinner, J. L. [Theoretical Chemistry Institute and Department of Chemistry, 1101 University Ave., University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
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.
Theoretical study of core-loss electron energy-loss spectroscopy at graphene nanoribbon edges
NASA Astrophysics Data System (ADS)
Fujita, N.; Hasnip, P. J.; Probert, M. I. J.; Yuan, J.
2015-08-01
A systematic study of simulated atomic-resolution electronic energy-loss spectroscopy (EELS) for different graphene nanoribbons (GNRs) is presented. The results of ab initio studies of carbon 1s core-loss EELS on GNRs with different ribbon edge structures and different hydrogen terminations show that theoretical core-loss EELS can distinguish key structural features at the atomic scale. In addition, the combination of polarized core-loss EELS with symmetry resolved electronic partial density of states calculations can be used to identify the origins of all the primary features in the spectra. For example, the nature of the GNR edge structure (armchair, zigzag, etc) can be identified, along with the degree of hydrogenation. Hence it is possible to use the combination of ab initio calculations with high resolution, high energy transmission core-loss EELS experiments to determine the local atomic arrangement and chemical bonding states (i.e. a structural fingerprint) in GNRs, which is essential for future practical applications of graphene.
NASA Astrophysics Data System (ADS)
Perkowski, P.
2009-06-01
Indium tin oxide (ITO) layers are widely used to make electrodes in measuring cells, because these layers are transparent and electrooptical investigations can be performed using such prepared cells. It was found during the dielectric spectroscopy measurements, performed for smectic liquid crystalline mixture, that it is not possible to detect some important relaxation modes in paraelectric SmA*, ferroelectric SmC*, and antiferroelectric SmCA* phases for the frequencies higher than 300 kHz. The measuring cell does not allow to measure relaxations, because its own dielectric behaviour covers the dielectric response of a liquid crystalline medium. One can observe the spurious contribution for high frequency part of the dielectric spectrum, due to the finite resistance of ITO layers. The theoretical model was introduced, which shows how to calculate relaxations related to liquid crystals from dielectric response of the empty and filled measuring cell. The proof of strong influence of cell properties on effective (measuring) values of dielectric permittivities was shown.
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.
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
Zhang, Jian; Zhou, Bin; Sun, Zhenrong; Wang, Xue B.
2015-01-01
Proposed in theory and confirmed to exist, anion–? interactions have been recognized as new and important non-covalent binding forces. Despite extensive theoretical studies, numerous crystal structural identifications, and a plethora of solution phase investigations, intrinsic anion–? interaction strengths that are free from complications of condensed phases’ environments, have not been directly measured in the gas phase. Herein we present a joint photoelectron spectroscopic and theoretical study on this subject, in which tetraoxacalix[2]arene[2]triazine 1, an electron-deficient and cavity self-tunable macrocyclic was used as a charge-neutral molecular host to probe its interactions with a series of anions with distinctly different shapes and charge states (spherical halides Cl?, Br?, I?, linear thiocyanate SCN?, trigonal planar nitrate NO??, pyramidic iodate IO??, and tetrahedral sulfate SO?²?). The binding energies of the resultant gaseous 1:1 complexes (1•Cl?,1•Br?, 1•I?, 1•SCN?, 1•NO??, 1•IO?? and 1•SO?²?) were directly measured experimentally, exhibiting substantial non-covalent interactions with pronounced anion specific effects. The binding strengths of Cl?, NO??, IO?? with 1 are found to be strongest among all singly charged anions, amounting to ca. 30 kcal/mol, but only about 40% of that between 1 and SO?²?. Quantum chemical calculations reveal that all anions reside in the center of the cavity of 1 with anion–? binding motif in the complexes’ optimized structures, where 1 is seen to be able to self-regulate its cavity structure to accommodate anions of different geometries and three-dimensional shapes. Electron density surface and natural bond orbital charge distribution analysis further support anion–? binding formation. The calculated binding energies of the anions and 1 nicely reproduce the experimentally estimated electron binding energy increase. This work illustrates that size-selective photoelectron spectroscopy combined with theoretical calculations represent a powerful technique to probe intrinsic anion–? interactions and has potential to provide quantitative guest-host molecular binding strengths and unravel fundamental insights in specific anion recognitions.
Investigation of transient heat current from first principles using complex absorbing potential
NASA Astrophysics Data System (ADS)
Yu, Zhizhou; Zhang, Lei; Xing, Yanxia; Wang, Jian
2014-09-01
We report on a first-principles investigation of transient heat current through molecular devices under steplike pulse of external and gate voltages. Using the nonequilibrium Green's function (NEGF) approach, an exact solution of transient heat current is obtained that goes beyond the wide-band limit. Combining with density-functional theory (DFT), we propose a time-dependent NEGF-DFT formalism to study the transient heat current under a steplike pulse for molecular devices from first principles. Anticipating the huge computational cost in the transient regime, we develop an algorithm to speed up the calculation using the complex absorbing potential (CAP). By adding the CAP to replace the Hamiltonian of leads, the effective self-energy of the Green's function becomes independent of energy, allowing analytic calculation of the triple integrations in the exact solution of transient heat current using the theorem of residue. With this linear scaling algorithm, the computational complexity is greatly reduced, and a first-principles calculation of transient heat current of molecular devices becomes possible. As an example, we apply our NEGF-DFT-CAP formalism for a molecular device, the Di-thiol benzene molecule connected by two semi-infinite aluminum leads, and we calculate the transient heat current under an upward gate voltage pulse. The enhancement of heat current is observed.
NASA Astrophysics Data System (ADS)
Tajima, Nobuo; Kaneko, Tomoaki; Nara, Jun; Takahisa, Ohno
2015-03-01
Graphene has attracted considerable research interest owing to its potential application to future electronic devices. Large area and high quality graphene is needed for device applications. Chemical vapor deposition (CVD) using a copper surface with a hydrocarbon source is one of the practical methods to produce graphene. This method is appropriate for creating large area graphene with low cost, and the graphene growth control to obtain a high quality product is a remaining challenge. The carbon atom nucleation and cluster growth processes in the CVD reactions have been studied extensively as key steps that affect the graphene growth behavior. We have been studying the carbon atom reactions in these processes by theoretical approaches In the present study, we have focused on the later stage of CVD reaction, that is, carbon atom reactions at graphene edges by which carbon clusters grow in the Cu-CVD We have found that these reactions have energy barriers of ~1 eV. First principles simulation code PHASE http://www.ciss.iis.u-tokyo.ac.jp/riss/english/project/device/) was used in the theoretical calculations.
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.
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 Å3, 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 Å3, 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).
Schmidt, Wolf Gero
2013-01-01
PHYSICAL REVIEW B 87, 195208 (2013) Optical response of stoichiometric and congruent lithium niobate from first-principles calculations A. Riefer,* S. Sanna, A. Schindlmayr, and W. G. Schmidt of ferroelectric LiNbO3 are calculated from first principles. The calculations are based on the electronic
Widom, Michael
First-principles calculations of cohesive energies in the Al-Co binary alloy system M. Mihalkovic2.hP28 LT ; AlCo.cP2 LT ; and Co.hP2 LT . Goedecke and Ellner7 interchange the relative composition 2 -Al13Co4. We apply first-principles total energy calculations to com- pare cohesive energies
Krasheninnikov, Arkady V.
2014-01-01
PHYSICAL REVIEW B 89, 035120 (2014) Electronic stopping power from first-principles calculations electronic stopping power Se of energetic ions in graphitic targets from first principles. By treating core into the dependence of the electronic stopping power Se on projectile velocity have been obtained with the explicit
PHYSICAL REVIEW B 88, 045117 (2013) First-principles study of CaFe2As2 under pressure
Widom, Michael
2013-01-01
PHYSICAL REVIEW B 88, 045117 (2013) First-principles study of CaFe2As2 under pressure Michael Widom 2013) We perform first-principles calculations on CaFe2As2 under hydrostatic pressure. Our total disorder compared to chemical substitution. CaFe2As2, the smallest-volume member of this family
Simons, Jack
Quantum Manifestations of Graphene Edge Stress and Edge Instability: A First-Principles Study Bing have performed first-principles calculations of graphene edge stresses, which display two interest- ing.05.Tp Graphene, a two-dimensional (2D) single layer of car- bon atoms, has attracted tremendous
Stephens, R
1978-08-01
Design details are given for a high-sensitivity Zeeman-modulated spectrometer using commercial hollow-cathode lamp sources. Experimental detection limits are given for 6 elements, and the values compared with theoretical values calculated on the basis of signals limited only by photon noise. Some conclusions are drawn regarding the sensitivity limits accessible to atomic-absorption spectroscopy with present technology. PMID:18962295
Tunable topological electronic structures in Sb(111) bilayers: A first-principles study
NASA Astrophysics Data System (ADS)
Chuang, Feng-Chuan; Hsu, Chia-Hsiu; Chen, Chia-Yu; Huang, Zhi-Quan; Ozolins, Vidvuds; Lin, Hsin; Bansil, Arun
2013-01-01
Electronic structures and band topology of a single Sb(111) bilayer in the buckled honeycomb configuration are investigated using first-principles calculations. A nontrivial topological insulating phase can be induced by tensile strain, indicating the possibility of realizing the quantum spin Hall state for Sb thin films on suitable substrates. The presence of buckling provides an advantage in controlling the band gap through an out-of-plane external electric field, making a topological phase transition with six spin-polarized Dirac cones at the critical point. With a tunable gap and reversible spin polarization, Sb thin films are promising candidates for spintronic applications.
Interplay of strain, polarization and magnetic ordering in complex oxides from first principles
NASA Astrophysics Data System (ADS)
Eklund, Carl-Johan
We study mechanisms of structural and magnetic phase transitions in crystalline oxides from first principles. The focus is on epitaxial stabilization in perovskites and on magnetoelastic coupling and frustration in spinels. These materials and phenomena are of great interest for basic science and have important roles to play in the design and discovery of new functional materials. The effects of epitaxial strain on the structure of the perovskite oxide CaTiO3 are investigated. Particular attention is paid to the stabilization of a ferroelectric phase related to the polar instability found in previous first-principles studies of calcium titanate in the ideal cubic perovskite structure. At 1.5% strain, we find an epitaxial orientation transition between the ab-ePbnm phase, favoured for compressive strains, and the c-ePbnm phase. For larger tensile strains, a polar instability, which was hidden in the equilibrium bulk structure, develops in the c-ePbnm phase and an epitaxial-strain-induced ferroelectric phase is obtained with polarization along a [110] direction with respect to the primitive perovskite lattice vectors of the square substrate. A ferroelectric rhombohedral R3c phase, with a different combination of octahedral rotations, is also found to be competitive in energy for large tensile strains, and might be observable under the application of additional perturbations, such as a small degree of cation substitution. We present an ongoing project to construct a first-principles effective Hamiltonian to investigate the transition from the high-temperature cubic phase to a low-temperature low-symmetry phase observed in the spinel structure oxides CdCr2O4 and ZnCr2O4. The local modes included in the expansion are the chromium displacements, distortions of the cadmium- or zinc-centred tetrahedra, and the homogeneous strain. The magnetostructural coupling of these degrees of freedom to the spins of the chromium ions is included in the effective Hamiltonian parametrization and first-principles determination using a symmetry analysis. The role of the magnetostructural coupling in the phase transition is analysed and discussed.
d0 Ferromagnetism in oxygen-doped CuCl: First principles study
NASA Astrophysics Data System (ADS)
Adli, W.; Ferhat, M.
