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1

Spectroscopy of organic semiconductors from first principles  

NASA Astrophysics Data System (ADS)

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.

Sharifzadeh, Sahar; Biller, Ariel; Kronik, Leeor; Neaton, Jeffery

2011-03-01

2

Study of the theoretical tensile strength of Fe by a first-principles computational tensile test  

NASA Astrophysics Data System (ADS)

This paper employs a first-principles total-energy method to investigate the theoretical tensile strengths of bcc and fcc Fe systemically. It indicates that the theoretical tensile strengths are shown to be 12.4, 32.7, 27.5 GPa for bcc Fe, and 48.1, 34.6, 51.2 GPa for fcc Fe in the [001], [110] and [111] directions, respectively. For bcc Fe, the [001] direction is shown to be the weakest direction due to the occurrence of a phase transition from ferromagnetic bcc Fe to high spin ferromagnetic fcc Fe. For fcc Fe, the [110] direction is the weakest direction due to the formation of an instable saddle-point 'bct structure' in the tensile process. Furthermore, it demonstrates that a magnetic instability will occur under a tensile strain of 14%, characterized by the transition of ferromagnetic bcc Fe to paramagnetic fcc Fe. The results provide a good reference to understand the intrinsic mechanical properties of Fe as a potential structural material in the nuclear fusion Tokamak.

Liu, Yue-Lin; Zhang, Ying; Hong, Rong-Jie; Lu, Guang-Hong

2009-05-01

3

Exploiting periodic first-principles calculations in NMR spectroscopy of disordered solids.  

PubMed

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

Ashbrook, Sharon E; Dawson, Daniel M

2013-09-17

4

Half-metallic graphene nanodots: A comprehensive first-principles theoretical study  

NASA Astrophysics Data System (ADS)

A comprehensive first-principles theoretical study of the electronic properties and half-metallic nature of finite rectangular graphene nanoribbons is presented. We identify the bisanthrene isomer of the C28H14 molecule to be the smallest graphene derivative to present a spin-polarized ground state. Even at this quantum dot level, the spins are predicted to be aligned antiferromagnetically at the two zigzag edges of the system. As a rule of thumb, we find that zigzag graphene edges that are at least three consecutive units long will present spin polarization if the width of the system is 1nm or wider. Room temperature detectability of the magnetic ordering is predicted for ribbons with zigzag edges 1nm and longer. For the longer systems studied, spin wave structures appear in some high spin multiplicity states. Energy gap oscillations with the length of the zigzag edge are observed. The amplitude of these oscillations is found to be smaller than that predicted for infinite ribbons. The half-metallic nature of the ribbons under an external in-plane electric field is found to be preserved even for finite and extremely short systems.

Hod, Oded; Barone, Verónica; Scuseria, Gustavo E.

2008-01-01

5

Probing the surface structure of hydroxyapatite using NMR spectroscopy and first principles calculations.  

PubMed

The surface characteristics of hydroxyapatite (HA) are probed using a combination of NMR spectroscopy and first principles calculations. The NMR spectrum is taken from a bone sample and the first principles calculations are performed using a plane-wave density functional approach within the pseudopotential approximation. The computational work focuses on the (100) and (200) surfaces, which exhibit a representative range of phosphate, hydroxyl and cation bonding geometries. The shielding tensors for the 31P, 1H and 17O nuclei are calculated from the relaxed surface structures using an extension of the projector augmented-wave method. The calculated 31P chemical shifts for the surface slab are found to be significantly different from the bulk crystal and are consistent with the NMR data from bone and also synthetically prepared nanocrystalline samples of HA. Rotational relaxations of the surface phosphate ions and the sub-surface displacement of other nearby ions are identified as causing the main differences. The investigation points to further calculations of other crystallographic surfaces and highlights the potential of using NMR with ab initio modelling to fully describe the surface structure and chemistry of HA, which is essential for understanding its reactivity with the surrounding organic matrix. PMID:18183321

Chappell, Helen; Duer, Melinda; Groom, Nicholas; Pickard, Chris; Bristowe, Paul

2008-01-28

6

Hydrogen donors in SnO2 studied by infrared spectroscopy and first-principles calculations  

NASA Astrophysics Data System (ADS)

Hydrogen is a potentially important source of n -type conductivity in oxide materials. We have investigated hydrogen in tin oxide (SnO2) , a wide-band-gap semiconductor with applications as a transparent conductor and in gas sensors. Infrared (IR) spectroscopy and electrical measurements indicate that hydrogen binds to a host oxygen atom and increases the conductivity. First-principles calculations confirm that interstitial hydrogen acts as a shallow donor (Hi+) . Our calculations also indicate that Hi+ diffuses easily and combines with Sn vacancies into stable (VSn-H)-3 complexes, with the calculated O-H frequencies in agreement with the experimental values. These results suggest that interstitial hydrogen acts as a shallow, mobile donor in a range of oxide materials.

Hlaing Oo, W. M.; Tabatabaei, S.; McCluskey, M. D.; Varley, J. B.; Janotti, A.; van de Walle, C. G.

2010-11-01

7

Chemical-state analysis of organic semiconductors using soft X-ray absorption spectroscopy combined with first-principles calculation.  

PubMed

The chemical states of organic semiconductors were investigated by total-electron-yield soft X-ray absorption spectroscopy (TEY-XAS) and first-principles calculations. The organic semiconductors, pentacene (C(22)H(14)) and pentacenequinone (C(22)H(12)O(2)), were subjected to TEY-XAS and the experimental spectra obtained were compared with the 1s core-level excited spectra of C and O atoms, calculated by a first-principles planewave pseudopotential method. Excellent agreement between the measured and the calculated spectra were obtained for both materials. Using this methodology, we examined the chemical states of the aged pentacene, and confirmed that both C-OH and C?O chemical bonds are generated by exposure to air. This result implies that not only oxygen but also humidity causes pentacene oxidation. PMID:22217144

Natsume, Yutaka; Kohno, Teiichiro; Minakata, Takashi; Konishi, Tokuzo; Gullikson, Eric M; Muramatsu, Yasuji

2012-02-16

8

Electron energy loss spectroscopy of polytetrafluoroethylene: experiment and first principles calculations.  

PubMed

We have performed electron energy-loss spectroscopy (EELS) on a 200 kV transmission electron microscope (TEM) equipped with a monochromator to investigate molecular conformation of polytetrafluoroethylene (PTFE). The experimental spectra show several unique features in the low-loss region and the onset of carbon K-edge for PTFE. Density function theory (DFT) methods are employed to calculate the low-loss and core-loss spectra of PTFE with consideration of the effects of phase transitions, chain orientation and polarization. The shape and width of the characteristic peaks of the experimental spectra are well reproduced in DFT calculations. By comparing the spectra from experiments and theory, the detailed information about the conformational dependence of EEL spectra for PTFE can be obtained. In the present work, we have demonstrated an application of combining high-resolution EELS and DFT calculations in both low-loss and core-loss regions to discriminate changes of chain conformation and orientation for the polymer with complex phase transition behavior. PMID:24296695

Wang, Chen; Duscher, Gerd; Paddison, Stephen J

2014-02-01

9

First-principles study of He effects in a bcc Fe grain boundary: site preference, segregation and theoretical tensile strength.  

PubMed

We perform a first-principles calculation to investigate the effects of He in an Fe ?5(310)/[001] grain boundary (GB) with the SIESTA code, for which the reliability of the pseudopotential and the basis set are systematically tested. We calculate the formation and segregation energies for different substitutional and interstitial cases in order to determine the site preference and the segregation properties of He in the Fe GB. It is demonstrated that the He segregation either breaks (substitution) or weakens (interstitial) the surrounding interfacial Fe-Fe bonds, leading to the GB tensile strength reduction. PMID:21403193

Zhang, Lei; Shu, Xiaolin; Jin, Shuo; Zhang, Ying; Lu, Guang-Hong

2010-09-22

10

First Principles Study and Theoretical Analysis of a Single Molecular Diode by p-n di-block molecules  

NASA Astrophysics Data System (ADS)

The concept of a single molecular diode was first proposed by Aviram and Ratner, and there have been many studies of synthesis D-?-A or p-n di-block molecules and measurements of the current-voltage (I-V) characteristics for relating molecular junctions. Recently, the I-V measurement in a symmetric tetraphenyl junction and non-symmetric dipyrimidinyl -diphenyl diblock junction was performed, and clear rectification was found in the latter system, which resembles the p-n junction by the covalent connection between electron-deficient bypyrimidinyl and electron-rich biphenyl moieties, though an applied bias is much lower than the resonant level. In this presentation, we performed the first principles calculations of electron transport for the above tetraphenyl and dipyrimidinyl -diphenyl diblock junctions by the self-consistent nonequilibrium Green's function theory with the use of our HiRUNE program module. We carried out the systematic analysis of the rectification behavior and identified the change of electron-pathway in the bridge molecule relating to p-n junction based on the first principles data. The relation between the rectifying action and molecular conformation, particularly, the torsion of diblock, will be discussed.

Nakamura, Hisao; Asai, Yoshihiro; Hihath, Josh; Tao, Nongjian

2011-03-01

11

Valence Band Density of States of Cu3Si Studied by Soft X-Ray Emission Spectroscopy and a First-Principle Molecular Orbital Calculation  

NASA Astrophysics Data System (ADS)

A systematic study of the valence band structure of Cu3Si has been performed by soft X-ray emission spectroscopy and a first-principle molecular orbital calculation using the discrete-variational (DV)-X? cluster model. The existence of Cu 4s, 4p states in the valence band and their important contributions to the valence band as that of Cu 3d are indicated together with previously reported ones. The high-binding energy peak in the Si L2,3 emission spectrum is considered to originate mainly from the Si-Si 3s bonding state but also have a certain contribution of Si 3s bonding state with Cu 4s, 4p. On the other hand, the low-binding energy peaks in the Si L2,3 emission band are attributed to both the antibonding states of Si 3s and the bonding states of Si 3d with Cu 4s, 4p and Cu 3d. The bonding states of Si 3s with Cu 4s, 4p and Cu 3d are expected to exist in the lower part of the valence band for ?\\prime-Cu3Si on the basis of the theoretical calculations. As for Si p states, the high-binding energy peak and the low-binding energy peak in the Si K? emission spectrum should be attributed to the Si 3p bonding state and antibonding state with Cu 3d and Cu 4s, 4p, respectively, according to the theoretical calculations. A comparison is made between experimental spectra and theoretical density of states.

An, Zhenlian; Kamezawa, Chihiro; Hirai, Masaaki; Kusaka, Masahiko; Iwami, Motohiro

2002-12-01

12

Electronic structures of hexagonal RMnO3 ( R=Gd , Tb, Dy, and Ho) thin films: Optical spectroscopy and first-principles calculations  

NASA Astrophysics Data System (ADS)

We investigated the electronic structure of multiferroic hexagonal RMnO3 ( R=Gd , Tb, Dy, and Ho) thin films using both optical spectroscopy and first-principles calculations. One of the difficulties in explaining the electronic structures of hexagonal RMnO3 is that they exist in nature with limited rare earth ions (i.e., R=Sc , Y, and Ho-Lu), so a systematic study in terms of the different R ions has been lacking. Recently, our group succeeded in fabricating hexagonal RMnO3 ( R=Gd , Tb, and Dy) using the epitaxial stabilization technique [Adv. Mater. (Weinheim Ger.) 18, 3125 (2006)]. Using artificially stabilized hexagonal RMnO3 , we extended the optical spectroscopic studies on the hexagonal multiferroic manganite system. We observed two optical transitions located near 1.7 and 2.3eV , in addition to the predominant absorption above 5eV . With the help of first-principles calculations, we attributed the low-lying optical absorption peaks to interband transitions from the oxygen states hybridized strongly with different Mn orbital symmetries to the Mn3d3z2-r2 state. As the ionic radius of the rare earth ion increased, we observed a systematic increase of the lowest peak position, which became more evident when compared with previously reported results. We explained this systematic change in terms of a flattening of the MnO5 triangular bipyramid.

Choi, Woo Seok; Kim, Dong Geun; Seo, Sung Seok A.; Moon, Soon Jae; Lee, Daesu; Lee, Jung Hyuk; Lee, Ho Sik; Cho, Deok-Yong; Lee, Yun Sang; Murugavel, Pattukkannu; Yu, Jaejun; Noh, Tae W.