2014-07-01
Based on our first-principles all-electron and pseudopotential calculations, CuCl doped by a nonmagnetic d0 oxygen element is predicted to be ferromagnetic. O-doped (3%) CuCl shows half-metallic ferromagnetism with a net total magnetic moment of 1.0?B. Furthermore the unusual d0 ferromagnetism in O-doped CuCl can be explained in terms of O(2p)-Cu(3d) like hybridization mechanism. These results suggest that O doped CuCl may be a promising half-metallic material free from magnetic precipitate and may find application in the field of spintronics.
A First Principles Investigation of the Electronic Structure of Actinide Oxides
Petit, Leon; Svane, Axel; Szotek, Zdzislawa; Temmerman, Walter M; Stocks, George Malcolm
2010-04-01
The ground state electronic structures of the actinide oxides AO, A{sub 2}O{sub 3} and AO{sub 2} (A=U, Np, Pu, Am, Cm, Bk, Cf) are determined from first-principles calculations using the self-interaction corrected local spin-density approximation. Our study reveals a strong link between preferred oxidation number and degree of localization. The ionic nature of the actinide oxides emerges from the fact that those oxides where the ground state is calculated to be metallic do not exist in nature, as the corresponding delocalized f-states favour the accommodation of additional O atoms into the crystal lattice.
First-Principles Study of the Doping Effects in Bilayer Graphene
Mao, Y. L.; Stocks, George Malcolm; Zhong, Jianxin
2010-01-01
We used first-principles calculations to study the doping effects in bilayer graphene, focusing on Au substitute doping in the upper layer of graphene. We found that Au doping in the upper layer maintains the lattice structure of the lower graphene layer. Our study on binding energy shows that the Au-doped bilayer structure is stable with Au atom tightly confined in a small region between the upper and lower layers. Charge density analysis indicates that charge is transferred from the Au donor to the carbon atoms in the lower layer, increasing the carrier density in the lower graphene.
First principles calculations of oxygen adsorption on the UN(0 0 1) surface
NASA Astrophysics Data System (ADS)
Zhukovskii, Yu. F.; Bocharov, D.; Kotomin, E. A.; Evarestov, R. A.; Bandura, A. V.
2009-01-01
Fabrication, handling and disposal of nuclear fuel materials require comprehensive knowledge of their surface morphology and reactivity. Due to unavoidable contact with air components (even at low partial pressures), UN samples contain considerable amount of oxygen impurities affecting fuel properties. In this study we focus on reactivity of the energetically most stable (0 0 1) substrate of uranium nitride towards the atomic oxygen as one of initial stages for further UN oxidation. The basic properties of O atoms adsorbed on the UN(0 0 1) surface are simulated here combining the two first principles calculation methods based on the plane wave basis set and that of the localized orbitals.
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 Theory of van der Waals Forces between Macroscopic Bodies
NASA Astrophysics Data System (ADS)
Yannopapas, Vassilios; Vitanov, Nikolay V.
2007-09-01
We present a first-principles method for the determination of the van der Waals interactions for a collection of finite-sized macroscopic bodies. The method is based on fluctuational electrodynamics and a rigorous multiple-scattering method for the electromagnetic field. As such, the method takes fully into account retardation, many-body, multipolar, and near-fields effects. By application of the method to the case of two metallic nanoparticles, we demonstrate the breakdown of the standard 1/r2 distance law as the van der Waals force decays exponentially with distance when the nanoparticles are too close or too far apart.
Emergence of Simple Patterns in Complex Atomic Nuclei from First Principles
NASA Astrophysics Data System (ADS)
Dytrych, T.; Launey, K. D.; Draayer, J. P.; Maris, P.; Vary, J. P.; Langr, D.; Oberhuber, T.
2015-09-01
We study the structure of low-lying states in 6Li, 6He, 8Be, 8B, 12C, and 16O, using ab initio symmetry-adapted no-core shell model. The results of our study demonstrate that collective modes in light nuclei emerge from first principles. We investigate the impact of the symmetry-adapted model space on spectroscopic properties and, in the case of the ground state of 6Li, on elastic electron scattering charge form factor. The results confirm that only a small symmetry-adapted subspace of the complete model space is needed to accurately reproduce complete-space observables and the form factor momentum dependence.
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 study of water on copper and noble metal (110) surfaces
Ren, Jun; Meng, Sheng [Department of Physics and School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 (United States)
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.
NASA Astrophysics Data System (ADS)
Zhao, P.; Liu, D. S.; Wang, P. J.; Zhang, Z.; Fang, C. F.; Ji, G. M.
2011-02-01
By applying non-equilibrium Green’s function (NEGF) formalism combined with first-principles density functional theory (DFT), we have investigated the electronic transport properties of the anthraquinone-based molecular switch. The molecule that comprises the switch can be converted between the hydroquinone (HQ) and anthraquinone (AQ) forms via redox reactions. The transmission spectra of these two forms are remarkably distinctive. Our results show that the current through the HQ form is significantly larger than that through the AQ form, which suggests that this system has attractive potential application in future molecular switch technology.
First-principles calculation of the structural stability of 6d transition metals
Oestlin, A. [Department of Materials Science and Engineering, Applied Materials Physics, KTH Royal Institute of Technology, Stockholm SE-100 44 (Sweden); Vitos, L. [Department of Materials Science and Engineering, Applied Materials Physics, KTH Royal Institute of Technology, Stockholm SE-100 44 (Sweden); Department of Physics and Materials Science, Uppsala University, P.O. Box 516, SE-75120 Uppsala (Sweden); Research Institute for Solid State Physics and Optics, P.O. Box 49, H-1525 Budapest (Hungary)
2011-09-15
The phase stability of the 6d transition metals (elements 103-111) is investigated using first-principles electronic-structure calculations. Comparison with the lighter transition metals reveals that the structural sequence trend is broken at the end of the 6d series. To account for this anomalous behavior, the effect of relativity on the lattice stability is scrutinized, taking different approximations into consideration. It is found that the mass-velocity and Darwin terms give important contributions to the electronic structure, leading to changes in the interstitial charge density and, thus, in the structural energy difference.
Spin crossover in solid and liquid (Mg,Fe)O from first-principles molecular dynamics
NASA Astrophysics Data System (ADS)
Stixrude, L. P.; Holmstrom, E.
2014-12-01
Ferropericlase, (Mg,Fe)O, is a major constituent of the Earth's lowermantle. Understanding the properties of this component is importantnot only in the solid state, but also in the molten state, as theplanet almost certainly hosted an extensive magma ocean in Hadeantimes. Using first-principles molecular dynamics simulations,thermodynamic integration, and adiabatic switching, we present a phasediagram of the spin crossover, i.e., the pressure-induced collapse ofmagnetic moments, in both solid and liquid (Mg,Fe)O. Furthermore, wecompute the thermodynamic properties and the electrical conductivityof the crystal and melt over a wide range of conditions. We thenpresent the geophysical implications of our findings.
First-principles study of spin transport in Fe-SiCNT-Fe magnetic tunnel junction
NASA Astrophysics Data System (ADS)
Choudhary, Sudhanshu; Jalu, Surendra
2015-08-01
We report first-principles calculations of spin-dependent quantum transport in Fe-SiCNT-Fe magnetic tunnel junction (MTJ). Perfect spin filtration effect and substantial tunnel magnetoresistance are obtained, which suggests SiCNTs as a suitable candidate over CNTs for implementing 1D MTJs. The calculated tunnel magnetoresistance is several hundred percent at zero bias voltage, it reduces to nearly zero after the bias voltage of about 1 V. When the orientation of magnetic configurations of both electrodes is parallel, the zero bias spin injection factor is staggering 99% and remains reasonably high in the range of 60%-75% after the bias voltage of 0.6 V.
Gerra, G; Tagantsev, A K; Setter, N
2007-05-18
We present an approach to the size effect problem in ferroelectric-electrode systems which combines first-principles calculations and phenomenological theory. The parameters of the model can be extracted from calculations on ultrathin films, while experimentally verifiable predictions can be made on thick films. We illustrate the approach for the case of SrRuO3/BaTiO3/SrRuO3 heterostructures with asymmetric interfaces. This enables us to provide a quantitative description of a number of manifestations of such asymmetry in films of technologically meaningful thickness. PMID:17677739
Radl, Stefan; Khinast, Johannes G
2007-08-01
Bubble flows in non-Newtonian fluids were analyzed using first-principles methods with the aim to compute and predict mass transfer coefficients in such fermentation media. The method we used is a Direct Numerical Simulation (DNS) of the reactive multiphase flow with deformable boundaries and interfaces. With this method, we are able for the first time to calculate mass transfer coefficients in non-Newtonian liquids of different rheologies without any experimental data. In the current article, shear-thinning fluids are considered. However, the results provide the basis for further investigations, such as the study of viscoelastic fluids. PMID:17216657
First principles study of structural stability and electronic structure of CdS nanoclusters
Datta, Soumendu; Saha-Dasgupta, Tanusri; Sarma, D D
2008-01-01
Using first-principles density functional calculations, we have studied the structural stability of stoichiometric as well as non-stoichiometric CdS nanoclusters at ambient pressure with diameters ranging up to about 2.5 nm. Our study reveals that the relative stability of the two available structures for CdS, namely zinc blende and wurtzite, depends sensitively on the details like surface geometry and/or surface chemistry. The associated band gap also exhibits non-monotonic behavior as a function of cluster size. Our findings may shed light on reports of experimentally observed structures and associated electronic structures of CdS nanoclusters found in the literature.
D. V. Suetin; I. R. Shein; A. L. Ivanovskii
2009-03-31
First principles FLAPW- GGA calculations have been performed to predict the structural, electronic, cohesive and magnetic properties for hexagonal WC doped with all 3d metals. The optimized lattice parameters, density of states, cohesive and formation energies have been obtained and analyzed for ternary solid solutions with nominal compositions W0.875M0.125C (where M = Sc, Ti, ..., Ni, Cu). In addition, the magnetic properties of these solid solutions have been examined, and magnetization has been established for W0.875Co0.125C.
Elastic properties of Ca-based metallic glasses predicted by first-principles simulations
NASA Astrophysics Data System (ADS)
Widom, M.; Sauerwine, B.; Cheung, A. M.; Poon, S. J.; Tong, P.; Louca, D.; Shiflet, G. J.
2011-08-01
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.
NASA Astrophysics Data System (ADS)
Khachai, H.; Khenata, R.; Haddou, A.; Bouhemadou, A.; Boukortt, A.; Soudini, B.; Boukabrine, F.; Abid, H.
2009-11-01
We present ab initio calculations for CaS, CaSe and CaTe, in the B1 (NaCl) and B2 (CsCl) phase, by means of accurate first principle total energy calculations using the all-electron full-potential linear muffin-tin orbital method FP-LMTO.The calculations are presented within the local density approximation (LDA). Results are given for structural, electronic and elastic. Good agreement is found with experimental data. The efficiency and the accuracy of this method are so appreciated to predict some properties of this kind of materials.
First-principle and empirical modelling of the global-scale ionosphere
NASA Technical Reports Server (NTRS)
Schunk, R. W.; Szuszczewicz, Edward P.
1988-01-01
The SUNDIAL program offers a unique opportunity to study ionospheric behavior on a global scale. As part of this program, data pertaining to solar, interplanetary, magnetospheric, ionospheric, and thermospheric conditions are collected simultaneously from a large number of satellite and ground-based sites spread around the world. In the coming years, these data should lead to a major improvement in both empirical and first-principle ionospheric models. As a benchmark against which to compare future progress, the present state of empirical and numerical ionospheric modeling is discussed. The discussion covers the capabilities and limitations of the existing models as well as the direction of future modeling efforts.
First Principles Phase Diagram Calculation For AlxGa1-xN
NASA Astrophysics Data System (ADS)
Nicklas, Jeremy; Wilkins, John
2008-03-01
First principles phase diagram calculations were performed for the wurtzite and zincblende structures of the quasibinary system AlN-GaN. The cluster expansion method using the code ATAT was performed without and with excess vibrational contributions to the free energy, Fvib. The ab initio calculations were performed with VASP using the PAW pseudopotentials with PBE for the exchange and correlation energies. Preliminary results show miscibility gaps for both structures with a decrease in the consolute points, (XC,TC), when including Fvib. The wurtzite structure is predicted to be approximately symmetric while the zincblende is predicted to be quite assymetric.
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.%.
First-principles study of Li ion diffusion in LiFePO4
Chuying Ouyang; Siqi Shi; Zhaoxiang Wang; Xuejie Huang; Liquan Chen
2004-01-01
The diffusion mechanism of Li ions in the olivine LiFePO4 is investigated from first-principles calculations. The energy barriers for possible spatial hopping pathways are calculated with the adiabatic trajectory method. The calculations show that the energy barriers running along the c axis are about 0.6, 1.2, and 1.5 eV for LiFePO4, FePO4, and Li0.5FePO4, respectively. However, the other migration pathways
NASA Astrophysics Data System (ADS)
Bahadur, Amar; Verma, Mohan L.; Mishra, Madhukar
2015-04-01
Using first principle calculation, we investigate the structural, electronic and magnetic properties of silicon doped zigzag boron nitride nanoribbon (ZBNNR). Our results show that the shift in position of silicon doping with respect to the ribbon edge causes change in the structural geometry, electronic structure and magnetization of ZBNNR. The band gap of silicon doped ZBNNR is found to become narrower as compared to that of perfect ZBNNR. We find that band gap and magnetic moment of ZBNNR can be tuned by substitutional silicon doping position and doping concentration.
Unusual compression behavior of TiO2 polymorphs from first principles
NASA Astrophysics Data System (ADS)
Zhou, Xiang-Feng; Dong, Xiao; Qian, Guang-Rui; Zhang, Lixin; Tian, Yonjun; Wang, Hui-Tian
2010-08-01
The physical mechanisms behind the reduction in the bulk modulus of a high-pressure cubic TiO2 phase are revealed by first-principles calculations. An unusual and abrupt change occurs in the dependence of energy on pressure at 43 GPa, indicating a pressure-induced phase transition from columbite TiO2 to a modified fluorite TiO2 with a Pca21 symmetry. Oxygen atom displacement in Pca21 TiO2 unexpectedly reduces the bulk modulus by 34% relative to fluorite TiO2 . This discovering provides a direct evidence for understanding the compressive properties of such groups of homologous materials.
First principle calculations of hexyl thiolate monolayer on Au(1 1 1)
NASA Astrophysics Data System (ADS)
Wu, Taiquan; Cao, Dan; Wang, Xinyan; Jiao, Zhiwei; Chen, Miaogen; Luo, Honglei; Zhu, Ping
2015-03-01
The first-principle technique has been employed to determine the structure of hexyl thiolate molecular chains, monolayers and the adsorption system. CASTEP calculation shows that hexyl thiolate monolayer is a self-assembly system. And the molecular orientation of the hexyl thiolate on the surface is not symmetrical, they have the simplex structure. The electron density confirms the result. Hexyl thiolate monolayer is adsorbed on the Au(1 1 1)-(?3 × ?3)R30° surface in the bridge site with the angle between the Ssbnd C6 bond and the surface is 65°. The structural parameters in the adsorption system are the same to those in the monolayer.
NASA Astrophysics Data System (ADS)
Liu, Xiaojie; Wang, Cai-Zhuang; Lin, Hai-Qing; Chang, Kai; Chen, Jian; Ho, Kai-Ming
2015-01-01
Interaction between K adatoms on graphene is investigated by first-principles calculations based on density function theory and analytical analyses based on the k .p perturbation theory. The calculation shows that there is a strong repulsion between K adatoms. The main origin of this strong repulsion is not from the dipole-dipole interaction as suggested for K adatoms on graphite surface, but comes from the screened Coulomb interaction. Potassium adatom on graphene donates its s electron and becomes K+. The positively charged K adatom induces electron density oscillation on graphene which is responsible for the screened Coulomb repulsion between the K adatoms.
First-principles theory of quantum well resonance in double barrier magnetic tunnel junctions.
Wang, Yan; Lu, Zhong-Yi; Zhang, X-G; Han, X F
2006-08-25
Quantum well (QW) resonances in Fe(001)/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions are calculated from first principles. By including the Coulomb blockade energy due to the finite size islands of the middle Fe film, we confirm that the oscillatory differential resistance observed in a recent experiment [T. Nozaki, Phys. Rev. Lett. 96, 027208 (2006)10.1103/PhysRevLett.96.027208] originates from the QW resonances from the Delta1 band of the Fe majority-spin channel. The primary source of smearing at low temperatures is shown to be the variation of the Coulomb blockade energy. PMID:17026335
First-Principles Study of Structural and Electronic Properties of Germanene
NASA Astrophysics Data System (ADS)
Behera, Harihar; Mukhopadhyay, Gautam
2011-07-01
The ground state structural and electronic properties of germanene (the germanium analogue of graphene) are investigated using first-principles calculations. On structure optimization, the grapheme-like honeycomb structure of germanene turns out as buckled (buckling parameter ? = 0.635 Å) in contrast with graphene's planar structure (buckling parameter ? = 0.0 Å). In spite of this, germanene has similar electronic structure as that of graphene. While corroborating the reported results, we newly predict the in-plane contraction of hexagonal Ge with (thermal) stretching along the "c" axis, akin to a phenomenon observed in graphite.
Energy investigations on the adhesive properties of Al/TiC interfaces: First-principles study
NASA Astrophysics Data System (ADS)
Wu, Xiaozhi; Sun, Ting; Wang, Rui; Liu, Lili; Liu, Qing
2014-09-01
The work of adhesion and interface energy of Al(100)/TiC(100) and Al(110)/TiC(110) coherent interface systems have been investigated by first-principles methods. To account for the effects of misfit dislocations on the semicoherent interfaces, the Peierls-Nabarro model combined with generalized stacking fault energy is employed to determine interface energy. The energy level diagrams are introduced to describe the adhesive properties of coherent and semicoherent interfaces. It is found that misfit dislocations can reduce the adhesion of interface, and the reduction is increasing with the unstable stacking fault energy.
NASA Astrophysics Data System (ADS)
Hu, Ting; Zhou, Jian; Dong, Jinming
2013-01-01
The vibrational properties and Raman spectra of graphene nanoribbons with six different edges have been studied by using the first-principles calculations. It is found that edge reconstruction leads to the emergence of localized vibrational modes and new topological defect modes, making the different edges identified by polarized Raman spectra. The radial breathing-like modes are found to be independent of the edge structures, while the G-band-related modes are affected by different edge structures. Our results suggest that the polarized Raman spectrum could be a powerful experimental tool for distinguishing the GNRs with different edge structures due to their different vibrational properties.
First-principles study of the mobility of SrTiO3
NASA Astrophysics Data System (ADS)
Himmetoglu, Burak; Janotti, Anderson; Peelaers, Hartwin; Alkauskas, Audrius; Van de Walle, Chris G.
2014-12-01
We investigate the electronic and vibrational spectra of SrTiO3, as well as the coupling between them, using first-principles calculations. We compute electron-phonon scattering rates for the three lowest-energy conduction bands and use Boltzmann transport theory to calculate the room-temperature mobility of SrTiO3. The results agree with experiment and highlight the strong impact of longitudinal optical phonon scattering. Our analysis provides important insights into the key factors that determine room-temperature mobility, such as the number of conduction bands and the nature and frequencies of longitudinal phonons. Such insights provide routes to engineering materials with enhanced mobilities.
First-principles studies of SnS2 nanotubes: a potential semiconductor nanowire.
Chang, Hyunju; In, Eunjeong; Kong, Ki-Jeong; Lee, Jeong-O; Choi, Youngmin; Ryu, Beyong-Hwan
2005-01-13
First principles calculations are used to predict the stability and electronic structures of SnS(2) nanotubes. Optimization of several structures and their corresponding strain energies confirm the stability of SnS(2) nanotube structures. Band structure calculations show that SnS(2) nanotubes could have moderate band gaps regardless of their chirality. It suggests that SnS(2) nanotubes would be well-suited to use as semiconductor wires in nanoelectronic devices if they are synthesized. Adsorption of NH(3) onto SnS(2) is also investigated and discussed with regard to potential sensor application. PMID:16850978
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.
First principles calculation of the effect of Coulomb collisions in partially ionized gases
Donkó, Z. [Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box No. 49, H-1525 Budapest (Hungary)] [Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box No. 49, H-1525 Budapest (Hungary)
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.
Half metallic ferromagnetism in alkali metal nitrides MN (M = Rb, Cs): A first principles study
Murugan, A., E-mail: rrpalanichamy@gmail.com; Rajeswarapalanichamy, R., E-mail: rrpalanichamy@gmail.com; Santhosh, M., E-mail: rrpalanichamy@gmail.com; Sudhapriyanga, G., E-mail: rrpalanichamy@gmail.com [Department of Physics, N.M.S.S.V.N College, Madurai, Tamilnadu-625019 (India); Kanagaprabha, S. [Department of Physics, Kamaraj College, Tuticorin, Tamil Nadu-628003 (India)
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.
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.
NASA Astrophysics Data System (ADS)
Hofer, W. A.; Redinger, J.; Biedermann, A.; Varga, P.
2000-11-01
Scanning tunneling microscopy (STM) scans on Fe(100) are compared with first principles calculations of the tunneling current based on the transfer Hamiltonian method. Experimentally, we find a reversal of corrugation for separations between sample and tip below 400 pm. In the simulations we can reproduce these topographies only in a distance range above 400 pm. The approach therefore fails to describe the observed corrugation reversal. We suggest that this failure is due to the quenching of surface states by the approaching STM tip.
NASA Astrophysics Data System (ADS)
Jakse, N.; Pasturel, A.
2007-11-01
We report results of first principles molecular dynamics simulations that confirm early speculations on the presence of liquid-liquid phase transition in undercooled silicon. However, we find that structural and electronic properties of both low-density liquid (LDL) and high-density liquid (HDL) phases are quite different from those obtained by empirical calculations, the difference being more pronounced for the HDL phase. The discrepancy between quantum and classical simulations is attributed to the inability of empirical potentials to describe changes in chemical bonds induced by density and temperature variations.