2008-01-01

13

A first principles theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in aqueous ionic solutions: D2O in hydration shells of Cl- ions  

NASA Astrophysics Data System (ADS)

A theoretical study of vibrational spectral diffusion and hydrogen bond dynamics in aqueous ionic solutions is presented from first principles without employing any empirical potential models. The present calculations are based on ab initio molecular dynamics for trajectory generation and wavelet analysis of the simulated trajectories for time dependent frequency calculations. Results are obtained for two different deuterated aqueous solutions: the first one is a relatively dilute solution of a single Cl- ion and the second one is a concentrated solution of NaCl (~3M) dissolved in liquid D2O. It is found that the frequencies of OD bonds in the anion hydration shell, i.e., those which are hydrogen bonded to the chloride ion, have a higher stretch frequency than those in the bulk water. Also, on average, the frequencies of hydration shell OD modes are found to increase with increase in the anion-water hydrogen bond distance. On the dynamical side, when the vibrational spectral diffusion is calculated exclusively for the hydration shell water molecules in the first solution, the dynamics reveals three time scales: a short-time relaxation (~200 fs) corresponding to the dynamics of intact ion-water hydrogen bonds, a slower relaxation (~3 ps) corresponding to the lifetimes of chloride ion-water hydrogen bonds, and another longer-time constant (~20 ps) corresponding to the escape dynamics of water from the anion hydration shell. Existence of such three time scales for hydration shell water molecules was also reported earlier for water containing a single iodide ion using classical molecular dynamics [B. Nigro et al., J. Phys. Chem. A 110, 11237 (2006)]. Hence, the present study confirms the basic results of this earlier work using a different methodology. However, when the vibrational spectral diffusion is calculated over all the OD modes, only two time scales of ~150 fs and ~2.7 ps are found without the slowest component of ~20 ps. This is likely because of the very small weight that the hydration shell water molecules carry to the overall spectral diffusion in the solution containing a single ion. For the concentrated solution also, the slowest component of ~20 ps is not found in the spectral diffusion of all water molecules because a distinct separation between the hydration shell and bulk water in terms of their stretch frequencies does not hold at this high concentration regime. The present first principles results are compared with those of the available experiments and classical simulations.

Mallik, Bhabani S.; Semparithi, A.; Chandra, Amalendu

2008-11-01

14

Identification of boron clusters in silicon crystal by B1s core-level X-ray photoelectron spectroscopy: A first-principles study  

SciTech Connect

We carried out a comprehensive study on the B1s core-level X-ray photoelectron spectroscopy (XPS) binding energies for B clusters in crystalline Si using a first-principles calculation with careful evaluation of the local potential boundary condition for the model system, where convergence within 0.1 eV was confirmed for the supercell size. For ion-implanted samples, we identified experimental peaks due to B clusters and threefold B as icosahedral B{sub 12} and <001>B-Si defects, respectively. For as-doped samples prepared by plasma doping, it was found that the calculated XPS binding energies for complexes of vacancies and B atoms were consistent with the experimental spectra.

Yamauchi, Jun [Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522 (Japan); Yoshimoto, Yoshihide [Department of Applied Mathematics and Physics, Graduate School of Engineering, Tottori University, 4-101 Koyama-Minami, Tottori 680-8552 (Japan); Suwa, Yuji [Central Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama 350-0395 (Japan)

2011-11-07

15

Understanding Metal Directed-Assembly Growth of Single-Crystal Organic Nanowires: Time-Resolved, In-Situ X-Ray Diffraction and First-Principles Theoretical Study  

SciTech Connect

The deterministic growth of oriented crystalline organic nanowires (CONs) from the vapor-solid chemical reaction (VSCR) between small-molecule reactants and metal nanoparticles has been demonstrated in several studies to date, however the growth mechanism has not yet been conclusively understood. Here, the VSCR growth of M-TCNQF4 (where M is Cu- or Ag-) nanowires is investigated both experimentally and theoretically with time-resolved, in-situ x-ray diffraction (XRD) and first-principles atomistic calculations, respectively, to understand how metals (M) direct the assembly of small molecules into CONs, and what determines the selectivity of a metal for an organic vapor reactant in the growth process. Analysis of the real-time growth kinetics data using a modified Avrami model indicates that the formation of CONs from VSCR follows a one-dimensional ion diffusion-controlled tip growth mechanism wherein metal ions diffuse from a metal film through the nanowire to its tip where they react with small molecules to continue growth. The experimental data and theoretical calculations indicate that the selectivity of different metals to induce nanowire growth depends strongly upon effective charge transfer between the organic molecules and the metal. Specifically, the experimental finding that Cu ions can exchange and replace Ag ions in Ag-TCNQF4 to form Cu-TCNQF4 nanowires is explained by the significantly stronger chemical bond between Cu and TCNQF4 molecules than for Ag, due to the strong spin-dependent electronic contribution of Cu. Understanding how to control the VSCR growth process may enable the synthesis of novel organic nanowires with axial or coaxial p/n junctions for organic nanoelectronics and solar energy harvesting.

Xiao, Kai [ORNL; Yoon, Mina [ORNL; Rondinone, Adam Justin [ORNL; Payzant, E Andrew [ORNL; Geohegan, David B [ORNL

2012-01-01

16

Anharmonic phonons of NaZr2(PO4)3 studied by Raman spectroscopy, first-principles calculations, and x-ray diffraction  

NASA Astrophysics Data System (ADS)

Raman spectroscopy, x-ray diffraction, and ab initio calculations are carried out to gain insight into the thermal expansion properties of NaZr2(PO4)3. In situ Raman studies at high pressure in a diamond anvil cell indicate that two low-energy phonons corresponding to a combination of PO4 tetrahedral librations and Zr translations contribute negatively to its thermal expansion, and temperature-dependent Raman studies reveal the modes that are truly anharmonic. To complement the spectroscopic measurements the phonon spectrum is computed and mode assignments are carried out employing first-principles calculations. The computed atomic displacements corresponding to the lowest-energy Raman mode indicates that it involves PO4 rotations and Zr translations, and its Gruneisen parameter is found to be negative, in agreement with our measured value. The thermal expansion coefficient calculated using mode Gruneisen parameters obtained from computed phonon spectra at different volumes (7.5×10-6/K) is in good agreement with the reported value (4.5×10-6/K). In situ x-ray diffraction measurements in a diamond anvil cell are carried out up to 20 GPa. There are clear indications of a phase transformation above 5.5 GPa, and the transformation is reversible with little hysteresis. From our x-ray diffraction measurements and density functional theory calculations the bulk modulus of rhombohedral NaZr2(PO4)3 is determined to be 47 and 45 GPa, respectively, and these values are used to obtain the respective Gruneisen parameters.

Kamali, K.; Ravindran, T. R.; Ravi, C.; Sorb, Y.; Subramanian, N.; Arora, A. K.

2012-10-01

17

First principles theoretical study of the hole-assisted conversion of CO to CO2 on the anatase TiO2(101) surface  

NASA Astrophysics Data System (ADS)

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.

Wanbayor, Raina; Deák, Peter; Frauenheim, Thomas; Ruangpornvisuti, Vithaya

2011-03-01

18

Theoretical investigations on KCl xBr 1-x, KCl xI 1-x and KBr xI 1-x: A first-principles study  

NASA Astrophysics Data System (ADS)

Using first-principles total energy calculations within the full-potential linearized augmented plane wave (FP-LAPW) method, we have investigated the structural, electronic and thermodynamic properties of potassium halides (KCl xBr 1-x, KCl xI 1-x and KBr xI 1-x), with x concentrations varying from 0% up to 100%. The effect of composition on lattice constants, bulk modulus, band gap and dielectric function was investigated. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed for the three alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and coworkers. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ? Hm as well as the phase diagram.

Amrani, B.; Kazempoor, A.; Khosravizadeh, Sh.; El Haj Hassan, F.; Akbarzadeh, H.

2008-08-01

19

Speciation in Aqueous MgSO4 Fluid at High Pressures and Temperatures Studied by First-Principles Modeling and Raman Spectroscopy  

NASA Astrophysics Data System (ADS)

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.

Jahn, S.; Schmidt, C.

2008-12-01

20

Reversal of the lattice structure in SrCoO(x) epitaxial thin films studied by real-time optical spectroscopy and first-principles calculations.  

PubMed

Using real-time spectroscopic ellipsometry, we directly observed a reversible lattice and electronic structure evolution in SrCoO(x) (x=2.5-3) epitaxial thin films. Drastically different electronic ground states, which are extremely susceptible to the oxygen content x, are found in the two topotactic phases: i.e., the brownmillerite SrCoO2.5 and the perovskite SrCoO3. First-principles calculations confirmed substantial differences in the electronic structure, including a metal-insulator transition, which originate from the modification in the Co valence states and crystallographic structures. More interestingly, the two phases can be reversibly controlled by changing the ambient pressure at greatly reduced temperatures. Our finding provides an important pathway to understanding the novel oxygen-content-dependent phase transition uniquely found in multivalent transition metal oxides. PMID:24033069

Choi, Woo Seok; Jeen, Hyoungjeen; Lee, Jun Hee; Seo, S S Ambrose; Cooper, Valentino R; Rabe, Karin M; Lee, Ho Nyung

2013-08-30

21

Optical phonons in the wurtzstannite Cu2ZnGeS4 semiconductor: Polarized Raman spectroscopy and first-principle calculations  

NASA Astrophysics Data System (ADS)

The vibrational properties of the wurtzstannite Cu2ZnGeS4 are studied experimentally by polarized Raman scattering in off-resonant and resonant conditions and theoretically by ab initio lattice dynamics calculations. Twenty-nine modes from 45 Raman active theoretically predicted have been experimentally detected and identified, including polar A1(TO),A1(LO), and B1(TO+LO)/B2(TO+LO) and nonpolar A2 symmetry phonon modes from measurements on (2 1 0) and (0 0 1) crystallographic planes of Cu2ZnGeS4 single crystals. The lattice dynamics calculations provide a full picture of the zone center phonon spectrum and allow the assignment of experimentally observed lines to first- and second-order lattice vibrations. Using resonance Raman conditions, a strong enhancement of the A1(LO) modes with the highest longitudinal-transversal spiting is observed.

Guc, M.; Litvinchuk, A. P.; Levcenko, S.; Izquierdo-Roca, V.; Fontané, X.; Valakh, M. Ya.; Arushanov, E.; Pérez-Rodríguez, A.

2014-05-01

22

?-Si3N4/CeO2-x Interface Investigated via Atomic Resolution Z-contrast Imaging, Electron Energy-Loss Spectroscopy and First-Principles Methods  

NASA Astrophysics Data System (ADS)

The addition of rare-earth oxides, typically forming intergranular glassy films in ceramics, has long been known to markedly affect toughness and creep resistance of Si3N4. The present work investigates the bonding characteristics of cerium at the interface between Si3N4 grains and the secondary ceria phases with aberration-corrected scanning transmission electron microscopy techniques. The obtained Z-contrast images and the electron energy-loss spectra taken at the interface of Si3N4/CeO2-x suggest that the arrangement of Ce at the interface depends on the thickness of the intergranular film, the electronic structure of the rare earth element, as well as the termination of Si3N4. Possible reasons for these observed structural and electronic variations at the interface, and their agreement with the theoretical predictions of two stoichiometric terminations of Si3N4(101&-circ;0) surface will be briefly discussed.

Walkosz, Weronika; Klie, Robert F.; Ogut, Serdar; Mikijelj, Biljana; Pennycook, Stephen J.; Idrobo, Juan C.

2009-03-01

23

TS-1 from first principles.  

PubMed

First principles studies on periodic TS-1 models at Ti content corresponding to 1.35% and 2.7% in weight of TiO(2) are presented. The problem of Ti preferential siting is addressed by using realistic models corresponding to the TS-1 unit cell [TiSi(95)O(192)] and adopting for the first time a periodic DFT approach, thus providing an energy scale for Ti in the different crystallographic sites in nondefective TS-1. The structure with Ti in site T3 is the most stable, followed by T4 (+0.3 kcal/mol); the less stable structure, corresponding to Ti in T1, is 5.6 kcal/mol higher in energy. The work has been extended to investigate models with two Ti's per unit cell [Ti(2)Si(94)O(192)] (2.7%). The possible existence of Ti-O-Ti bridges, formed by two corner-sharing TiO(4) tetrahedra, is discussed. By using cluster models cut from the optimized periodic DFT structures, both vibrational (DFT) and electronic excitation spectra (TDDFT) have been calculated and favorably compared with the experimental data available on TS-1. Interesting features emerged from excitation spectra: (i) Isolated tetrahedral Ti sites show a Beer-Lambert behavior, with absorption intensity proportional to concentration. Such a behavior is gradually lost when two Ti's occupy sites close to each other. (ii) The UV-vis absorption in the 200-250 nm region can be associated with transitions from occupied states delocalized on the framework oxygens to empty d states localized on Ti. Such extended-states-to-local-states transitions may help the interpretation of the photovoltaic activity recently detected in Ti zeolites. PMID:19785451

Gamba, Aldo; Tabacchi, Gloria; Fois, Ettore

2009-12-31

24

Chamber Clearing First Principles Modeling  

SciTech Connect

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.