NASA Astrophysics Data System (ADS)
Kondo, Kei-Ichi
2010-09-01
We give a theoretical framework to obtain a low-energy effective theory of quantum chromodynamics (QCD) towards a first-principle derivation of confinement/deconfinement and chiral-symmetry breaking/restoration crossover transitions. In fact, we demonstrate that an effective theory obtained using simple but nontrivial approximations within this framework enables us to treat both transitions simultaneously on equal footing. A resulting effective theory is regarded as a modified and improved version of nonlocal Polyakov-loop extended Nambu-Jona-Lasinio (nonlocal PNJL) models proposed recently by Hell, Rössner, Cristoforetti, and Weise, and Sasaki, Friman, and Redlich, extending the original (local) PNJL model by Fukushima and others. A novel feature is that the nonlocal NJL coupling depends explicitly on the temperature and Polyakov loop, which affects the entanglement between confinement and chiral-symmetry breaking, together with the cross term introduced through the covariant derivative in the quark sector considered in the conventional PNJL model. The chiral-symmetry breaking/restoration transition is controlled by the nonlocal NJL interaction, while the confinement/deconfinement transition in the pure gluon sector is specified by the nonperturbative effective potential for the Polyakov loop obtained recently by Braun, Gies, Marhauser, and Pawlowski. The basic ingredients are a reformulation of QCD based on new variables and the flow equation of the Wetterich type in the Wilsonian renormalization group. This framework can be applied to investigate the QCD phase diagram at finite temperature and density.
NASA Astrophysics Data System (ADS)
Ma, Ling; Zhang, Jian-Min; Xu, Ke-Wei; Ji, Vincent
2014-09-01
As a candidate for hydrogen storage medium, geometric stability and hydrogen capacity of Ca-decorated graphene with topological defects are investigated using the first-principle based on density functional theory (DFT), specifically for the experimentally realizable single carbon vacancy (SV), 585 double carbon vacancy (585 DCV) and 555-777 double carbon vacancy (555-777 DCV) defects. It is found that Ca atom can be stabilized on above defective graphenes since Ca's binding energy on vacancy defect is much larger than its cohesive energy. Up to six H2 molecules can stably bind to a Ca atom on defective graphene with the average adsorption energies of 0.17-0.39 eV/H2. The hybridization of the Ca-3d orbitals with H2-?orbitals and the electrostatic interaction between the Ca cation and the induced H2 dipole both contribute to the H2 molecules binding. Double-side Ca-decorated graphene with 585 DCV and 555-777 DCV defects can theoretically reach a gravimetric capacity of 5.2 wt% hydrogen, indicating that Ca-decorated defective graphene can be used as a promising material for high density hydrogen storage.
Andrés, Juan; Gracia, Lourdes; Gouveia, Amanda Fernandes; Ferrer, Mateus Meneghetti; Longo, Elson
2015-10-01
Morphology is a key property of materials. Owing to their precise structure and morphology, crystals and nanocrystals provide excellent model systems for joint experimental and theoretical investigations into surface-related properties. Faceted polyhedral crystals and nanocrystals expose well-defined crystallographic planes depending on the synthesis method, which allow for thoughtful investigations into structure-reactivity relationships under practical conditions. This feature article introduces recent work, based on the combined use of experimental findings and first-principles calculations, to provide deeper knowledge of the electronic, structural, and energetic properties controlling the morphology and the transformation mechanisms of different metals and metal oxides: Ag, anatase TiO2, BaZrO3, and ?-Ag2WO4. According to the Wulff theorem, the equilibrium shapes of these systems are obtained from the values of their respective surface energies. These investigations are useful to gain further understanding of how to achieve morphological control of complex three-dimensional crystals by tuning the ratio of the surface energy values of the different facets. This strategy allows the prediction of possible morphologies for a crystal and/or nanocrystal by controlling the relative values of surface energies. PMID:26377834
Kondo, Kei-Ichi [Department of Physics, Chiba University, Chiba 263-8522 (Japan); Department of Physics, University of Tokyo, Tokyo 113-0033 (Japan)
2010-09-15
We give a theoretical framework to obtain a low-energy effective theory of quantum chromodynamics (QCD) towards a first-principle derivation of confinement/deconfinement and chiral-symmetry breaking/restoration crossover transitions. In fact, we demonstrate that an effective theory obtained using simple but nontrivial approximations within this framework enables us to treat both transitions simultaneously on equal footing. A resulting effective theory is regarded as a modified and improved version of nonlocal Polyakov-loop extended Nambu-Jona-Lasinio (nonlocal PNJL) models proposed recently by Hell, Roessner, Cristoforetti, and Weise, and Sasaki, Friman, and Redlich, extending the original (local) PNJL model by Fukushima and others. A novel feature is that the nonlocal NJL coupling depends explicitly on the temperature and Polyakov loop, which affects the entanglement between confinement and chiral-symmetry breaking, together with the cross term introduced through the covariant derivative in the quark sector considered in the conventional PNJL model. The chiral-symmetry breaking/restoration transition is controlled by the nonlocal NJL interaction, while the confinement/deconfinement transition in the pure gluon sector is specified by the nonperturbative effective potential for the Polyakov loop obtained recently by Braun, Gies, Marhauser, and Pawlowski. The basic ingredients are a reformulation of QCD based on new variables and the flow equation of the Wetterich type in the Wilsonian renormalization group. This framework can be applied to investigate the QCD phase diagram at finite temperature and density.
NASA Astrophysics Data System (ADS)
Wippermann, Stefan; Pérez León, Carmen; Drees, Holger; Marz, Michael; Hoffmann-Vogel, Regina
2015-03-01
Stepped well-ordered surfaces are important nanotemplates for the fabrication of one-dimensional nanostructures with intriguing electronic properties. The vicinal Si(7710) surface is an important model system of this kind. It contains (7x7) reconstructed areas equivalent to the well characterized and understood Si(111)-(7x7) surface. Thereby this system essentially contains its own calibration, providing an ideal testbed for surface characterization techniques and understanding in depth the rich morphology of the structural features present in this system. Here we present a joint experimental and theoretical investigation of the structural properties of the vicinal Si(7710) surface. We carried out Kelvin probe force microscopy (KPFM) measurements with unprecedented atomic resolution, and first principles calculations of the local work function as a function of the lateral position of the tip above the surface. These calculations allowed us to interpret the experimental KPFM data in terms of specific structural features and electronic properties of surface states, such as e. g. defects, dangling bond angles and occupations of dangling bonds. R. H.-V. acknowledges ERC starting grant NANOCONTACTS No. ERC 2009-Stg 239838.
First-Principles Study on Structural and Thermoelectric Properties of Al- and Sb-Doped Mg2Si
NASA Astrophysics Data System (ADS)
Hirayama, Naomi; Iida, Tsutomu; Funashima, Hiroki; Morioka, Shunsuke; Sakamoto, Mariko; Nishio, Keishi; Kogo, Yasuo; Takanashi, Yoshifumi; Hamada, Noriaki
2015-06-01
We theoretically investigate the structural and thermoelectric properties of magnesium silicide (Mg2Si) incorporating Al or Sb atoms as impurities using first-principles calculations. We optimized the structural properties through variable-cell relaxation using a pseudopotential method based on density functional theory. The result indicates that the lattice constant can be affected by the insertion of impurity atoms into the system, mainly because the ionic radii of these impurities differ from those of the matrix constituents Mg and Si. We then estimate, on the basis of the optimized structures, the site preferences of the impurity atoms using a formation energy calculation. The result shows a nontrivial concentration-dependence of the site occupation, such that Al tends to go into the Si, Mg, and interstitial sites with comparable formation energies at low doping levels (<2 at.%); it can start to substitute for the Mg sites preferentially at higher doping levels (<4 at.%). Sb, on the other hand, shows a strong preference for the Si sites at all impurity concentrations. Furthermore, we obtain the temperature-dependence of the thermoelectromotive force (Seebeck coefficient) of the Al- and Sb-doped Mg2Si using the full-potential linearized augmented-plane-wave method and the Boltzmann transport equation.
First-principles study of hydrogen-bonded molecular conductor ? -H3(Cat-EDT-TTF/ST)2
NASA Astrophysics Data System (ADS)
Tsumuraya, Takao; Seo, Hitoshi; Kato, Reizo; Miyazaki, Tsuyoshi
2015-07-01
We theoretically study hydrogen-bonded molecular conductors synthesized recently, ? -H3(Cat-EDT-TTF) 2 and its diselena analog, ? -H3(Cat-EDT-ST) 2, by first-principles density functional theory calculations. In these crystals, two H(Cat-EDT-TTF/ST) units share a hydrogen atom with a short O-H-O hydrogen bond. The calculated band structure near the Fermi level shows a quasi-two-dimensional character with a rather large interlayer dispersion due to the absence of insulating layers, in contrast with conventional molecular conductors. We discuss effective low-energy models based on H(Cat-EDT-TTF/ST) units and its dimers, respectively, where the microscopic character of the orbitals composing them are analyzed. Furthermore, we find a stable structure which is different from the experimentally determined structure, where the shared hydrogen atom becomes localized to one of the oxygen atoms, in which charge disproportionation between the two types of H(Cat-EDT-TTF) units is associated. The calculated potential energy surface for the H atom is very shallow near the minimum points; therefore the probability of the H atom can be delocalized between the two O atoms.
NASA Astrophysics Data System (ADS)
Chang, H. J.; Chen, L. F.; Zhu, X. F.
2012-10-01
The rapid solidification processes of Ca50Mg20Cu30 liquid alloy have been simulated by first principle molecular dynamics simulation based on the density functional theory. The local structural evolution of the alloy is analysed using Honeycutt Andersen (HA) bond-type index and bond-angle distribution methods. The electronic properties of this amorphous solid are also studied. The simulated coordination numbers are very close to the theoretical values according to the efficient cluster packing model (ECP) model. The interaction between Ca-Cu atomic pairs is strongest in this alloy. The HA bond-type result shows that a large quantity of pentagonal bipyramids are formed in undercooled alloy liquid and become most common polyhedral local structures of the amorphous solid, which suppress the crystallization and increase the glass forming ability of Ca-Mg-Cu alloy. The bond-angle distribution indicates that the close packing of the three neighbor atoms and the pentagon configurations become the primary short range order (SRO) as temperature decreases. The electronic density demonstrates that the covalent bond of Ca-Mg, Ca-Cu, Mg-Cu, and Cu-Cu pairs is existed in this alloy. This chemical SRO also benefits the glass transition.
NASA Astrophysics Data System (ADS)
Liu, Qi-Jun; Zhang, Ning-Chao; Liu, Fu-Sheng; Liu, Zheng-Tang
2014-07-01
Titanium dioxide is well known as a semiconductor material, which attracts a great deal of attention for promising applications in many fields due to its outstanding physical and chemical properties. To investigate the structural, elastic, mechanical, electronic and optical properties of various TiO2 phases systematically, we present the ultrasoft pseudopotential planewave method within local density approximation and generalized gradient approximation, as well as the norm-conserving pseudopotential within hybrid functional B3LYP by first-principles calculations on fluorite, pyrite, rutile, anatase, hollandite, brookite, columbite, cotunnite, bronze and baddeleyite TiO2 phases. The structural parameters of ten phases are calculated, which are shown to be consistent with previous theoretical and experimental data. We obtain the elastic constants of ten phases and then estimate the bulk, shear and Young’s moduli, Poisson’s coefficient and Lamé’s constants using the Voigt-Reuss-Hill approximation. The energy band structures, density of states and charge populations of ten phases were obtained and indicated there is covalency in TiO2. Moreover, the complex dielectric function, refractive index and extinction coefficient of the ten phases were calculated; this data can aid future experimental research.
The role of Bi vacancies in the electrical conduction of BiFeO?: a first-principles approach.
Xu, Qiang; Sobhan, Mushtaq; Yang, Qian; Anariba, Franklin; Ong, Khuong Phuong; Wu, Ping
2014-07-28
We employ first-principles methods to study the mechanism controlling the electrical conduction in BiFeO3 (BFO). We find that under oxygen-rich conditions, Bi vacancies (V(Bi)) have lower defect formation energy than O vacancies (V(O)) (-0.43 eV vs. 3.35 eV), suggesting that V(Bi) are the acceptor defects and control the conductivity of BFO, making it a p-type semiconductor. In order to obtain further insight into the conduction mechanism, we calculate the effect of donor (Sn(4+)) and acceptor (Pb(2+)) impurities in BFO. Results indicate that Sn impurities prefer to substitute Fe sites to form shallow donor defects, which compensate the acceptor levels derived from V(Bi). Meanwhile, Pb atoms favour the substitution of Bi sites to form acceptor defects, reducing the overall concentration of holes (h(+)). Theoretical findings were later surveyed by current-voltage characteristics of Sn- or Pb-doped BFO nanofibers. This study is of general interest in carrier transport in charge compensation semiconductors, and of particular relevance within the context of defect-mediated conductivity in BFO. PMID:24879577
First principle study of elastic and thermodynamic properties of FeB{sub 4} under high pressure
Zhang, Xinyu E-mail: jiaqianqin@gmail.com Ning, Jinliang; Sun, Xiaowei; Li, Xinting; Ma, Mingzhen; Liu, Riping E-mail: jiaqianqin@gmail.com; Qin, Jiaqian E-mail: jiaqianqin@gmail.com
2013-11-14
The elastic properties, elastic anisotropy, and thermodynamic properties of the lately synthesized orthorhombic FeB{sub 4} at high pressures are investigated using first-principles density functional calculations. The calculated equilibrium parameters are in good agreement with the available experimental and theoretical data. The obtained normalized volume dependence of high pressure is consistent with the previous experimental data investigated using high-pressure synchrotron x-ray diffraction. The complete elastic tensors and crystal anisotropies of the FeB{sub 4} are also determined in the pressure range of 0–100?GPa. By the elastic stability criteria and vibrational frequencies, it is predicted that the orthorhombic FeB{sub 4} is stable up to 100 GPa. In addition, the calculated B/G ratio reveals that FeB{sub 4} possesses brittle nature in the range of pressure from 0 to 100?GPa. The calculated elastic anisotropic factors suggest that FeB{sub 4} is elastically anisotropic. By using quasi-harmonic Debye model, the compressibility, bulk modulus, the coefficient of thermal expansion, the heat capacity, and the Grüneisen parameter of FeB{sub 4} are successfully obtained in the present work.
NASA Astrophysics Data System (ADS)
Yu, Zhi-Qiang; Xu, Zhi-Mou; Wu, Xing-Hui
2014-10-01
We perform the first-principles calculations within the framework of density functional theory to determine the electronic structure and optical properties of MgxZn1?xS bulk crystal. The results indicate that the electronic structure and optical properties of MgxZn1?xS bulk crystal are sensitive to the Mg impurity composition. In particular, the MgxZn1?xS bulk crystal displays a direct band structure and the band gap increases from 2.05 eV to 2.91 eV with Mg dopant composition value x increasing from 0 to 0.024. The S 3p electrons dominate the top of valence band, while the Zn 4s electrons and Zn 3p electrons occupy the bottom of conduction band in MgxZn1?xS bulk crystal. Moreover, the dielectric constant decreases and the optical absorption peak obviously has a blue shift. The calculated results provide important theoretical guidance for the applications of MgxZn1?xS bulk crystal in optical detectors.
Cao, Xiaoxiao; Su, Yan; Zhao, Jijun
2015-07-01
Natural gas mixtures are inclusion compounds composed of major light hydrocarbon gaseous molecules (CH4, C2H6, C3H6, and C3H8). Previous ab initio calculations were mainly limited by the cluster models. For the first time, we report first-principles calculations on the stability and vibrational properties of the gas molecules inside the crystalline lattice of type II clathrate. In accordance with our calculations, the larger the size of guest molecule, the more stable the clathrate hydrate for small-sized alkane guest molecules (CnHm, n ? 3, m ? 8). The interaction energy per guest molecule gradually increases as the number of guest molecules increase for both sII pure and sII mixed hydrates. In addition, the vibrational frequencies of guest molecules trapped in sII hydrate are also simulated. The C-C stretching frequency shows a blue shift as the amount of guest molecules increase. Our theoretical results prove to be valuable insight for identifying the types of guest molecules from experimental spectroscopic data. PMID:26073359
NASA Astrophysics Data System (ADS)
Wang, Jingyang; Zhou, Yanchun; Lin, Zhijun; Liao, Ting
2005-08-01
We have investigated the electronic structure, chemical bonding, and equations of state of Zr2Al3C5 by means of the ab initio pseudopotential total energy method. The chemical bonding displays layered characteristics and is similar to that of nanolaminate ternary aluminum carbides Ti2AlC and Ti3AlC2 . Zr2Al3C5 could be fundamentally described as strong covalent bonding among Al-C-Zr-C-Zr-C-Al atomic chains being interleaved and mirrored by AlC2 blocks. The interplanar cohesion between covalent atomic chains and AlC2 blocks is very weak based on first-principles cohesion energy calculations. Inspired by the structure-property relationship of Ti2AlC and Ti3AlC2 , it is expected that Zr2Al3C5 will have easy machinability, damage tolerance, and oxidation resistance besides the merits of refractory ZrC. Zr2Al3C5 has a theoretical bulk modulus of 160GPa and illustrates elastic anisotropy under pressure below 20GPa .
NASA Astrophysics Data System (ADS)
Lei, Shu-Lai; Li, Bin; Huang, Jing; Li, Qun-Xiang; Yang, Jin-Long
2013-07-01
The carrier doping effects on the magnetic properties of defective graphene with a hydrogen chemisorbed single-atom vacancy (H-GSV) are investigated by performing extensive spin-polarized first-principles calculations. Theoretical results show that the quasi-localized pz-derived states around the Fermi level are responsible for the weakened magnetic moment (MM) and magnetic stabilized energy (MSE) of the H-GSV under carrier doping. The mechanism of reduced MSE in the carrier doped H-GSV can be well understood by the Heisenberg magnetic coupling model due to the response of these pz-derived states to the carrier doping. Within the examined range of carrier doping concentration, the total MM of H-GSV is always larger than 1.0?B with ?B representing the Bohr magneton, which is mainly contributed by the localized sp2 states of the unsaturated C atom around the vacancy. These findings of H-GSV provide fundamental insight into defective graphene and help to understand the related experimental observations.
NASA Astrophysics Data System (ADS)
Andrés, Juan; Gracia, Lourdes; Fernandes Gouveia, Amanda; Meneghetti Ferrer, Mateus; Longo, Elson
2015-10-01
Morphology is a key property of materials. Owing to their precise structure and morphology, crystals and nanocrystals provide excellent model systems for joint experimental and theoretical investigations into surface-related properties. Faceted polyhedral crystals and nanocrystals expose well-defined crystallographic planes depending on the synthesis method, which allow for thoughtful investigations into structure–reactivity relationships under practical conditions. This feature article introduces recent work, based on the combined use of experimental findings and first-principles calculations, to provide deeper knowledge of the electronic, structural, and energetic properties controlling the morphology and the transformation mechanisms of different metals and metal oxides: Ag, anatase TiO2, BaZrO3, and ?-Ag2WO4. According to the Wulff theorem, the equilibrium shapes of these systems are obtained from the values of their respective surface energies. These investigations are useful to gain further understanding of how to achieve morphological control of complex three-dimensional crystals by tuning the ratio of the surface energy values of the different facets. This strategy allows the prediction of possible morphologies for a crystal and/or nanocrystal by controlling the relative values of surface energies.
Study of Pb-doped Ge2Sb2Te5 in crystalline phase using first principle calculations
NASA Astrophysics Data System (ADS)
Singh, Janpreet; Singh, Gurinder; Kaura, Aman; Tripathi, S. K.
2015-08-01
To improve the phase change characteristics of Ge2Sb2Te5 (GST), doping is used as one of the effective methods. 4.4 atomic % of Pb doped GST has been studied using first principle calculations. No effect of doping on Te-Ge and Te-Sb bond length has been observed, but the Te-Te bond gets shrink with Pb doping. Due to which the Sb2Te3 segregates as a second phase, with increased doping concentration of Pb in GST alloy. Using such type of calculation, we can calculate the desirable concentration of dopant atoms to prepare the desired material. We can control any segregation in required material with pre-theoretical calculations. The metallic nature of Pd doped GST has been discussed with band structure plots. The metallic character of alloys calculated as in this paper will be helpful to understand the tuning of conductivity of phase change materials, which helps to enhance the phase change properties.
The first-principles study on Zr3Al and Sc3Al in L12 structure
NASA Astrophysics Data System (ADS)
Ar?kan, Nihat
2013-05-01
The elastic, electronic, and phonon properties of the intermetallic compounds Zr3Al and Sc3Al in the L12 structure have been investigated in detail by employing an ab initio pseudopotential method and a linear-response technique within a generalized gradient approximation (GGA) of the density-functional theory (DFT) scheme. The calculated ground-state properties such as lattice constants and bulk modulus agree well with the previous theoretical calculations. The numerical first-principles calculations of the elastic constants have been used to calculate C11, C12, and C44 for Zr3Al and Sc3Al. The electronic band structures of Zr3Al and Sc3Al show that at the Fermi level, a major part of the contribution comes from Zr 4d (Sc 3d) states. The phonon-dispersion curves and phonon total and partial density of states based on the linear-response method have been investigated for both materials. Temperature variations of specific heat capacity in the range of 0-500 K are obtained using the quasi-harmonic model.
NASA Astrophysics Data System (ADS)
Zhang, Yu; Tang, Fu-Ling; Xue, Hong-Tao; Lu, Wen-Jiang; Liu, Jiang-Fei; Huang, Min
2015-02-01
Using first-principles plane-wave calculations within density functional theory, we theoretically studied the atomic structure, bonding energy and electronic properties of the perfect Mo (110)/MoSe2 (100) interface with a lattice mismatch less than 4.2%. Compared with the perfect structure, the interface is somewhat relaxed, and its atomic positions and bond lengths change slightly. The calculated interface bonding energy is about -1.2 J/m2, indicating that this interface is very stable. The MoSe2 layer on the interface has some interface states near the Fermi level, the interface states are mainly caused by Mo 4d orbitals, while the Se atom almost have no contribution. On the interface, Mo-5s and Se-4p orbitals hybridize at about -6.5 to -5.0 eV, and Mo-4d and Se-4p orbitals hybridize at about -5.0 to -1.0 eV. These hybridizations greatly improve the bonding ability of Mo and Se atom in the interface. By Bader charge analysis, we find electron redistribution near the interface which promotes the bonding of the Mo and MoSe2 layer.