Loosmore, G

2009-06-09

25

First-Principles Study of Silicon Nanowires with Different Surfaces  

Microsoft Academic Search

A first-principles study of silicon nanowires with different surface terminators is presented, where in the midst of the hydrogen, fluorin, and oxygen cases investigated, the O-terminated silicon nanowire is introduced into the theoretical calculation for the first time. The results show that surface modification affects the valence band significantly while it has little impact on the conduction band and that

Mingzhi Gao; Siyu You; Yan Wang

2008-01-01

26

Methods for First-Principles Alloy Thermodynamics  

NASA Astrophysics Data System (ADS)

Traditional first-principles calculations excel at providing formation energies at absolute zero, but obtaining thermodynamic information at nonzero temperatures requires suitable sampling of all the excited states visited in thermodynamic equilibrium, which would be computationally prohibitive via brute-force quantum mechanical calculations alone. In the context of solid-state alloys, this issue can be addressed via the coarse-graining concept and the cluster expansion formalism. This process generates simple, effective Hamiltonians that accurately reproduce quantum mechanical calculation results and that can be used to efficiently sample configurational, vibrational, and electronic excitations and enable the prediction of thermodynamic properties at nonzero temperatures. Vibrational and electronic degrees of freedom are formally eliminated from the problem by writing the system's partition function in a nested form in which the inner sums can be readily evaluated to yield an effective Hamiltonian. The remaining outermost sum corresponds to atomic configurations and can be handled via Monte Carlo sampling driven by the resulting effective Hamiltonian, thereby delivering thermodynamic properties at nonzero temperatures. This article describes these techniques and their implementation in the alloy theoretic automated toolkit, an open-source software package. The methods are illustrated by applications to various alloy systems.

van de Walle, Axel

2013-11-01

27

Mechanical Behaviors of Alloys From First Principles  

NASA Astrophysics Data System (ADS)

Several mesoscale models have been developed to consider a number of mechanical properties and microstructures of Ti-V approximants to Gum Metal and steels from the atomistic scale. In Gum Metal, the relationships between phonon properties and phase stabilities are studied. Our results show that it is possible to design a BCC (beta-phase) alloy that deforms near the ideal strength, while maintaining structural stability with respect to the formation of the o and alpha'' phases. Theoretical diffraction patterns reveal the role of the soft N-point phonon and the BCC-to-HCP transformation path in post-deformation samples. The total energies of the path explain the formation of the giant faults and nano shearbands in Gum Metal. In the study of steels, we focus on the carbon-solute dislocation interactions. The analysis covers the Eshelby's model of point defects and first principles calculations. It is argued that the effects of chemistry and magnetism, omitted in the elasticity model, do not make major contributions to the segregation energy. The predicted solute atmospheres are in good agreement with atom probe measurements.

Hanlumyuang, Yuranan

28

The molecular structure and dynamics of 2-aminopyridine-3-carboxylic acid by X-ray diffraction at 100 K, inelastic neutron scattering, infrared, Raman spectroscopy and from first principles calculations  

NASA Astrophysics Data System (ADS)

Molecular structure of 2-aminopyridine-3-carboxylic acid was studied by single crystal X-ray diffraction at 100 K and inelastic neutron spectroscopy (INS) at 20 K. Infrared and Raman spectra at 293 K were also recorded. Molecular geometries and frequencies were calculated for crystal at local density approximation LDA and general gradient approximation GGA methods. The theoretical frequencies were compared with those observed on the INS, infrared and Raman spectral patterns.

Pawlukoj?, A.; Starosta, W.; Leciejewicz, J.; Natkaniec, I.; Nowak, D.

2007-03-01

29

Site-specific electronic structure analysis by channeling EELS and first-principles calculations.  

PubMed

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

Tatsumi, Kazuyoshi; Muto, Shunsuke; Yamamoto, Yu; Ikeno, Hirokazu; Yoshioka, Satoru; Tanaka, Isao

2006-01-01

30

First-principles studies of solvated molecules  

NASA Astrophysics Data System (ADS)

This work is an investigation of the spectroscopy and thermodynamics of solvated molecules using cyanoacetylene in helium and methane in water as our two main case studies. For the former system, information on the spectra was obtained, while the latter was used as a prototype to study the hydrophobic effect. The most important elements needed in the study of these systems, namely, the intermolecular potential energy surfaces (PESs) were developed from first principles, i.e., without the use of any experimental data. In the case of the CH4-H2O system, the PESs were also used to calculate the cross second virial coefficient, useful in the natural-gas industry. The predictions of the spectra of the He-HCCCN dimer made in this work have now been confirmed by experiments. By extending the results for this dimer using a simple model, the reduction in the rotational constant of HCCCN upon solvation in a superfluid helium nanodroplet was explained and the value of this constant was predicted. The work on the hydrophobic effect involved molecular dynamics simulations of methane-water mixtures. In some of these calculations, a polarization model was used to go beyond the popularly-used pairwise additivity approximation of the interaction energies for many-body systems. The results from the molecular simulations increase our understanding of the nature of the hydrophobic effect by describing the influence of temperature on this phenomenon and also shedding light on the relation between the hydrophobic effect and the molecular properties of the solute molecules. The need to use the Helmholtz free energy and not simply one of its components (the entropic one as is often done) to explain the aggregation of apolar molecules in water was also emphasized.

Akin-Ojo, Omololu

31

Rediscovering First Principles through Online Learning.  

ERIC Educational Resources Information Center

Describes an evaluation of Web-based instruction at the University of Houston-Clear Lake (Texas) that showed that the design team had been distracted from many first principles of instructional design by the creative chaos on the Web and discusses how self-reflection and role definitions allowed the team to overcome these disappointments and…

Kidney, Gary W.; Puckett, Edmond G.

2003-01-01

32

FIRST PRINCIPLES CALCULATIONS OF TOKAMAK ENERGY TRANSPORT  

Microsoft Academic Search

FIRST PRINCIPLES CALCULATIONS OF TOKAMAK ENERGY TRANSPORT. A predictive, physics based model of tokamak energy transport that is based on simulations of the gyrokinetic equation is presented. Calculations of core energy transport are compared with experimental results from JET, TFTR, JT60-U and DIII-D for a variety of discharge conditions. In the region of the plasma predicted to be unstable to

M. KOTSCHENREUTHER; W. DORLAND; Q. P. LIU; G. W. HAMMETT; M. A. BEER; S. A. SMITH; A. BONDESON

33

First principles calculations for lithiated manganese oxides.  

National Technical Information Service (NTIS)

First principles calculations within the local-spin-density-functional theory (LSDFF) framework are presented of densities of electronic states for MnO, LiMn02 in the monoclinic and orthorhombic structures, cubic LiMn204 spinel and M4n02 (delithiated spin...

R. Prasad

1998-01-01

34

First principles determination of dislocation properties.  

SciTech Connect

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.

Hamilton, John C.

2003-12-01

35

Boron Fullerenes: A First-Principles Study  

Microsoft Academic Search

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 B12 icosahedron 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.

Nevill Gonzalez Szwacki

2008-01-01

36

Boron Fullerenes: A First-Principles Study  

Microsoft Academic Search

A family of unusually stable boron cages was identified and examined using first-principles local-density functional method.\\u000a The structure of the fullerenes is similar to that of the B12 icosahedron and consists of six crossing double-rings. The energetically most stable fullerene is made up of 180 boron atoms.\\u000a A connection between the fullerene family and its precursors, boron sheets, is made.

Nevill Gonzalez Szwacki

2008-01-01

37

Epitaxial strain effects from first principles  

Microsoft Academic Search

Epitaxial strain can substantially enhance the spontaneous polarizations and Curie temperatures of ferroelectric thin films compared to the corresponding bulk materials. In this work we use first principles calculations to calculate the effect of epitaxial strain on the spontaneous polarization of the ferroelectrics BaTiO3, PbTiO3, and LiNbO3, and the multiferroic material BiFeO3. We show that the epitaxial strain dependence of

Claude Ederer; Nicola Spaldin

2006-01-01

38

First-principles FP-LAPW calculations and X-ray spectroscopy studies of the electronic structure of Zr3V3O and Zr3V3O0.6 oxides  

NASA Astrophysics Data System (ADS)

Electronic properties of Zr3V3O oxide, a very promising hydrogen-storage material, were studied both from theoretical and experimental points of view employing the full potential linearized augmented plane wave (FP-LAPW) method as well as X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). Total and partial densities of states of the constituting atoms of Zr3V3O have been derived from the FP-LAPW calculations. These data indicate that, the O 2p-like states are the dominant contributors in the bottom of the valence band, whilst the top of the valence band and the bottom of the conduction band of Zr3V3O are dominated by contributions of the V2 3d-like states, with slightly smaller contributions of the V1 3d-like states as well. Significant contributions of the Zr 4d-like states throughout the whole valence-band region and near the bottom of the conduction band are also characteristic of the electronic structure of Zr3V3O. The XPS valence-band spectra and the XES Zr L?2,15, V L? and O K? bands have been derived and compared on a common energy scale for Zr3V3O and Zr3V3O0.6 oxides. This comparison of the experimental spectra was found to be in excellent agreement with the results of the FP-LAPW calculations. In addition, the XPS Zr 3d, V 2p and O 1s core-level binding energies have been measured for Zr3V3O and Zr3V3O0.6 oxides.

Khyzhun, O. Yu.; Bekenev, V. L.; Karpets, M. V.; Zavaliy, I. Yu.

2012-11-01

39

First principles design of ductile refractory alloys  

SciTech Connect

The purpose of this work is to predict elastic and thermodynamic properties of Cr-based alloys based on first-principles calculations. The ultimate goal is to develop new materials for hightemperature applications in energy systems. In this study, we choose both Poisson ratio and Rice–Thomson parameter as computational screening tool for identifying ductilizing additives to the refractory alloys. In this report, we present our preliminary results on bulk modulus and enthalpy of mixing of 25 bcc Cr15X1 alloys.

Gao, M.C.; Rollett, A.D. (Carnegie-Mellon Univ., Pittsburgh, PA); Widom, M. (Carnegie-Mellon Univ., Pittsburgh, PA); Dogan, O.N.; King, P.E.

2008-03-09

40

Giant magnetoresistance calculated from first principles  

SciTech Connect

The Layer Korringa Kohn Rostoker-Coherent Potential Approximation technique was used to calculate the low temperature Giant Magnetoresistance from first principles for Co{vert_bar}Cu and permalloy{vert_bar}Cu superlattices. Our calculations predict large giant magnetoresistance ratios for Co{vert_bar}Cu and extremely large ratios for permalloy{vert_bar}Cu for current perpendicular to the layers. Mechanisms such as spin-orbit coupling which mix spin channels are expected to greatly reduce the GMR effect for permalloy{vert_bar}Cu.

Butler, W.H. [Oak Ridge National Lab., TN (United States); MacLaren, J.M. [Tulane Univ., New Orleans, LA (United States). Dept. of Physics; Zhang, X.G. [Univ. of Kentucky, Lexington, KY (United States). Center for Computational Sciences

1994-09-01

41

First principles calculations for lithiated manganese oxides.  

SciTech Connect

First principles calculations within the local-spin-density-functional theory (LSDFF) framework are presented of densities of electronic states for MnO, LiMnO{sub 2} in the monoclinic and orthorhombic structures, cubic LiMn{sub 2}O{sub 4} spinel and {lambda}-MnO{sub 2} (delithiated spinel), all in antiferromagnetic spin configurations. The changes in energy spectra as the Mn oxidation state varies between 2+ and 4+ are illustrated. Preliminary calculations for Co-doped LiMnO{sub 2} are presented, and the destabilization of a monoclinic relative to a rhombohedral structure is discussed.