Thore, A. Dahlqvist, M. E-mail: bjoal@ifm.liu.se Alling, B. E-mail: bjoal@ifm.liu.se Rosén, J. E-mail: bjoal@ifm.liu.se
2014-09-14
In this paper, we report the by first-principles predicted properties of the recently discovered magnetic MAX phase Mn?GaC. The electronic band structure and vibrational dispersion relation, as well as the electronic and vibrational density of states, have been calculated. The band structure close to the Fermi level indicates anisotropy with respect to electrical conductivity, while the distribution of the electronic and vibrational states for both Mn and Ga depend on the chosen relative orientation of the Mn spins across the Ga sheets in the Mn–Ga–Mn trilayers. In addition, the elastic properties have been calculated, and from the five elastic constants, the Voigt bulk modulus is determined to be 157?GPa, the Voigt shear modulus 93?GPa, and the Young's modulus 233?GPa. Furthermore, Mn?GaC is found relatively elastically isotropic, with a compression anisotropy factor of 0.97, and shear anisotropy factors of 0.9 and 1, respectively. The Poisson's ratio is 0.25. Evaluated elastic properties are compared to theoretical and experimental results for M?AC phases where M?=?Ti, V, Cr, Zr, Nb, Ta, and A?=?Al, S, Ge, In, Sn.
NASA Astrophysics Data System (ADS)
Mašín, Martin; Bergqvist, Lars; Kudrnovský, Josef; Kotrla, Miroslav; Drchal, Václav
2013-02-01
We present a theoretical study of the thermodynamical properties of a fcc-Cu(001) substrate covered by an iron-cobalt monolayer as well as by an incomplete iron layer. The effective two-dimensional Heisenberg Hamiltonian is constructed from first principles and the properties of exchange interactions are investigated. The Curie temperatures are estimated using the Monte Carlo (MC) simulations and compared with a simplified approach using the random-phase approximation (RPA) in connection with the virtual-crystal approach (VCA) to treat randomness in exchange integrals. Calculations indicate a weak maximum of the Curie temperature as a function of composition of the iron-cobalt overlayer. While a good quantitative agreement between RPA-VCA and MC was found for the iron-cobalt monolayer, the RPA-VCA approach fails quantitatively for low coverage due to the magnetic percolation effect. We also present a study of the effect of alloy disorder on the shape of magnon spectra of random overlayers.
Bueno, Marta; Camacho, Carlos J; Sancho, Javier
2007-09-01
The bioinformatics revolution of the last decade has been instrumental in the development of empirical potentials to quantitatively estimate protein interactions for modeling and design. Although computationally efficient, these potentials hide most of the relevant thermodynamics in 5-to-40 parameters that are fitted against a large experimental database. Here, we revisit this longstanding problem and show that a careful consideration of the change in hydrophobicity, electrostatics, and configurational entropy between the folded and unfolded state of aliphatic point mutations predicts 20-30% less false positives and yields more accurate predictions than any published empirical energy function. This significant improvement is achieved with essentially no free parameters, validating past theoretical and experimental efforts to understand the thermodynamics of protein folding. Our first principle analysis strongly suggests that both the solute-solute van der Waals interactions in the folded state and the electrostatics free energy change of exposed aliphatic mutations are almost completely compensated by similar interactions operating in the unfolded ensemble. Not surprisingly, the problem of properly accounting for the solvent contribution to the free energy of polar and charged group mutations, as well as of mutations that disrupt the protein backbone remains open. PMID:17523191
NASA Astrophysics Data System (ADS)
Wu, Hsuan-Chung; Peng, Yen-Chun; Chen, Chieh-Cheng
2013-01-01
This study evaluated the electronic and optical properties of Ga-doped ZnO with various concentrations of gallium, employing first principles calculations based on density functional theory and the Hubbard U (DFT + Ud + Up). The lattice constants and band gap of ZnO calculated in this study are in agreement with experimental values. Results show that donor concentration increases with an increase in Ga concentration; however, electrical conductivity is reduced when localized states close to the Fermi level and higher scattering probability of free electrons occur with high Ga concentration. Following the incorporation of Ga into ZnO (1.4-6.3 at.%), the average transmittance of light in both the visible and UV ranges exceeds that of ZnO. However, the stronger and wider donor states obtained from high doping levels (12.5-25 at.%) significantly decreases the average transmittance. Thus, selecting a suitable doping level is crucial to optimizing the photoelectric performance of Ga-doped ZnO. This study also provides a theoretical explanation for the factors influencing these properties.
NASA Astrophysics Data System (ADS)
Lu, Deyu; Liu, Ping
2014-03-01
DFT+U method has been widely employed in theoretical studies on various ceria systems to correct the delocalization bias in local and semi-local DFT functionals with moderate computational cost. To rationalize the Hubbard U of Ce 4f, we employed the first principles linear response method to compute Hubbard U for Ce in ceria clusters, bulks, and surfaces. We found that in contrast to the commonly used approach treating U as a constant, the Hubbard U varies in a wide range from 4.1 eV to 6.7 eV, and exhibits a strong correlation with the Ce coordination numbers and Ce-O bond lengths, rather than the Ce 4f valence state. The variation of the Hubbard U can be explained by the changes in the strength of local screening due to O --> Ce intersite transition. Our study represents a systematic, quantitative investigation of the relationship between the Hubbard U and the local atomic arrangement, enabling a DFT+environment-dependent U scheme that can have potential impact on catalysis research of strongly correlated systems. This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.
NASA Astrophysics Data System (ADS)
Rebola, Alejandro; Klie, Robert; Zapol, Peter; Ogut, Serdar
2013-03-01
The misfit-layered oxide Ca3Co4O9 (CCO) has recently been the subject of many experimental and some theoretical investigations due to its remarkable thermoelectric properties. CCO is composed of two incommensurate subsystems, a distorted rocksalt-type Ca2CoO3 layer sandwiched between hexagonal CoO2 layers. Taking into account that the composition ratio between these subsystems is very close to the golden mean, which is the limit of the sequence of the ratios of consecutive Fibonacci numbers F (n) , we model CCO from first principles[1] by using rational approximants of composition [Ca2CoO3]2 F (n)[CoO2]2 F (n + 1). In the present study, we use 3/2 and 5/3 rational approximants and PBE+U computations to calculate the ab initio phonon dispersion curves, related thermal properties, as well as ab initio electronic transport properties such as DC conductivity and thermopower within the relaxation time approximation by applying the Boltzmann transport theory. Results are compared with available experimental data and potential routes for increasing the thermopower of CCO are discussed.
Phase stability analysis of the InAs/GaAs (001) wetting layer from first principles
NASA Astrophysics Data System (ADS)
Thomas, John C.; Millunchick, Joanna Mirecki; Van der Ven, Anton; Modine, Normand A.
2014-05-01
The large atomic-size mismatch between In and Ga and the large lattice-mismatch strain between InAs and GaAs make the InAs/GaAs (001) growth interface a complex alloy system, the understanding of which can enhance control of device synthesis and nanostructure self-assembly. We present a detailed first-principles analysis of the full progression of surface reconstructions observed on the InAs/GaAs(001) wetting layer during early stages of In deposition. We use systematic techniques to identify the most likely surface reconstruction prototypes of the InAs wetting layer on GaAs(001) using density functional theory (DFT) and then develop several cluster expansion Hamiltonians in order to thoroughly explore surface alloy disorder due to species substitution of In, Ga, and As at the surface. We use these results to construct a first principles 0-K surface phase diagram of the InAs wetting layer on GaAs(001) and test the sensitivity of our predicted phase diagram to DFT approximations and convergence errors. We find two alloy configurations of the (4×3) structural prototype that are likely ground-state surface reconstructions, and simulated scanning tunneling micrographs (STM) of these reconstructions indicate that they can explain prominent features of experimentally obtained STM of the InAs/GaAs (4×3) surface.
NASA Astrophysics Data System (ADS)
Pozhar, Liudmila A.
2010-05-01
An equilibrium two-time temperature Green's function (TTGF)-based, quantum statistical mechanical approach has been used to derive from the first principles an explicit expression for the tensor of "local" refraction indices of spatially nonuniform systems in weak external electromagnetic (EM) fields in the linear approximation with regard to the field magnitudes. Written in terms of the TTGF-based, first-principle tensorial dielectric and magnetic susceptibilities, the obtained formula for the local tensor of refraction indices (TRI) is applicable to any system, including individual nanoscale objects, such as quantum dots and wires, magnetic nanostructures, composite materials, or spatially nonuniform, bulk magnetic materials. An explicit expression for the space-time Fourier transform (STFT) of the dielectric susceptibility tensor used in TRI is derived in terms of STFTs of the charge density—charge density TTGFs, while the corresponding STFT of the magnetic susceptibility tensor also includes STFTs of the microcurrent—microcurrent TTGFs. The STFTs of the equilibrium TTGFs featuring in the susceptibilities, and thus necessary to calculate TRI, can be obtained by equilibrium quantum statistical mechanical means, modeling and simulations, or from experimental data. Two TRI regimes of significant interest for applications that can be realized in spatially inhomogeneous magnetic systems have been identified.
First principles calculations of magnetic properties of Fe and Fe3C at finite temperature
NASA Astrophysics Data System (ADS)
Eisenbach, Markus; Brown, Gregory; Rusanu, Aurelian; Nicholson, Don M.
2011-03-01
We demonstrate a method to investigate finite temperature magnetism from first principles that harnesses massively parallel computers to obtain the free energy, specific heat, magnetization, susceptibility, and other quantities as function of temperature by combining classical Wang-Landau Monte-Carlo calculations with a first principles electronic structure code that allows the energy calculation of constrained magnetic states. Here we will present our calculations of finite temperature properties such as specific heat, magnetization and susceptibility of Fe and Fe3C using this approach where we find the Curie temperatures to be in good agreement with experiment at 980K and 425K respectively. This work was conducted at Oak Ridge National Laboratory (ORNL), which is managed by UT-Battelle for the U.S. Department of Energy (US DOE) under contract DE-AC05-00OR22725 and sponsored in parts by the Center for Nanophase Material Sciences, Scientific User Facilities Division, the Center for Defect Physics, an Energy Frontier Research Center funded by the US DOE Office of Basic Energy Sciences and by the US DOE Office of Energy Efficiency and Renewable Energy, Industrial Technologies Program. This research used resources of the Oak Ridge Leadership Computing Facility at ORNL, which is supported by the US DOE, Office of Science.
NASA Astrophysics Data System (ADS)
Kotomin, E. A.; Mastrikov, Yu. A.; Rashkeev, S. N.; Van Uffelen, P.
2009-09-01
Results are reported of first principles VASP supercell calculations of basic defect migration in UN nuclear fuels. The collinear interstitialcy mechanism of N migration is predicted to be energetically more favourable than direct [0 0 1] hops. It is also found that U and N vacancies have close migration energies, and O impurities accelerate migration of N vacancies nearby. These values are both in qualitative agreement with the effect of oxygen on the reduction of the activation energy for thermal creep reported in the literature, as well as in quantitative agreement with the experimental data when taking into account the uncertainties. The migration energies have been implemented in the thermal creep model of the TRANSURANUS fuel performance code. Therefore a concrete example is provided of how first principles computations can contribute directly to improve the design tools of advanced nuclear fuels, e.g. the predictions reveal a limited effect of oxygen on the thermo-mechanical performance of nitride fuels under fast breeder reactor (FBR) normal operating conditions.
Terahertz spectra of biotin based on first principle, molecular mechanical, and hybrid simulations.