Prasad, R.

1998-12-23

42

Hydrogenated BN monolayers: A first principles study  

NASA Astrophysics Data System (ADS)

In the present contribution we apply first principles calculations to investigate the electronic structures and stability of BN hydrogenated monolayers which include a substitutional carbon atom. For comparison, additional C hydrogenated structures are considered. The obtained results demonstrate that BN chair-like monolayers are more stable than boat-like configurations. It is found that the most stable structures present bond angles quite similar to the characteristic one observed for sp3 hybridization. Moreover, a net magnetic moment arises from the introduction of a substitutional carbon impurity. In addition, the results indicate that carbon substitutionals can induce a remarkable reduction of the work function.

Azevedo, S.; Kaschny, J. R.

2013-09-01

43

First-Principles pH Theory  

NASA Astrophysics Data System (ADS)

Despite being one of the most important macroscopic measures and a long history even before the quantum mechanics, the concept of pH has rarely been mentioned in microscopic theories, nor being incorporated computationally into first-principles theory of aqueous solutions. Here, we formulate a theory for the pH dependence of solution formation energy by introducing the proton chemical potential as the microscopic counterpart of pH in atomistic solution models. Within the theory, the general acid-base chemistry can be cast in a simple pictorial representation. We adopt density-functional molecular dynamics to demonstrate the usefulness of the method by studying a number of solution systems including water, small solute molecules such as NH3 and HCOOH, and more complex amino acids with several functional groups. For pure water, we calculated the auto- ionization constant to be 13.2 with a 95 % accuracy. For other solutes, the calculated dissociation constants, i.e., the so- called pKa, are also in reasonable agreement with experiments. Our first-principles pH theory can be readily applied to broad solution chemistry problems such as redox reactions.

Kim, Yong-Hyun; Zhang, S. B.

2006-03-01

44

Iron diffusion from first principles calculations  

NASA Astrophysics Data System (ADS)

The cores of Earth and other terrestrial planets are made up largely of iron1 and it is therefore very important to understand iron's physical properties. Chemical diffusion is one such property and is central to many processes, such as crystal growth, and viscosity. Debate still surrounds the explanation for the seismologically observed anisotropy of the inner core2, and hypotheses include convection3, anisotropic growth4 and dendritic growth5, all of which depend on diffusion. In addition to this, the main deformation mechanism at the inner-outer core boundary is believed to be diffusion creep6. It is clear, therefore, that to gain a comprehensive understanding of the core, a thorough understanding of diffusion is necessary. The extremely high pressures and temperatures of the Earth's core make experiments at these conditions a challenge. Low-temperature and low-pressure experimental data must be extrapolated across a very wide gap to reach the relevant conditions, resulting in very poorly constrained values for diffusivity and viscosity. In addition to these dangers of extrapolation, preliminary results show that magnetisation plays a major role in the activation energies for diffusion at low pressures therefore creating a break down in homologous scaling to high pressures. First principles calculations provide a means of investigating diffusivity at core conditions, have already been shown to be in very good agreement with experiments7, and will certainly provide a better estimate for diffusivity than extrapolation. Here, we present first principles simulations of self-diffusion in solid iron for the FCC, BCC and HCP structures at core conditions in addition to low-temperature and low-pressure calculations relevant to experimental data. 1. Birch, F. Density and composition of mantle and core. Journal of Geophysical Research 69, 4377-4388 (1964). 2. Irving, J. C. E. & Deuss, A. Hemispherical structure in inner core velocity anisotropy. Journal of Geophysical Research 116, B04307 (2011). 3. Buffett, B. A. Onset and orientation of convection in the inner core. Geophysical Journal International 179, 711-719 (2009). 4. Bergman, M. Measurements of electric anisotropy due to solidification texturing and the implications for the Earth's inner core. Nature 389, 60-63 (1997). 5. Deguen, R. & Cardin, P. Thermochemical convection in Earth's inner core. Geophysical Journal International 187, 1101-1118 (2011). 6. Reaman, D. M., Daehn, G. S. & Panero, W. R. Predictive mechanism for anisotropy development in the Earth's inner core. Earth and Planetary Science Letters 312, 437-442 (2011). 7. Ammann, M. W., Brodholt, J. P., Wookey, J. & Dobson, D. P. First-principles constraints on diffusion in lower-mantle minerals and a weak D'' layer. Nature 465, 462-5 (2010).

Wann, E.; Ammann, M. W.; Vocadlo, L.; Wood, I. G.; Lord, O. T.; Brodholt, J. P.; Dobson, D. P.

2013-12-01

45

Cumulene molecular wire conductance from first principles  

NASA Astrophysics Data System (ADS)

We present first principles calculations of current-voltage characteristics (IVC) and conductance of Au(111):S2 -cumulene- S2:Au(111) molecular wire junctions with realistic contacts. The transport properties are calculated using full self-consistent ab initio nonequilibrium Green’s function density-functional theory methods under external bias. The conductance of the cumulene wires shows oscillatory behavior depending on the number of carbon atoms (double bonds). Among all conjugated oligomers, we find that cumulene wires with odd number of carbon atoms yield the highest conductance with metalliclike ballistic transport behavior. The reason is the high density of states in broad lowest unoccupied molecular orbital levels spanning the Fermi level of the electrodes. The transmission spectrum and the conductance depend only weakly on applied bias, and the IVC is nearly linear over a bias region of ±1V . Cumulene wires are therefore potential candidates for metallic connections in nanoelectronic applications.

Prasongkit, J.; Grigoriev, A.; Wendin, G.; Ahuja, Rajeev

2010-03-01

46

First principles study of magnetism in nanographenes  

SciTech Connect

Magnetism in nanographenes (also know as polycyclic aromatic hydrocarbons, or PAHs) are studied with first principles density functional calculations. We find that an antiferromagnetic (AFM) phase appears as the PAH reaches a certain size. This AFM phase in PAHs has the same origin as the one in infinitely long zigzag-edged graphene nanoribbons, namely, from the localized electronic state at the zigzag edge. The smallest PAH still having an AFM ground state is identified. With increased length of the zigzag edge, PAHs approach an infinitely long ribbon in terms of (1) the energetic ordering and difference among the AFM, ferromagnetic (FM), and nonmagnetic (NM) phases and (2) the average local magnetic moment at the zigzag edges. These PAHs serve as ideal targets for chemical synthesis of nanographenes that possess magnetic properties. Moreover, our calculations support the interpretation that experimentally observed magnetism in activated carbon fibers originates from the zigzag edges of the nanographenes.

Jiang, Deen [ORNL; Sumpter, Bobby G [ORNL; Dai, Sheng [ORNL

2007-01-01

47

Susceptibilities for first principles band structures  

NASA Astrophysics Data System (ADS)

We present a parallel implementation of a new method for calculating the unenhanced susceptibility proposed by us recently. Our implementation uses the first principles LMTO band structure within the tight binding approach as input to calculate the joint density of states. The susceptibility is then obtained by integrating over the product of the joint density of states and a Lindhard function. Our program, which has a simple friendly user interface, runs on the PC with a quadputer board, a Meiko Surface running CSTools powered either by T800 or i860 compute boards and the Intel iPSC/860 hypercube in Daresbury. Our method incorporates the troublesome matrix elements naturally and our results on Pd and Ni show that the decrease in ?( q) as we go away from the Brillouin zone centre is due mainly to the matrix elements rather than to the band energies.

Crockford, D. J.; Yeung, W.

1993-04-01

48

Theoretical investigations in femtosecond coherence spectroscopy  

NASA Astrophysics Data System (ADS)

The generation of ultrashort laser pulses with durations shorter than typical nuclear vibrational timescales has led to a variety of spectroscopies that can be used to prepare specific molecular states and study their evolution in real time. The present thesis work is concerned with one such technique, namely femtosecond coherence spectroscopy, which is a pump-probe technique. The aim of this work, which is primarily theoretical, is to expose the physical nature of the molecular states generated by the pump pulse, as well as to study the behavior of the signals detected by the probe pulse. This thesis also involves the development of a highly efficient numerical technique called the maximum entropy method for the analysis of femtosecond and nanosecond time resolved kinetic data. Earlier treatments of the interaction of pulsed laser light with molecular systems were either based on a purely numerical approach, or were based on a semi- quantitative approach that yields analytical results under the semiclassical approximation. The present work is based on an analytical approach that yields fully quantum mechanical formulas and allows for a direct physical interpretation of the various processes involved. We consider a two level electronic system coupled to linearly displaced harmonic oscillators and derive, for the first time, fully quantum mechanical expressions for all the moments of the coordinate, momentum and phonon number operators for the vibrational density matrices associated with the ground and excited electronic states. The novel aspect of the analysis presented here is the connection made between the moments and equilibrium absorption and dispersion lineshapes in a manner analogous to the ``transform methods'' previously used to describe resonance Raman scattering. We also develop an effective linear response approach that is based on a displaced and squeezed state representation for the non-stationary states, in combination with a knowledge of the pump/reaction induced moments. The effective linear response approach is shown to be in excellent agreement with the conventional third order response approach. The present approach is applied to the interpretation of experimental amplitude and phase measurements on reactive and non-reactive samples of the heme protein myoglobin.

Kumar, Anand T. N.

49

Thermodynamics of magnetic systems from first principles  

NASA Astrophysics Data System (ADS)

Density functional calculations have proven to be a useful tool in the study of ground state properties of many materials. The investigation of finite temperature magnetism on the other hand has to rely usually on the usage of empirical models that allow the large number of evaluations of the system's Hamiltonian that are required to obtain the phase space sampling needed to obtain the free energy, specific heat, magnetization, susceptibility, and other quantities as function of temperature. We have demonstrated a solution to this problem that harnesses the computational power of today's large massively parallel computers by combining a classical Monte-Carlo calculations with our first principles multiple scattering electronic structure code (LSMS) for constrained magnetic states. Here we will present recent advances in our method that improve the convergence as well as applications to 3d element based ferromagnets. This research was performed at Oak Ridge National Lab 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 the Division of Materials Science and Engineering, Office of Basic Energy Science of

Eisenbach, Markus; Brown, Gregory; Rusanu, Aurelian; Nicholson, Don M.

2012-02-01

50

First Principle Modeling of Energetic Storm Particles  

NASA Astrophysics Data System (ADS)

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.

Shiota, Daikou; Kataoka, Ryuho; Sugiyama, Tooru; Kusano, Kanya

51

Urban growth simulation from ``first principles''  

NASA Astrophysics Data System (ADS)

General and mathematically transparent models of urban growth have so far suffered from a lack in microscopic realism. Physical models that have been used for this purpose, i.e., diffusion-limited aggregation, dielectric breakdown models, and correlated percolation all have microscopic dynamics for which analogies with urban growth appear stretched. Based on a Markov random field formulation we have developed a model that is capable of reproducing a variety of important characteristic urban morphologies and that has realistic microscopic dynamics. The results presented in this paper are particularly important in relation to ``urban sprawl,'' an important aspect of which is aggressively spreading low-density land uses. This type of growth is increasingly causing environmental, social, and economical problems around the world. The microdynamics of our model, or its ``first principles,'' can be mapped to human decisions and motivations and thus potentially also to policies and regulations. We measure statistical properties of macrostates generated by the urban growth mechanism that we propose, and we compare these to empirical measurements as well as to results from other models. To showcase the open-endedness of the model and to thereby relate our work to applied urban planning we have also included a simulated city consisting of a large number of land use classes in which also topographical data have been used.

Andersson, Claes; Lindgren, Kristian; Rasmussen, Steen; White, Roger

2002-08-01

52

Phonon Engineering in Metals from First Principles  

NASA Astrophysics Data System (ADS)

The electron-phonon interaction in metallic systems controls the electronic transport properties, including both electrical and thermal resistivity. The effect of compressive strain on the electron-phonon interaction in metals is investigated using first-principles density functional theory, and we propose various ways to ``engineer'' this interaction for various technological applications. In particular, we show that by applying compressive strain on the FCC crystals of Al, Cu, Ag and Au, the net electron-phonon scattering rate decreases and likewise the electrical resistivity decreases with increasing pressure. This trend is corroborated by experimental measurements of the resistance of a 0.5 mm diameter high-purity Al wire pressurized up to 2 GPa in a solid-media pressure apparatus at room temperature. The rate of the decrease in electrical resistivity as a function of pressure as determined by experiment is matched by the rate predicted by theory. Our simulations show that Al nanowires have the same response to strain as the bulk crystal; the net electron-phonon scattering can be reduced through compressive strain. Modifying the electron-phonon interaction in metallic structures shows great promise for future nano-electronic devices.

Lanzillo, Nicholas; Thomas, J.; Watson, E. B.; Washington, M.; Nayak, Saroj K.

2013-03-01

53

First-Principles Investigation on Boron Nanostructures  

NASA Astrophysics Data System (ADS)

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.

Tang, Hui

54

First-Principles Study of Silicon Nanowires with Different Surfaces  

NASA Astrophysics Data System (ADS)

A first-principles study of silicon nanowires with different surface terminators is presented, where in the midst of the hydrogen, fluorin, and oxygen cases investigated, the O-terminated silicon nanowire is introduced into the theoretical calculation for the first time. The results show that surface modification affects the valence band significantly while it has little impact on the conduction band and that the dominant effect differs between the two crystalline types used in the calculation. In the [100] nanowire with oxygen and fluorin as the terminator, the mixing between the states reduces the hole effective mass (EM), which makes the [100] direction suitable for metal-oxide-semiconductor field effect transistors (MOSFET). In the [110] case, a large strain induces property deterioration in the oxygen-terminated nanowire, while the fluorin-terminated nanowire shows some good behavior. The close correlation between the conductive bands of the strained bulk Si and [110] nanowires verifies the calculation results.

Gao, Mingzhi; You, Siyu; Wang, Yan

2008-04-01

55

First principles theory of piezoelectricity in ferroelectrics  

NASA Astrophysics Data System (ADS)

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.

Cohen, Ronald

2000-03-01

56

First-principles study of decagonal quasicrystals  

NASA Astrophysics Data System (ADS)

This thesis studies the energetics of decagonal quasicrystals using ab-initio methods. First, we extend the generalized pseudopotential theory (GPT) to treat ternary transition metal (TM) aluminides. The GPT interactions are decomposed to pair and many-body interactions that allow efficient calculations of total energies. In aluminum-rich systems treated at the pair-potential level, one practical limitation is a transition-metal over-binding that creates an unrealistic TM-TM attraction at short separations. An additional potential term is added for systems with short TM atom separations, formally folding repulsive contributions of the three- and higher-body interactions into the pair potentials. To do this, we have performed numerical ab-initio total-energy calculations using an Al-Co-Ni compound in a particular quasicrystalline approximant structure. The results allow us to fit a short-ranged, many-body correction of the form a( r0/r)b to the GPT pair potentials for Co-Co, Co-Ni, and Ni-Ni interactions. We employ the corrected potentials to predict the structure of a decagonal quasicrystal from first-principles considerations. The resulting structure obeys a nearly deterministic decoration of tiles on a hierarchy of length scales related by powers of tau, the golden mean. Second, an investigation of matching rules in Al-Co-Cu quasicrystals using a form of tile Hamiltonian (TH) reveals several results. Phason flips that replace a star-hexagon pair with a pair of boats lower the energy. In Penrose tilings, quasiperiodicity is forced by arrow matching rules on rhombus edges. The edge orientation in Al-Co-Cu is due to Co/Cu chemical ordering. Tile edges meet in vertices with 72° or 144° angles. We find strong interactions between edge orientations at 72° vertices that force a type of matching rule. Comparisons between the ab-initio methods and pair potentials are presented. Lastly, we explore the applicability of the locally self-consistent multiple scattering method (LSMS) in the energy calculations of our quasicrystal models. This is an O(N) all-electron method, which makes calculations both faster and more accurate, in principle, than other available ab-initio methods.

Al-Lehyani, Ibrahim Hamdan

57

First principles studies of semiconductor epitaxial growth  

NASA Astrophysics Data System (ADS)

This thesis conducts investigations mainly on the structures, energetics, and recations of semiconductor as well as oxide surfaces using first principles cluster model approach. The first part of the research work addresses the issues in the epitaxial growth of Hgsb{1-x}Cdsb{x}Te (MCT) materials. Hg divalent compounds were studied thoroughly using a variety of quantum chemical methods in order to understand the energetics of Hg precursors for growth. The (001) growth surfaces were then examined in detail using cluster model calculations. Based on these results, a novel metal-organic molecular beam epitaxial (MOMBE) growth strategy with favorable energetics for growing MCT using Hsb2C=CH-CHsb2-Hg-Cequiv C-CHsb3 is proposed. It is hoped that with this new growth strategy, the Hg vacancy and p-doping problems that currently exist in growth can be avoided. The second part of the thesis discusses the molecular beam epitaxial (MBE) growth of cubic GaN on the (001) surface using various N sources. Surface reconstructions and the interactions of gas-phase atomic and molecular nitrogens with the surface were elucidated using cluster models. Using these results an energy phase diagram for the growth of GaN has been constructed. It suggests that excited state molecular Nsb2\\ (sp3Sigmasbsp{u}{+}) is the most favorable of all N species for growth of high quality GaN because it can undergo a dissociative chemisorption process. Ground state atomic N\\ (sp4S) is also good for growth. The doublet excited states N\\ (sp2D and sp2P) might cause surface N abstraction, leading to N vacancies in the material. Finally, a Fe(OH)sb3(Hsb2O)sb3 GVB cluster model of crystalline alpha-Fesb2Osb3 was developed. This simple model can describe the local geometry and bonding of Fe in the bulk oxide. Using quantum mechanical calculations, the orientation of the oleic imidazoline (OI) molecule bonding to the oxide surface has been determined. OI class of molecules are used extensively for corrosion inhibitor in oil field pipeline applications. It is found in this work that OI can make very strong bonding to the Fe of the iron oxide. In aqueous environments they can replace water on the pipe surface to form a protective layer to prevent corrosion.

Tsai, Bao-Liang

58

Residential Care: Back to First Principles.  

ERIC Educational Resources Information Center

Residential care must be redefined, free from jargon and rhetoric. Over the past 20 years, the social welfare approach, which encompasses the medical model, has dominated legislative and practical thinking about residential care. This theoretical thinking reached its culmination in the concept of the therapeutic community. The therapeutic…

Burns, David A.

59

Current status of single-molecule spectroscopy: Theoretical aspects  

Microsoft Academic Search

We survey the current status of single-molecule spectroscopy in the view point of theoretical aspects. After an explanation of basic concepts in single-molecule spectroscopy, we focus on the following topics: (1) line shape phenomena in disordered media, (2) photon counting statistics for time-dependent fluctuations in single-molecule spectroscopy, (3) fluorescence intensity fluctuations for nonergodic systems, (4) time-resolved single-molecule fluorescence for conformational

Younjoon Jung; Eli Barkai; Robert J. Silbey

2002-01-01

60

First Principle Study of Sodium Nanoclusters  

NASA Astrophysics Data System (ADS)

The structural and electronic properties of small Nan (n = 2-5) nanoclusters have been investigated by employing an ab-initio self-consistent density functional theory in the local density approximation. The total energy, binding energy, bond length and HOMO-LUMO gap are calculated in large energy interval for various isomeric forms of sodium nanoclusters. The results are compared with the other theoretical calculations.

Saxena, Prabodh Sahai; Srivastava, Pankaj; Shrivastava, Ashwani Kumar

2011-12-01

61

Theoretical Calculations of Atomic Data for Spectroscopy  

NASA Technical Reports Server (NTRS)

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.

Bautista, Manuel A.

2000-01-01

62

Theoretical standards in x-ray spectroscopies  

SciTech Connect

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).

Not Available

1992-01-01

63

First principles study of hydroxyapatite surface.  

PubMed

The biomineral hydroxyapatite (HA) [Ca10(PO4)6(OH)2] is the main mineral constituent of mammal bone. We report a theoretical investigation of the HA surface. We identify the low energy surface orientations and stoichiometry under a variety of chemical environments. The surface most stable in the physiologically relevant OH-rich environment is the OH-terminated (1000) surface. We calculate the work function of HA and relate it to the surface composition. For the lowest energy OH-terminated surface we find the work function of 5.1 eV, in close agreement with the experimentally reported range of 4.7 eV-5.1 eV [V. S. Bystrov, E. Paramonova, Y. Dekhtyar, A. Katashev, A. Karlov, N. Polyaka, A. V. Bystrova, A. Patmalnieks, and A. L. Kholkin, J. Phys.: Condens. Matter 23, 065302 (2011)]. PMID:23902010

Slepko, Alexander; Demkov, Alexander A

2013-07-28

64

First principles study of hydroxyapatite surface  

NASA Astrophysics Data System (ADS)

The biomineral hydroxyapatite (HA) [Ca10(PO4)6(OH)2] is the main mineral constituent of mammal bone. We report a theoretical investigation of the HA surface. We identify the low energy surface orientations and stoichiometry under a variety of chemical environments. The surface most stable in the physiologically relevant OH-rich environment is the OH-terminated (1000) surface. We calculate the work function of HA and relate it to the surface composition. For the lowest energy OH-terminated surface we find the work function of 5.1 eV, in close agreement with the experimentally reported range of 4.7 eV-5.1 eV [V. S. Bystrov, E. Paramonova, Y. Dekhtyar, A. Katashev, A. Karlov, N. Polyaka, A. V. Bystrova, A. Patmalnieks, and A. L. Kholkin, J. Phys.: Condens. Matter 23, 065302 (2011)].

Slepko, Alexander; Demkov, Alexander A.

2013-07-01

65

First principles study of biomineral hydroxyapatite  

NASA Astrophysics Data System (ADS)

Hydroxyapatite (HA) [Ca10(PO4)6(OH)2] is one of the most abundant materials in mammal bone. It crystallizes within the spaces between the tropocollagen chains and strengthens the bone tissue. The mineral content of human bone increases with age reaching a maximum value from which it starts to decrease leading to diseases such as osteomalacia. Therefore, an emergent application of this study is bone repair and the production of synthetic bone. Despite its importance, little is known about the growth of HA crystallites in bones. Nor is it well understood how the HA attaches to protein chains and interacts with the surrounding aqueous solution. Using density functional theory (DFT) we calculate the theoretical ground state structure, electronic and vibration properties of hexagonal HA. We find several low energy structures and analyze the energy barriers for spontaneous phase transitions. We calculate the phonon density of states and study the surface energetics for different orientations. We identify the surfaces with highest reactivity using the frontier orbital approach and analyze interactions between these surfaces and water molecules/amino acids.

Slepko, Alexander

2010-03-01

66

First Principles Calculations for X-ray Resonant Spectra and Elastic Properties  

SciTech Connect

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}.

Yongbin Lee

2006-05-01

67

Electronic and vibrational properties of nickel sulfides from first principles  

NASA Astrophysics Data System (ADS)

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.

Wang, Jeng-Han; Cheng, Zhe; Brédas, Jean-Luc; Liu, Meilin

2007-12-01

68

First principles molecular dynamics study of filled ice hydrogen hydrate.  

PubMed

We investigated structural changes, phase diagram, and vibrational properties of hydrogen hydrate in filled-ice phase C(2) by using first principles molecular dynamics simulation. It was found that the experimentally reported "cubic" structure is unstable at low temperature and/or high pressure: The "cubic" structure reflects the symmetry at high (room) temperature where the hydrogen bond network is disordered and the hydrogen molecules are orientationally disordered due to thermal rotation. In this sense, the "cubic" symmetry would definitely be lowered at low temperature where the hydrogen bond network and the hydrogen molecules are expected to be ordered. At room temperature and below 30 GPa, it is the thermal effects that play an essential role in stabilizing the structure in "cubic" symmetry. Above 60 GPa, the hydrogen bonds in the framework would be symmetrized and the hydrogen bond order-disorder transition would disappear. These results also suggest the phase behavior of other filled-ice hydrates. In the case of rare gas hydrate, there would be no guest molecules' rotation-nonrotation transition since the guest molecules keep their spherical symmetry at any temperature. On the contrary methane hydrate MH-III would show complex transitions due to the lower symmetry of the guest molecule. These results would encourage further experimental studies, especially nuclear magnetic resonance spectroscopy and neutron scattering, on the phases of filled-ice hydrates at high pressures and/or low temperatures. PMID:22938248

Zhang, Jingyun; Kuo, Jer-Lai; Iitaka, Toshiaki

2012-08-28

69

First principles study of O defects in CdSe  

NASA Astrophysics Data System (ADS)

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.

T-Thienprasert, J.; Limpijumnong, S.; Du, M.-H.; Singh, D. J.