Bykhovski, Alexei; Woolard, Dwight
2013-07-01
Terahertz (THz) absorption of biotin was simulated using the first principle and the density functional theory (DFT) both in the harmonic approximation and with corrections for the anharmonicity. Anharmonicity corrections were calculated using two different approaches. First, the perturbation theory-based first principle calculations were performed to include third- and fourth-order anharmonicity corrections in atomic displacements to harmonic vibrational states. Second, the atom-centered density matrix propagation molecular dynamics model that provides a good energy conservation was used to calculate the atomic trajectories, velocities, and a dipole moment time history of biotin at low and room temperatures. Predicted low-THz lines agree well with the experimental spectra. The influence of the polyethylene (PE) matrix embedment on the THz spectra of biotin at the nanoscale was studied using the developed hybrid DFT/molecular mechanical approach. While PE is almost transparent at THz frequencies, additional low-THz lines are predicted in the biotin/PE system, which reflects a dynamic interaction between biotin and a surrounding PE cavity. PMID:25055303
First-Principles Investigation of Electronic Excitation Dynamics in Water under Proton Irradiation
NASA Astrophysics Data System (ADS)
Reeves, Kyle; Kanai, Yosuke
2015-03-01
A predictive and quantitative understanding of electronic excitation dynamics in water under proton irradiation is of great importance in many technological areas ranging from utilizing proton beam therapy to preventing nuclear reactor damages. Despite its importance, an atomistic description of the excitation mechanism has yet to be fully understood. Identifying how a high-energy proton dissipates its kinetic energy into the electronic excitation is crucial for predicting atomistic damages, later resulting in the formation of different chemical species. In this work, we use our new, large-scale first-principles Ehrenfest dynamics method based on real-time time-dependent density functional theory to simulate the electronic response of bulk water to a fast-moving proton. In particular, we will discuss the topological nature of the electronic excitation as a function of the proton velocity. We will employ maximally-localized functions to bridge our quantitative findings from first-principles simulations to a conceptual understanding in the field of water radiolysis.
First-Principles Calculations, Experimental Study, and Thermodynamic Modeling of the Al-Co-Cr System
Liu, Xuan L.; Gheno, Thomas; Lindahl, Bonnie B.; Lindwall, Greta; Gleeson, Brian; Liu, Zi-Kui
2015-01-01
The phase relations and thermodynamic properties of the condensed Al-Co-Cr ternary alloy system are investigated using first-principles calculations based on density functional theory (DFT) and phase-equilibria experiments that led to X-ray diffraction (XRD) and electron probe micro-analysis (EPMA) measurements. A thermodynamic description is developed by means of the calculations of phase diagrams (CALPHAD) method using experimental and computational data from the present work and the literature. Emphasis is placed on modeling the bcc-A2, B2, fcc-?, and tetragonal-? phases in the temperature range of 1173 to 1623 K. Liquid, bcc-A2 and fcc-? phases are modeled using substitutional solution descriptions. First-principles special quasirandom structures (SQS) calculations predict a large bcc-A2 (disordered)/B2 (ordered) miscibility gap, in agreement with experiments. A partitioning model is then used for the A2/B2 phase to effectively describe the order-disorder transitions. The critically assessed thermodynamic description describes all phase equilibria data well. A2/B2 transitions are also shown to agree well with previous experimental findings. PMID:25875037
NASA Astrophysics Data System (ADS)
Kobayashi, Hajime; Tokita, Yuichi
2015-03-01
Charge transfer rates near pentacene grain boundaries are derived by calculating the site energies and transfer integrals of 37 pentacene molecules using first-principles calculations. The site energies decrease considerably near the grain boundaries, and electron traps of up to 300 meV and hole barriers of up to 400 meV are generated. The charge transfer rates across the grain boundaries are found to be reduced by three to five orders of magnitude with a grain boundary gap of 4 Å because of the reduction in the transfer integrals. The electron traps and hole barriers also reduce the electron and hole transfer rates by factors of up to 10 and 50, respectively. It is essential to take the site energies into consideration to determine charge transport near the grain boundaries. We show that the complex site energy distributions near the grain boundaries can be represented by an equivalent site energy difference, which is a constant for any charge transfer pass. When equivalent site energy differences are obtained for various grain boundary structures by first-principles calculations, the effects of the grain boundaries on the charge transfer rates are introduced exactly into charge transport simulations, such as the kinetic Monte Carlo method.
Implicit solvent model for linear-scaling first-principles electronic structure calculations
NASA Astrophysics Data System (ADS)
Helal, Hatem H.; Payne, Mike; Mostofi, Arash A.
2009-03-01
Density functional theory (DFT) enables first-principles calculations that exhibit cubic scaling of the computational time required with respect to the number of atoms in the system. This presents an unavoidable difficulty when first-principles accuracy is needed for the study of large-scale biological systems. The ONETEP program reformulates DFT so that the required computational effort scales only linearly with system size, recently demonstrated for up to 32,000 atoms on 64 cores.ootnotetextN. D. M. Hine, P. D. Haynes, A. A. Mostofi, C.-K. Skylaris and M. C. Payne, submitted to J. Chem. Phys. (2008). Further complicating DFT based studies of biomolecular systems is the need for an accurate representation of the electrostatic environment. Rather than introducing explicit solvent molecules into the system, which would be computationally prohibitive, we present our recent efforts to integrate an implicit solvent modelootnotetextD. A. Scherlis et al., J. Chem. Phys. 124, 074103 (2006). with ONETEP in order to study systems in solution consisting of many thousands of atoms. We report preliminary results of our methodology with a study of the DNA nucleosome core particle.
Structures and magnetic properties of Co-Zr-B magnets studied by first-principles calculations
NASA Astrophysics Data System (ADS)
Zhao, Xin; Ke, Liqin; Nguyen, Manh Cuong; Wang, Cai-Zhuang; Ho, Kai-Ming
2015-06-01
The structures and magnetic properties of Co-Zr-B alloys near the composition of Co5Zr with B at. % ?6% were studied using adaptive genetic algorithm and first-principles calculations. The energy and magnetic moment contour maps as a function of chemical composition were constructed for the Co-Zr-B magnet alloys through extensive structure searches and calculations. We found that Co-Zr-B system exhibits the same structure motif as the "Co11Zr2" polymorphs, and such motif plays a key role in achieving strong magnetic anisotropy. Boron atoms were found to be able to substitute cobalt atoms or occupy the "interruption" sites. First-principles calculations showed that the magnetocrystalline anisotropy energies of the boron-doped alloys are close to that of the high-temperature rhombohedral Co5Zr phase and larger than that of the low-temperature Co5.25Zr phase. Our calculations provide useful guidelines for further experimental optimization of the magnetic performances of these alloys.
First principles theory for surface plasmon generation and decay to hot carriers
NASA Astrophysics Data System (ADS)
Sundararaman, Ravishankar; Narang, Prineha; Jermyn, Adam; Atwater, Harry A.; Goddard, William A., III
2014-03-01
Plasmonic resonances provide a promising pathway for efficiently capturing infrared photons from solar radiation and boosting photo-catalytic activity via local temperature enhancements and hot carrier generation. Previous calculations of plasmon decay to excited carriers employing a fully quantized model Hamiltonian [2] indicate strong plasmon polarization dependence and momentum anisotropy of the generated carriers, in contrast with classical theories. An accurate first principles calculation for this process must account for microscopic details at the atomic scale for the electronic states as well as the effect of the 10-100 nm length scale particle and antennae geometries on the plasmon resonances. Here, we present a first-principles multi-scale model of plasmonics combining electronic density-functional theory with electromagnetic models on longer length-scales, and investigate the role of electronic structure and geometry on plasmonic light absorption, decay and hot carrier generation. This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.
Zhu Weihua Xiao Heming
2007-12-15
A detailed first-principles study of the structural and vibrational properties of crystalline silver azide under hydrostatic pressure of 0-500 GPa has been performed with density functional theory in the generalized gradient approximation. The crystal structure is relaxed to allow ionic configurations, cell shape, and volume to change without any symmetry constraints. It is found that the silver azide crystal remains orthorhombic structure with Ibam space group for pressures up to 7 GPa, where there is a transition to an I4/mcm tetragonal symmetry. The lattice parameter and electronic structure are investigated as functions of pressure. The calculated vibrational frequencies at ambient pressure are in agreement with available experimental data. We also discuss the pressure-induced frequency shifts for the internal and lattice modes of silver azide crystal upon compression. - Graphical abstract: Silver azide attracts scientific and technological interest as an energetic material. There are sufficient difficulties to study it experimentally at extreme conditions due to their explosive nature. This paper presents a detailed first-principles study of the structural and vibrational properties of crystalline silver azide under hydrostatic pressure of 0-500 GPa.
First Principle Ab-initio Study of TiO2
NASA Astrophysics Data System (ADS)
Ekuma, Chinedu; Jarrell, Mark; Moreno, Juana; Bagayoko, Diola
2012-02-01
We report results from first principle computations of electronic properties of rutile TiO2 within the local density functional approximation (LDA). Our first principle, non-relativistic and ground state calculations employed a local density functional approximation (LDA) potential and the linear combination of atomic orbitals (LCAO) -- utilizing a self-consistently obtained, optimized basis set. We solved self-consistently both the Kohn-Sham equation and the equation giving the ground state charge density in terms of the wave functions of the occupied states. Our calculated band structure shows that there is significant O2p-Ti3d hybridization in the valence bands. These bands are well separated from the conduction bands by an indirect band gap of 2.95 eV, from ? to R. Consequently, this work predicts that rutile TiO2 is an indirect band gap material, as all other gaps from our calculations are larger than 2.95 eV. A slightly larger, direct band gap of 3.05 eV is found at the ? point, in excellent agreement with experiment. Our structural optimization led to lattice parameters of 4.65 å and 2.97 å for ao and co, respectively, with a u parameter of 0.3051, and a bulk modulus of 215 GPa.
First principles molecular dynamics of metal/water interfaces under bias potential
NASA Astrophysics Data System (ADS)
Pedroza, Luana; Brandimarte, Pedro; Rocha, Alexandre; Fernandez-Serra, Marivi
2014-03-01
Understanding the interaction of the water-metal system at an atomic level is extremely important in electrocatalysts for fuel cells, photocatalysis among other systems. The question of the interface energetics involves a detailed study of the nature of the interactions between water-water and water-substrate. A first principles description of all components of the system is the most appropriate methodology in order to advance understanding of electrochemically processes. In this work we describe, using first principles molecular dynamics simulations, the dynamics of a combined surface(Au and Pd)/water system both in the presence and absence of an external bias potential applied to the electrodes, as one would come across in electrochemistry. This is accomplished using a combination of density functional theory (DFT) and non-equilibrium Green's functions methods (NEGF), thus accounting for the fact that one is dealing with an out-of-equilibrium open system, with and without van der Waals interactions. DOE Early Career Award No. DE-SC0003871.
Liu, Xuan L; Gheno, Thomas; Lindahl, Bonnie B; Lindwall, Greta; Gleeson, Brian; Liu, Zi-Kui
2015-01-01
The phase relations and thermodynamic properties of the condensed Al-Co-Cr ternary alloy system are investigated using first-principles calculations based on density functional theory (DFT) and phase-equilibria experiments that led to X-ray diffraction (XRD) and electron probe micro-analysis (EPMA) measurements. A thermodynamic description is developed by means of the calculations of phase diagrams (CALPHAD) method using experimental and computational data from the present work and the literature. Emphasis is placed on modeling the bcc-A2, B2, fcc-?, and tetragonal-? phases in the temperature range of 1173 to 1623 K. Liquid, bcc-A2 and fcc-? phases are modeled using substitutional solution descriptions. First-principles special quasirandom structures (SQS) calculations predict a large bcc-A2 (disordered)/B2 (ordered) miscibility gap, in agreement with experiments. A partitioning model is then used for the A2/B2 phase to effectively describe the order-disorder transitions. The critically assessed thermodynamic description describes all phase equilibria data well. A2/B2 transitions are also shown to agree well with previous experimental findings. PMID:25875037
First principles calculation of the shift current photovoltaic effect in ferroelectrics.