2012-08-01

70

Electronic and vibrational properties of nickel sulfides from first principles.  

PubMed

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

Wang, Jeng-Han; Cheng, Zhe; Brédas, Jean-Luc; Liu, Meilin

2007-12-01

71

Predicted boron-carbide compounds: A first-principles study  

NASA Astrophysics Data System (ADS)

By using developed particle swarm optimization algorithm on crystal structural prediction, we have explored the possible crystal structures of B-C system. Their structures, stability, elastic properties, electronic structure, and chemical bonding have been investigated by first-principles calculations with density functional theory. The results show that all the predicted structures are mechanically and dynamically stable. An analysis of calculated enthalpy with pressure indicates that increasing of boron content will increase the stability of boron carbides under low pressure. Moreover, the boron carbides with rich carbon content become more stable under high pressure. The negative formation energy of predicted B5C indicates its high stability. The density of states of B5C show that it is p-type semiconducting. The calculated theoretical Vickers hardnesses of B-C exceed 40 GPa except B4C, BC, and BC4, indicating they are potential superhard materials. An analysis of Debye temperature and electronic localization function provides further understanding chemical and physical properties of boron carbide.

Wang, De Yu; Yan, Qian; Wang, Bing; Wang, Yuan Xu; Yang, Jueming; Yang, Gui

2014-06-01

72

Predicted boron-carbide compounds: A first-principles study.  

PubMed

By using developed particle swarm optimization algorithm on crystal structural prediction, we have explored the possible crystal structures of B-C system. Their structures, stability, elastic properties, electronic structure, and chemical bonding have been investigated by first-principles calculations with density functional theory. The results show that all the predicted structures are mechanically and dynamically stable. An analysis of calculated enthalpy with pressure indicates that increasing of boron content will increase the stability of boron carbides under low pressure. Moreover, the boron carbides with rich carbon content become more stable under high pressure. The negative formation energy of predicted B5C indicates its high stability. The density of states of B5C show that it is p-type semiconducting. The calculated theoretical Vickers hardnesses of B-C exceed 40 GPa except B4C, BC, and BC4, indicating they are potential superhard materials. An analysis of Debye temperature and electronic localization function provides further understanding chemical and physical properties of boron carbide. PMID:24929411

Wang, De Yu; Yan, Qian; Wang, Bing; Wang, Yuan Xu; Yang, Jueming; Yang, Gui

2014-06-14

73

First Principles Structure Calculations Using the General Potential Lapw Method  

NASA Astrophysics Data System (ADS)

We have developed a completely general first principles self-consistent full-potential linearized-augmented-plane -wave (LAPW) method program within the density functional formalism to calculate electronic band structure, total energy, pressure and other quantities. No symmetry assumptions are used for the crystal structure. Shape unrestricted charge densities and potentials are calculated inside muffin -tin (MT) spheres as well as in the interstitial regions. All contributions to the Hamiltonian matrix elements are completely taken into account. The core states are treated fully relativistically using the spherical part of the potential only. Scalar relativistic effects are included for the band-states, and spin-orbit coupling is included using a second variation procedure. Both core states and valence states are treated self-consistently, the frozen core approximation is not required. The fast Fourier transformation method is used wherever it is applicable, and this greatly improves the efficiency. This state-of-the-art program has been tested extensively to check the accuracy and convergence properties by comparing calculated electronic band structures, ground state properties, equations of state and cohesive energies for bulk W and GaAs with other theoretical calculations and experimental results. It has been successfully applied to calculate and predict structural and metal-insulator phase transitions for close-packed crystal BaSe and BaTe and the geometric structure of the d-band metal W(001) surface. The results are in generally good agreement with experiment.

Wei, Su-Huai

74

First-principles calculations for point defects in solids  

NASA Astrophysics Data System (ADS)

Point defects and impurities strongly affect the physical properties of materials and have a decisive impact on their performance in applications. First-principles calculations have emerged as a powerful approach that complements experiments and can serve as a predictive tool in the identification and characterization of defects. The theoretical modeling of point defects in crystalline materials by means of electronic-structure calculations, with an emphasis on approaches based on density functional theory (DFT), is reviewed. A general thermodynamic formalism is laid down to investigate the physical properties of point defects independent of the materials class (semiconductors, insulators, and metals), indicating how the relevant thermodynamic quantities, such as formation energy, entropy, and excess volume, can be obtained from electronic structure calculations. Practical aspects such as the supercell approach and efficient strategies to extrapolate to the isolated-defect or dilute limit are discussed. Recent advances in tractable approximations to the exchange-correlation functional (DFT +U, hybrid functionals) and approaches beyond DFT are highlighted. These advances have largely removed the long-standing uncertainty of defect formation energies in semiconductors and insulators due to the failure of standard DFT to reproduce band gaps. Two case studies illustrate how such calculations provide new insight into the physics and role of point defects in real materials.

Freysoldt, Christoph; Grabowski, Blazej; Hickel, Tilmann; Neugebauer, Jörg; Kresse, Georg; Janotti, Anderson; Van de Walle, Chris G.

2014-01-01

75

Theoretical atomic spectroscopy (A handbook for astronomers and physicists)  

NASA Astrophysics Data System (ADS)

The book contains the fundamentals of the theory and methods of calculation of the energy and radiation characteristics of atoms and ions. In particular, attention is given to the energy spectra of multielectron atoms and ions, description and calculation of electron transitions in atoms and ions, recombination processes, and astrophysical applications of theoretical spectroscopy. Some programs for calculating atomic parameters are described.

Rudzikas, Zenon B.; Hikitin, Aleksei A.; Kholtygin, Aleksandr F.

76

First principles pseudopotential calculations on aluminum and aluminum alloys.  

National Technical Information Service (NTIS)

Recent advances in computational techniques have led to the possibility of performing first principles calculations of the energetics of alloy formation on systems involving several hundred atoms. This includes impurity concentrations in the 1% range as w...

J. W. Davenport N. Chetty R. B. Marr S. Narasimhan J. E. Pasciak

1993-01-01

77

Theoretical Modeling of Various Spectroscopies for Cuprates and Topological Insulators  

NASA Astrophysics Data System (ADS)

Spectroscopies resolved highly in momentum, energy and/or spatial dimensions are playing an important role in unraveling key properties of wide classes of novel materials. However, spectroscopies do not usually provide a direct map of the underlying electronic spectrum, but act as a complex 'filter' to produce a 'mapping' of the underlying energy levels, Fermi surfaces (FSs) and excitation spectra. The connection between the electronic spectrum and the measured spectra is described as a generalized 'matrix element effect'. The nature of the matrix element involved differs greatly between different spectroscopies. For example, in angle-resolved photoemission (ARPES) an incoming photon knocks out an electron from the sample and the energy and momentum of the photoemitted electron is measured. This is quite different from what happens in K-edge resonant inelastic X-ray scattering (RIXS), where an X-ray photon is scattered after inducing electronic transitions near the Fermi energy through an indirect second order process, or in Compton scattering where the incident X-ray photon is scattered inelastically from an electron transferring energy and momentum to the scattering electron. For any given spectroscopy, the matrix element is, in general, a complex function of the phase space of the experiment, e.g. energy/polarization of the incoming photon and the energy/momentum/spin of the photoemitted electron in the case of ARPES. The matrix element can enhance or suppress signals from specific states, or merge signals of groups of states, making a good understanding of the matrix element effects important for not only a robust interpretation of the spectra, but also for ascertaining optimal regions of the experimental phase space for zooming in on states of the greatest interest. In this thesis I discuss a comprehensive scheme for modeling various highly resolved spectroscopies of the cuprates and topological insulators (TIs) where effects of matrix element, crystal structure, strong electron correlations (for cuprates) and spin-orbit coupling (for TIs) are included realistically in material-specific detail. Turning to the cuprates, in order to obtain a realistic description of various spectroscopies, one must include not only the effects of the matrix elements and the complexity of the crystal structure, but also of strong electronic correlations beyond the local density approximation (LDA)-based conventional picture, so that the physics of kinks, pseudogaps and superconductivity can be taken into account properly. In this connection, a self-consistent, intermediate coupling scheme informed by material-specific, first-principles band structures has been developed, where electron correlation effects beyond the LDA are incorporated via appropriate self-energy corrections to the electron and hole one-particle Green's functions. Here the antiferromagnetic (AFM) order is used as the simplest model of a competing order. A number of salient features of the resulting electronic spectrum and its energy, momentum and doping dependencies are in accord with experimental observations in electron as well as hole doped cuprates. This scheme thus provides a reasonable basis for undertaking a comprehensive, beyond-LDA level of modeling of various spectroscopies. The specific topics considered here are: (i) Origin of high-energy kink or the waterfall effect found in ARPES; (ii) Identification of the three energy scales observed in RIXS spectra as the pseudogap, charge transfer gap, and Mott gap; (iii) Evolution of the electron momentum densities with holedoping as seen in Compton scattering experiments. For three dimensional topological insulators, the ARPES and scanning tunneling microscopy (STM) spectra has been analyzed using a tight-binding model as well as a k · p model. The spin-orbit coupling, which is essential to produce the characteristic features of the surface states of a TI, is included realistically in the above models. In our generalized k · p model Dresselhaus spin-orbit coupling term extends up to fifth order to reproduce the c

Basak, Susmita

78

First-principles modeling of the infrared spectrum of hydrous fine-grained minerals.  

NASA Astrophysics Data System (ADS)

Infrared (IR) spectroscopy is widely used to investigate the finely divided minerals occurring in soils and weathering environments. The resulting information ranges from the detection and identification of specific or minor mineral constituents to the determination of the stacking order and ordering pattern of substituting cations in clay minerals. However, the straightforward interpretation of IR spectra is often difficult, in particular when considering divided and poorly ordered minerals, such as clays. On the one hand, it is experimentally difficult to measure the polarization properties of very small particles. On the other hand, there is not always an evident relation between the microscopic atomic structure and the vibrational modes observed with IR spectroscopy. An additional complexity of powder spectra also arises from the influence of the shape of particles on the infrared spectra. The first-principles calculation of vibrational spectra thus represents an ideal tool to establish unambiguous relationships between the IR spectrum and the microscopic structure of minerals. This is done by computing the phonon vibrational modes of the mineral from density functional perturbation theory. The same method is also used to obtain the low-frequency dielectric tensor and thus determine the influence of the shape of particles on the IR spectrum (via long-range electrostatic interactions). By combining the theoretical and experimental investigation of selected clays and other fine-grained minerals, we will show that macroscopic parameters may strongly affect their IR spectra and have to be taken into account to interpret the absorption bands in terms of vibrational modes. Examples will include kaolinite-group minerals, serpentine minerals and gibbsite.

Balan, E.; Lazzeri, M.; Mauri, F.

2006-05-01

79

First-principles studies of the surface reaction of acetylene with HSi(001)(1×1)  

Microsoft Academic Search

Recent experiments have shown that it is possible to grow a monolayer of 1-alkynes on hydrogenated Si(001)-(1×1) surfaces by thermal reaction of the organic compound with the Si surface. Infrared spectroscopy and x-ray reflectivity measurements have indicated that the alkynes form two Si?C bonds to the surface (per reacting molecule). We present the results of a first-principles study of the

E. Martínez-Guerra; Noboru Takeuchi

2007-01-01

80

Phase transition and thermodynamic properties of ThO2 from first-principles calculations  

NASA Astrophysics Data System (ADS)

Within the framework of the quasi-harmonic approximation, the thermodynamic properties and the phase transition of ThO2 from the cubic structure to the orthorhombic structure are studied using the first-principles projector-augmented wave method. The vibrational contribution to Helmholtz free energy is evaluated from the first-principles phonon density of states and the Debye-Grüneisen model. The calculated results reveal that at ambient temperature, the phase transition from the cubic phase to the orthorhombic phase occurs at 26.49 GPa, which is in agreement with the experimental and theoretical data. With increasing temperature, the transition pressure decreases almost linearly above room temperature. The predicted heat capacity and linear thermal expansion coefficient of cubic ThO2 are in good consistence with the experimental data. By comparing the experimental results with the calculation results from the first-principles and Debye-Grüneisen model, it is found that the thermodynamic properties of ThO2 are depicted well by the first-principles phonon treatment after including the an-harmonic correction to quasi-harmonic free energy.

Wang, Fan-Hou; Li, Qiang; Huang, Duo-Hui; Cao, Qi-Long; Yang, Jun-Sheng; Gao, Zeng-Hui

2013-08-01

81

First principles pseudopotential calculation of electron energy loss near edge structures of lattice imperfections  

Microsoft Academic Search

Theoretical calculations of electron energy loss near edge structures (ELNES) of lattice imperfections, particularly a Ni(111)\\/ZrO2(111) heterointerface and an Al2O3 stacking fault on the {1?1¯?0?0} plane, are performed using a first principles pseudopotential method. The present calculation can qualitatively reproduce spectral features as well as chemical shifts in experiment by employing a special pseudopotential designed for the excited atom with

Teruyasu Mizoguchi; Katsuyuki Matsunaga; Eita Tochigi; Yuichi Ikuhara

82

X-ray magnetic circular dichroism in UGe2: first-principles calculations  

Microsoft Academic Search

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of UGe2 at the U N4,5, N2,3 and Ge K and L2,3 edges are investigated theoretically from first principles, using the fully relativistic spin-polarized Dirac linear muffin-tin orbital (LMTO) band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as the LSDA+U method. The

V N Antonov; B N Harmon; A N Yaresko

2007-01-01

83

First-principles calculations of electronic structure and optical properties of strained Mg 2 Si  

Microsoft Academic Search

A detailed theoretical study on structural, electronic and optical properties of Mg2Si under the isotropic lattice deformation\\u000a was performed based on the first-principles pseudopotential method. The results show that the isotropic lattice deformation\\u000a results in a linear decrease in the energy gap for the direct ?15-?1 and indirect ?15-L1 transitions from 93% to 113%, while the indirect band gap ?15-X1

Qian Chen; Quan Xie; FengJuan Zhao; DongMeng Cui; XuZhen Li

2010-01-01

84

First principles pseudopotential calculations on aluminum and aluminum alloys  

Microsoft Academic Search

Recent advances in computational techniques have led to the possibility of performing first principles calculations of the energetics of alloy formation on systems involving several hundred atoms. This includes impurity concentrations in the 1% range as well as realistic models of disordered materials (including liquids), vacancies, and grain boundaries. The new techniques involve the use of soft, fully nonlocal pseudopotentials,

J. W. Davenport; N. Chetty; R. B. Marr; S. Narasimhan; J. E. Pasciak; R. F. Peierls; M. Weinert

1993-01-01

85

Coarse graining approach to First principles modeling of structural materials  

NASA Astrophysics Data System (ADS)

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 efficient coarse graining (CG) approach to calculate local electronic properties of large MD-generated structures from the first 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 field. The local moments in the neighborhood of the defect cores are calculated with first-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 significantly and makes large-scale structures amenable to first principles study. Work is sponsored by the USDoE, Office 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 Office of Science of the USDoE under Contract No. DE-AC05-00OR22725.

Odbadrakh, Khorgolkhuu; Nicholson, Don; Rusanu, Aurelian; Samolyuk, German; Wang, Yang; Stoller, Roger; Zhang, Xiaoguang; Stocks, George

2013-03-01

86

First principles studies of complex oxide surfaces and interfaces  

Microsoft Academic Search

Oxides enter our everyday life and exhibit an impressive variety of physical and chemical properties. The understanding of their behaviour, which is often determined by the electronic and atomic structures of their surfaces and interfaces, is a key question in many fields, such as geology, environmental chemistry, catalysis, thermal coatings, microelectronics, and bioengineering. In the last decade, first principles methods,

Claudine Noguera; Fabio Finocchi; Jacek Goniakowski

2004-01-01

87

First-principles study of semiconductor and metal surfaces  

Microsoft Academic Search

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

Sungho Kim

2005-01-01

88

First principles simulations of fluid water: The radial distribution functions  

Microsoft Academic Search

We apply a recently developed first principles but simplified molecular dynamics method to the simulation of water at different conditions. The computational simplicity of this method allows its application to systems containing a significant number of molecules, yet still taking explicitly into account the quantum electronic structure of the system. In the present work we simulate a system of 216

Jose´ Ortega; James P. Lewis; Otto F. Sankey

1997-01-01

89

Recent progress in first-principles studies of magnetoelectric multiferroics  

Microsoft Academic Search

Materials that combine magnetic and ferroelectric properties have generated increasing interest over the last few years, due to both their diverse properties and their potential utility in new types of magnetoelectric device applications. In this review we discuss recent progress in the study of such magnetoelectric multiferroics which has been achieved using computational first-principles methods based on density functional theory.

Claude Ederer; Nicola A. Spaldin

2005-01-01

90

Theoretical study of the spectroscopy of Al2  

NASA Technical Reports Server (NTRS)

The present study of the singlet and triplet states of Al2 below about 30,000/cm at the multireference configuration-interaction level attempts to identify and characterize the band systems in both the singlet and triplet manifolds. The spectroscopy of Al2 can be understood in terms of an X 3Pi(u) ground state. Calculations suggest that the assignments of the E and F systems recently noted in a jet-cooled beam are correct, although remaining differences between theoretical and experimental spectroscopic constants and radiative lifetimes preclude a definitive assignment.

Langhoff, Stephen R.; Bauschlicher, Charles W., Jr.

1990-01-01

91

First-principles study of acetylene adsorption on Si(100):??The end-bridge structure  

NASA Astrophysics Data System (ADS)

We present the results of a first-principles study of the adsorption of acetylene on Si(100). In particular, we focus on a recent controversial issue concerning the occurrence of the “end-bridge” configuration in which an acetylene molecule bridges two adjacent surface Si dimers. At variance with recent theoretical calculations, we find that the lowest-energy “end-bridge” structure is stable and nonmetallic both at 0.5 ML and at 1.0 ML coverage. This leads to the conclusion that this configuration can be one of those observed in experiments. The results of first-principles calculations of the optical properties of different configurations of acetylene on Si(100) are also presented and discussed.

Silvestrelli, Pier Luigi; Pulci, Olivia; Palummo, Maurizia; del Sole, Rodolfo; Ancilotto, Francesco

2003-12-01

92

New Optical Evaluation Approach for Parabolic Trough Collectors: First-Principle OPTical Intercept Calculation  

SciTech Connect

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.

Zhu, G.; Lewandowski, A.

2012-11-01

93

Linear Scaling First-Principles Molecular Dynamics with Controlled Accuracy  

SciTech Connect

In our quest for accurate linear scaling first-principles molecular dynamics methods for pseudopotential DFT calculations, we investigate the accuracy of real-space grid approaches, with finite differences and spherical localization regions. We examine how the positions of the localization centers affect the accuracy and the convergence rate of the optimization process. In particular we investigate the accuracy of the atomic forces computation compared to the standard O(N{sup 3}) approach. We show the exponential decay of the error on the energy and forces with the size of the localization regions for a variety of realistic physical systems. We propose a new algorithm to automatically adapt the localization centers during the ground state computation which allows for molecular dynamics simulations with diffusion processes. The combination of algorithms proposed lead to a genuine linear scaling First-Principles Molecular Dynamics method with controlled accuracy. We illustrate our approach with examples of microcanonical molecular dynamics with localized orbitals.

Gygi, F; Fattebert, J

2004-03-10

94

First-Principles Molecular Dynamics at a Constant Electrode Potential  

NASA Astrophysics Data System (ADS)

A simulation scheme for performing first-principles molecular dynamics at a constant electrode potential is presented, opening the way for a more realistic modeling of voltage-driven devices. The system is allowed to exchange electrons with a reservoir at fixed potential, and dynamical equations for the total electronic charge are derived by using the potential energy of the extended system. In combination with a thermostat, this potentiostat scheme reproduces thermal fluctuations of the charge with the correct statistics, implying a realistic treatment of the potential as a control variable. Practically, the dynamics of the charge are decoupled from the electronic structure calculations, making the scheme easily implementable in existing first-principles molecular dynamics codes. Our approach is demonstrated on a test system by considering various test cases.

Bonnet, Nicéphore; Morishita, Tetsuya; Sugino, Osamu; Otani, Minoru

2012-12-01

95

First-principles molecular dynamics at a constant electrode potential.  

PubMed

A simulation scheme for performing first-principles molecular dynamics at a constant electrode potential is presented, opening the way for a more realistic modeling of voltage-driven devices. The system is allowed to exchange electrons with a reservoir at fixed potential, and dynamical equations for the total electronic charge are derived by using the potential energy of the extended system. In combination with a thermostat, this potentiostat scheme reproduces thermal fluctuations of the charge with the correct statistics, implying a realistic treatment of the potential as a control variable. Practically, the dynamics of the charge are decoupled from the electronic structure calculations, making the scheme easily implementable in existing first-principles molecular dynamics codes. Our approach is demonstrated on a test system by considering various test cases. PMID:23368585

Bonnet, Nicéphore; Morishita, Tetsuya; Sugino, Osamu; Otani, Minoru

2012-12-28

96

First principles molecular dynamics of molten NaCl  

NASA Astrophysics Data System (ADS)

First principles Hellmann-Feynman molecular dynamics (HFMD) results for molten NaCl at a single state point are reported. The effect of induction forces on the structure and dynamics of the system is studied by comparison of the partial radial distribution functions and the velocity and force autocorrelation functions with those calculated from classical MD based on rigid-ion and shell-model potentials. The first principles results reproduce the main structural features of the molten salt observed experimentally, whereas they are incorrectly described by both rigid-ion and shell-model potentials. Moreover, HFMD Green-Kubo self-diffusion coefficients are in closer agreement with experimental data than those predicted by classical MD. A comprehensive discussion of MD results for molten NaCl based on different ab initio parametrized polarizable interionic potentials is also given.

Galamba, N.; Costa Cabral, B. J.

2007-03-01

97

First-principles approaches to simulate lithiation in silicon electrodes  

NASA Astrophysics Data System (ADS)

Silicon is viewed as an excellent electrode material for lithium batteries due to its high lithium storage capacity. Various Si nanostructures, such as Si nanowires, have performed well as lithium battery anodes and have opened up exciting opportunities for the use of Si in energy storage devices. The mechanism of lithium insertion and the interaction between Li and the Si electrode must be understood at the atomic level; this understanding can be achieved by first-principles simulation. Here, first-principles computations of lithiation in silicon electrodes are reviewed. The review focuses on three aspects: the various properties of bulk Li-Si compounds with different Li concentrations, the electronic structure of Si nanowires and Li insertion behavior in Si nanowires, and the dynamic lithiation process at the Li/Si interface. Potential study directions in this research field and difficulties that the field still faces are discussed at the end.

Zhang, Qianfan; Cui, Yi; Wang, Enge

2013-10-01

98

Graphitization of diamond (111) studied by first principles molecular dynamics  

Microsoft Academic Search

Large scale first principles numerical simulations, performed on modern massively parallel computers, can be usefully applied to study the physics of semiconductor surface and interface systems. We report on a recent study of the surface-initiated diamond to graphite structural transition of crystalline carbon. Our investigation consisted of a series of fully ab initio molecular dynamic simulations of the diamond C(111)-(2

Alessandro De Vita; Giulia Galli; Andrew Canning; Roberto Car

1996-01-01

99

First-principles local pseudopotentials for group-IV elements  

Microsoft Academic Search

A simple scheme is proposed for generating first-principles local pseudopotentials, and applied to group-IV elements. The scheme is based on solving Kohn-Sham equations inversely, using the density from a nonlocal pseudopotential calculation. The generated local pseudopotentials have been applied in the calculations of dimers and solids including diamond and fcc structures of Si, Ge, and Sn. Fairly good results are

Bing Wang; M. J. Stott

2003-01-01

100

Alternative approach to separable first-principles pseudopotentials  

Microsoft Academic Search

We construct a first-principles pseudo-Hamiltonian using ideas borrowed from the linear augmented-plane-wave method. Space is subdivided into the core region around the nuclei and the valence region, which is just the remainder of space. A basis set consisting of atomic-type orbitals in the core region and plane waves in the valence region leads to Hamiltonian and overlap matrices that depend

S. Goedecker; K. Maschke

1990-01-01

101

Oxygen adsorption on Cu(100): First-principles pseudopotential calculations  

Microsoft Academic Search

We have studied the adsorption characteristics of atomic and molecular oxygen, incident on the Cu(100) surface. Our pseudopotential first-principles calculations yield trajectories for the O2 molecule without dissociation barriers at the entrance channel. We discuss the energetics of the O2 adsorption and dissociation in terms of the elbow plots which are two-dimensional cuts of the full six-dimensional potential-energy surface. The

M. Alatalo; S. Jaatinen; P. Salo; K. Laasonen

2004-01-01

102

First Principles Investigation of Surface Stacking Faults on Ru(0001)  

NASA Astrophysics Data System (ADS)

Using first-principles density-functional theory we study the surface structure and stacking-fault energy of the Ru(0001) surface. In particular, we will present the relaxed structure of the Ru(0001) surface with and without stacking faults. The fault energy and relaxation are compared with bulk Ru values. Calculations are performed using our recently developed scalable band-distributed parallel molecular dynamics code. Selected vibrational modes are also reported.