Young, Steve M; Rappe, Andrew M
2012-09-14
We calculate the bulk photovoltaic response of the ferroelectrics BaTiO(3) and PbTiO(3) from first principles by applying the "shift current" theory to the electronic structure from density functional theory. The first principles results for BaTiO(3) reproduce experimental photocurrent direction and magnitude as a function of light frequency, as well as the dependence of current on light polarization, demonstrating that shift current is the dominant mechanism of the bulk photovoltaic effect in BaTiO(3). Additionally, we analyze the relationship between response and material properties in detail. Photocurrent does not depend simply or strongly on the magnitude of material polarization, as has been previously assumed; instead, electronic states with delocalized, covalent bonding that is highly asymmetric along the current direction are required for strong shift current enhancements. The complexity of the response dependence on both external and material parameters suggests applications not only in solar energy conversion, but in photocatalysis and sensor and switch type devices as well. PMID:23005660
Quantum Mechanics and First-Principles Molecular Dynamics Selection of Polymer Sensing Materials
NASA Astrophysics Data System (ADS)
Blanco, Mario; Shevade, Abhijit V.; Ryan, Margaret A.
We present two first-principles methods, density functional theory (DFT) and a molecular dynamics (MD) computer simulation protocol, as computational means for the selection of polymer sensing materials. The DFT methods can yield binding energies of polymer moieties to specific vapor bound compounds, quantities that were found useful in materials selection for sensing of organic and inorganic compounds for designing sensors for the electronic nose (ENose) that flew on the International Space Station (ISS) in 2008-2009. Similarly, we present an MD protocol that offers high consistency in the estimation of Hildebrand and Hansen solubility parameters (HSP) for vapor bound compounds and amorphous polymers. HSP are useful for fitting measured polymer sensor responses with physically rooted analytical models. We apply the method to the JPL electronic nose (ENose), an array of sensors with conducting leads connected through thin film polymers loaded with carbon black. Detection relies on a change in electric resistivity of the polymer film as function of the amount of swelling caused by the presence of the analyte chemical compound. The amount of swelling depends upon the chemical composition of the polymer and the analyte molecule. The pattern is unique and it unambiguously identifies the compound. Experimentally determined changes in relative resistivity of fifteen polymer sensor materials upon exposure to ten vapors were modeled with the first-principles HSP model.
A Study on Bio-Compatible Piezoelectric Materials by First Principles Calculation
NASA Astrophysics Data System (ADS)
Uetsuji, Yasutomo; Imoto, Kouhei; Kumazawa, Sadaomi; Tsuchiya, Kazuyoshi; Ueda, Sei; Nakamachi, Eiji
Bio-compatible piezoelectric materials are becoming increasingly important for actuators and sensors in medical devices, that is Bio-MEMS such as health monitoring systems and drag delivery systems. In this study, we challenged to derive new piezoelectric materials with bio-compatibility by first principles calculation. Firstly, constituent elements of bio-compatible piezoelectric materials have been specified by HSAB method from the viewpoint of interaction energy with in-vivo molecules. Secondly, in order to create a perovskite-type crystal structure with good piezoelectric response, the combination of bio-compatible elements was selected to satisfy geometric stable condition defined by tolerance factor. As a result, we discovered 7 kinds of new piezoelectric materials. We focused on one of them, MgSiO3 that is known to be a mineral with perovskite-type crystal structure, and analyzed the stable cubic structure at paraelectric non-polar phase and the stable tetragonal structure at ferroelectric phase by first principles DFT. Additionally, structural phase transition of MgSiO3 has been investigated on the assumption of linear structural change from cubic structure to tetragonal one. DFT calculation indicated that MgSiO3 can change spontaneously to tetragonal structure with high tetragonality and polarization, and that MgSiO3 can present a good piezoelectricity.
Huang, Zuocai; Zhang, Lei; Pan, Wei, E-mail: panw@mail.tsinghua.edu.cn
2013-09-15
Pure zircon and scheelite LuVO{sub 4} were prepared by solid state reaction and high-pressure route, respectively. Structure, elastic constants, lattice dynamics and thermodynamics of LuVO{sub 4} polymorphs were studied by experiments and first principles calculation. Calculations here are in good agreement with the experimental results. The phonon dispersions of LuVO{sub 4} polymorphs were studied by the linear response method. The calculated phonon dispersions show that zircon and scheelite LuVO{sub 4} phases are dynamically stable. Raman-active frequencies were measured and assigned to different modes according to the calculations. The internal frequencies shift downward after phase transition from zircon to scheelite. Born effective charge tensors elements for both phases are analyzed. The finite temperature thermodynamic properties of LuVO{sub 4} polymorphs were calculated from the obtained phonon density of states by quasi-harmonic approach. - Graphical abstract: Lutetium orthovanadate polymorphs were synthesized by SSR and HP methods and their physical and chemical properties, including lattice dynamical properties, were determined by DFT calculations and experiments. Display Omitted - Highlights: • Pure zircon and scheelite LuVO{sub 4} polymorphs were synthesized by solid state reaction and high-pressure route. • Chemical and physical properties of LuVO4 polymorphs were studied by experiments and first principles calculation. • Raman-active frequencies were measured and assigned to different modes according to the calculations. • Lattice dynamics of polymorphs were discussed in details.
First-principles study of structural, electronic and optical properties of ZnF2
NASA Astrophysics Data System (ADS)
Wu, Jian-Bang; Cheng, Xin-Lu; Zhang, Hong; Xiong, Zheng-Wei
2014-07-01
The structural, electronic, and optical properties of rutile—, CaCl2-, and PdF2—ZnF2 are calculated by the plane-wave pseudopotential method within the density functional theory. The calculated equilibrium lattice constants are in reasonable agreement with the available experimental and other calculated results. The band structures show that the rutile—, CaCl2-, and PdF2—ZnF2 are all direct band insulator. The band gaps are 3.63, 3.62, and 3.36 eV, respectively. The contribution of the different bands was analyzed by the density of states. The Mulliken population analysis is performed. A mixture of covalent and weak ionic chemical bonding exists in ZnF2. Furthermore, in order to understand the optical properties of ZnF2, the dielectric function, absorption coefficient, refractive index, electronic energy loss spectroscopy, and optical reflectivity are also performed in the energy range from 0 to 30 eV. It is found that the main absorption parts locate in the UV region for ZnF2. This is the first quantitative theoretical prediction of the electronic and optical properties of ZnF2 compound, and it still awaits experimental confirmation.
Basire, M; Parneix, P; Calvo, F
2010-03-11
With a recently developed simulation method (Basire, M.; et al. J. Phys. Chem. A 2009, 113, 6947), the infrared vibrational spectra of several polyatomic molecules are calculated over a broad range of temperature, taking into account quantum, anharmonic, and couplings effects. Anharmonic force fields, generated from static first-principle calculations, are sampled in the microcanonical ensemble to provide energy-resolved absorption intensities and their finite temperature analogues after Laplace transformation. Effective anharmonic frequencies are characterized as a continuous function of temperature for vinyl fluoride, the N-acetyl-Phe-NH(2) peptide, and protonated naphthalene. These frequencies generally deviate increasingly from the harmonic value with increasing temperature, although the overestimation due to the harmonic approximation is particularly salient for high-frequency modes. Anharmonicities may also be sufficient to alter structural assignment of experimental spectra with respect to empirically scaled harmonic bands. These results emphasize some possible limitations and inaccuracies inherent to using such static scaling factors for correcting harmonic IR spectra. PMID:19961196
NASA Astrophysics Data System (ADS)
Liang, Liangbo
Since its fabrication in 2004, graphene has attracted huge attention due to its exceptional electronic properties, and is now considered as one of the most promising candidates to replace the current semiconductor technology as silicon approaches its miniaturization limit. However, the absence of an electronic band gap in pristine graphene makes it ill-suited for many electronic applications. Semiconducting character can be imparted by a variety of methods, including chemical or structural modifications. For instance, a band gap can be opened by confining the electronic wave function in one dimension by cutting graphene to form graphene nanoribbons (GNRs). To possess a band gap comparable to conventional semiconductors like silicon, GNRs are required to have a width less than 3 nm and must also display sharp edges, which remains a great experimental challenge. Recently, a breakthrough advance has been achieved with the controlled synthesis of atomically precise nanoribbons using a bottom-up approach where small aromatic molecules chemically assemble into high-quality subnanometer ribbons. This method not only allows for the synthesis of high-quality straight GNRs, but also for more complex structures like wiggle-like GNRs, called graphene nanowiggles (GNWs). In Part I of this thesis, first-principles density functional theory (DFT) calculations are carried out on a variety of GNWs to reveal their unusual electronic and magnetic properties that are absent in their individual GNRs components, such as tunable band gaps and versatile magnetic states. The relationship between the band gap and the geometry is dictated by the armchair or zigzag characters of the corresponding parallel and oblique sectors, enabling GNWs to offer a broader set of geometrical parameters to tune the electronic structures compared to GNRs. In addition, first-principles many-body Green's function calculations within the GW approximation are performed to yield a quantitative prediction of GNWs' electronic properties. The enhanced electron-electron interaction in the quasi-one-dimensional GNWs results in significant self-energy corrections to their DFT band gaps. Consequently, the quasiparticle band gaps are typically more than twice of the DFT band gaps and are within the most interesting range 0.0-3.7 eV. In Part II of this thesis, we venture beyond graphene-based systems and investigate graphene-like materials: transition metal dichalcogenides MX 2(M = Mo, W; X = S). Similar to graphite, they are also layered structures stacked by weak van der Waals (vdW) forces. Single-layer MoS2 and WS2 have been synthesized and found to show enhanced carrier charge mobilities and strong photoluminescence with direct band gaps, and thus they have been considered as replacements or complements to graphene for applications. Raman spectroscopy is often considered as one of the most popular tools to characterize them. Despite extensive experimental Raman studies on MoS 2 and WS2, it remains unclear how Raman intensities and especially intensity ratio of Raman modes E2g1 and A1g depend on the materials' thickness, due to the large spectrum of seemingly contradictory findings. In the final part of the thesis (Part III), we highlight the experimental collaboration project with Prof. Plummer's group from Louisiana State University: spin-dependent surface reconstruction of layered Fe-based superconductors CaFe2As2. Low energy electron diffraction, scanning tunneling microscopy and spectroscopy, and first-principles spin-polarized DFT are utilized to investigate the geometric, electronic, and magnetic structures of the stripe-ordered (1x2) surface of Ca(Fe1-xCox) 2As2 (x=0, 0.075). The surface is terminated with a 50% Ca layer. Compared to the bulk, the surface Ca layer has a large inward relaxation (˜ 0.5 A), and the underneath As-Fe2-As layer displays a significant buckling. First-principles calculations show that the (1x2) phase is stabilized by the bulk anti-ferromagnetic spin ordering through the spin-charge-lattice coupling. Strikingly, a superconducting gap (˜7 meV