Kim, Seong-Gon; Nunes, R. W.; Singh, D. J.

1998-03-01

103

First-principles study of the surfaces of zirconia  

Microsoft Academic Search

We have studied the surfaces of zirconia (ZrO2) by first-principles calculations using density functional theory. We predict surface energies and relaxations for the principal surfaces of different bulk phases of zirconia. We find that the stoichiometric tetragonal(111) and monoclinic(1¯11) are the most stable surfaces. We find a strong linear correlation between surface energies before and after relaxing the surface ions.

A. Christensen; Emily A. Carter

1998-01-01

104

First principles study of the Ga(1010)\\/water interface  

Microsoft Academic Search

Pure GaN is a water-splitting photocatalyst which works in UV light. A first-principles study shows monolayer of water molecules on the (1010) surface of wurtzite GaN have negligible barrier to be dissociated into OH^- which bind to Ga^+ and H^+ which bind to N^-.footnotetextX.Shen et al., J. Phys. Chem. C 113, 3365 (2009) We present an ab initio molecule dynamics

Jue Wang; Xiao Shen; Marivi Fernandez-Serra

2010-01-01

105

First principles studies of surface alloying in immiscible systems  

Microsoft Academic Search

The surface alloy formation of Au, Ag and Cu on the W(001) surface has been studied by using first principles total energy calculations. It is found that Au, Ag and Cu, which would not alloy with W in bulk, will form the c(2X2) substitutional surface alloy on the W(001) surface, though little bonding was found between Au (Ag, Cu) and

J. G. Che; K. M. Zhang; X. D. Xie

2000-01-01

106

First Principles Studies of Metal-Oxide Surfaces  

Microsoft Academic Search

The interest in first principles investigations of metal-oxide surfaces has been growing rapidly over the past 10 years. Several phenomena of vital importance for fundamental understanding of heterogeneous catalysis have been investigated, including the metal\\/metal-oxide interface and the interaction with adsorbates. This contribution reviews different implementations of the density functional theory frequently used in studies of metal-oxide properties. Computational results

Henrik Grönbeck

2004-01-01

107

First principles study of graphene nanoribbons and nanorectangles  

Microsoft Academic Search

We have studied the finite size effect on the electronic structure of graphene nanoribbons (GNRs) using first principles density functional techniques. In particular, we have computed the energy gap dependence on the width and length for zero-dimensional nanorectangles for both the armchair and zigzag ribbons; and compared to the one-dimensional (infinite length) cases. One-dimensional armchair ribbons are expected to be

Philip Shemella; Li Chen; Yu Zhou; Yiming Zhang; Sreekala Subbulakshmi; Pulickel Ajayan; Saroj Nayak

2008-01-01

108

Topological insulators from the perspective of first-principles calculations  

NASA Astrophysics Data System (ADS)

Topological insulators are new quantum states with helical gapless edge or surface states inside the bulk band gap.These topological surface states are robust against the weak time-reversal invariant perturbations, such as lattice distortions and non-magnetic impurities. Recently a variety of topological insulators have been predicted by theories, and observed by experiments. First-principles calculations have been widely used to predict topological insulators with great success. In this review, we summarize the current progress in this field from the perspective of first-principles calculations. First of all, the basic concepts of topological insulators and the frequently-used techniques within first-principles calculations are briefly introduced. Secondly, we summarize general methodologies to search for new topological insulators. In the last part, based on the band inversion picture first introduced in the context of HgTe, we classify topological insulators into three types with s-p, p-p and d-f, and discuss some representative examples for each type.

Zhang, Haijun; Zhang, Shou-Cheng

2013-02-01

109

First-principles study of graphene - carbon nanotube contacts  

NASA Astrophysics Data System (ADS)

The electron transport properties of carbon nanotube -- graphene junctions are investigated with first-principles total energy and electron transport calculations. By combining the advantageous material properties of graphene and nanotubes one can create all carbon hybrid architectures with properties that are particularly well suited to applications. The p-type Schottky barrier height is calculated in model junctions with (8,0) and (10,0) nanotubes in a top-contact configuration. Results indicate a lower barrier in carbon nanotube -- graphene junctions than in other carbon nanotue -- metal systems.

Cook, Brandon; Varga, Kalman

2012-02-01

110

Stability of graphene oxide phases from first-principles calculations  

NASA Astrophysics Data System (ADS)

We determine the energy diagram of graphene oxides (GOs) as a function of oxygen and hydrogen chemical potentials by systematic first-principles calculations. The diagram reveals that thermodynamically stable GOs can exist only in stringent growth conditions in the form of hydroxyl, epoxy, or mixed hydroxyl/epoxy phases. There is no mixed phase with sp2 carbon because of substrate relaxation, which is unique to two-dimensional system with little coupling in the third direction such as graphene. The mixed phase observed experimentally is interpreted instead, in terms of a kinetic stability of nonequilibrium grown GOs against phase separation.

Wang, Lu; Sun, Y. Y.; Lee, Kyuho; West, D.; Chen, Z. F.; Zhao, J. J.; Zhang, S. B.

2010-10-01

111

Twinned structure for shape memory: First-principles calculations  

NASA Astrophysics Data System (ADS)

We have performed first-principles calculations for the crystal structures of binary shape memory alloys NiTi, PdTi, and PtTi along with the transformation from austenite to martensite. A detailed analysis of the transition pathway shows that both the cubic B2 and the orthorhombic B19 evolve to a twinned structure followed by a transformation to the monoclinic B19' . In contrast to the low-energy body-centered orthorhombic structure for NiTi, the existence of this twinned state is essential for storing the shape memory in smart alloys.

Wang, Xiao-Qian

2008-09-01

112

First Principles Simulation of a Ceramic/ Metal Interface with Misfit  

SciTech Connect

The relaxed atomic structure of a model ceramic/metal interface, {l_brace}222{r_brace}MgO/Cu , is simulated, including lattice constant mismatch, using first principles local-density functional theory plane wave pseudopotential methods. The 399-atom computational unit cell contains 36 O and 49 Cu atoms per layer in accordance with the 7/6 ratio of MgO to Cu lattice constants. The atomic layers on both sides of the interface warp to optimize the local bonding. The interface adhesive energy is calculated. The interface electronic structure is found to vary appreciably with the local environment. (c) 2000 The American Physical Society.

Benedek, R. [Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (United States)] [Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (United States); Alavi, A. [School of Mathematics and Physics, Queen's University, Belfast, BT7 1NN, Northern Ireland (Ireland)] [School of Mathematics and Physics, Queen's University, Belfast, BT7 1NN, Northern Ireland (Ireland); Seidman, D. N. [Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (United States)] [Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208 (United States); Yang, L. H. [Condensed Matter Physics Division, Lawrence Livermore National Laboratory, Livermore, California 94551 (United States)] [Condensed Matter Physics Division, Lawrence Livermore National Laboratory, Livermore, California 94551 (United States); Muller, D. A. [Lucent Technologies, 700 Mountain Avenue, Murray Hill, New Jersey 07974 (United States)] [Lucent Technologies, 700 Mountain Avenue, Murray Hill, New Jersey 07974 (United States); Woodward, C. [Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson AFB, Dayton, Ohio 45433-7817 (United States)] [Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson AFB, Dayton, Ohio 45433-7817 (United States)

2000-04-10

113

First principles pseudopotential calculations on aluminum and aluminum alloys  

SciTech Connect

Recent advances in computational techniques have led to the possibility of performing first principles calculations of the energetics of alloy formation on systems involving several hundred atoms. This includes impurity concentrations in the 1% range as well as realistic models of disordered materials (including liquids), vacancies, and grain boundaries. The new techniques involve the use of soft, fully nonlocal pseudopotentials, iterative diagonalization, and parallel computing algorithms. This approach has been pioneered by Car and Parrinello. Here the authors give a review of recent results using parallel and serial algorithms on metallic systems including liquid aluminum and liquid sodium, and also new results on vacancies in aluminum and on aluminum-magnesium alloys.

Davenport, J.W.; Chetty, N.; Marr, R.B.; Narasimhan, S.; Pasciak, J.E.; Peierls, R.F.; Weinert, M.

1993-12-31

114

Ethanol adsorption on the Si (111) surface: First principles study  

NASA Astrophysics Data System (ADS)

Equilibrium atomic configurations and electron energy structure of ethanol adsorbed on the Si (111) surface are studied by the first principles density functional theory. Geometry optimization is performed by the total energy minimization method. Equilibrium atomic geometries of ethanol, both undissociated and dissociated, on the Si (111) surface are found and analysed. Reaction pathways and predicted transition states are discussed in comparison with available experimental data in terms of the feasibility of the reactions occurring. Analysis of atom and orbital resolved projected density of states indicates substantial modifications of the Si surface valence and conduction electron bands due to the adsorption of ethanol affecting the electronic properties of the surface.

Gavrilenko, Alexander V.; Bonner, Carl E.; Gavrilenko, Vladimir I.

2012-03-01

115

Collective modes in light nuclei from first principles.  

PubMed

Results for ab initio no-core shell model calculations in a symmetry-adapted SU(3)-based coupling scheme demonstrate that collective modes in light nuclei emerge from first principles. The low-lying states of 6Li, 8Be, and 6He are shown to exhibit orderly patterns that favor spatial configurations with strong quadrupole deformation and complementary low intrinsic spin values, a picture that is consistent with the nuclear symplectic model. The results also suggest a pragmatic path forward to accommodate deformation-driven collective features in ab initio analyses when they dominate the nuclear landscape. PMID:24483740

Dytrych, T; Launey, K D; Draayer, J P; Maris, P; Vary, J P; Saule, E; Catalyurek, U; Sosonkina, M; Langr, D; Caprio, M A

2013-12-20

116

Hybrid perovskites for photovoltaics: Insights from first principles  

NASA Astrophysics Data System (ADS)

The methylammonium lead iodide perovskites at the core of recently proposed solar cells with exceptionally large quantum conversion efficiency are studied by first-principles methods. Large absorption coefficients (0.03-0.04 nm-1 for wavelength ˜500 nm) and small effective masses suited for both n-type and p-type transport are obtained as a consequence of their peculiar structural and electronic characteristics. In particular, the presence of a direct gap between highly dispersed Pb(6s)-I(5p) valence bands and Pb(6p) conduction bands is the key ingredient at the basis of their excellent performance in photovoltaic applications.

Filippetti, A.; Mattoni, A.

2014-03-01

117

Large impurity effects in rubrene crystals: First-principles calculations  

SciTech Connect

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.

Tsetseris, L. [Aristotle University of Thessaloniki, Greece; Pantelides, Sokrates T. [Vanderbilt University

2008-01-01

118

First-principles study of nitrobenzene adsorption on graphene  

NASA Astrophysics Data System (ADS)

Based on first-principles calculations, the properties of nitrobenzene adsorption on graphene have been investigated. The results show that nitrobenzene prefers to be parallel to the plane of graphene with nitro closer to graphene than phenyl. Due to molecular adsorption, there is a doping band near Fermi energy. The adsorbed molecules will become perpendicular to the plane of graphene under a electrostatic field, and the doping band varies conspicuously with respect to Fermi energy. In addition, a 5000 steps MD is performed at T = 300 K to test the stability of nitrobenzene molecules adsorbed erectly on graphene under electric field.

Dai, Zhenhong; Zhao, Yinchang

2014-06-01

119

First principle study of unzipped boron nitride nanotubes  

NASA Astrophysics Data System (ADS)

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.

Azadi, Sam; Moradian, Rostam

2010-01-01

120

Ferroic Coupling in Layered Perovskites from First Principles  

NASA Astrophysics Data System (ADS)

Antiferrodistortive SrTiO3 can be driven ferroelectric with a modest amount of biaxial strain. The n = 1 Ruddlesden-Popper remains paraelectric and undistorted for the same strain state. Elucidating the manner in which ferroic instabilities emerge as the number of perovskite blocks, n, increases is a fundamentally interesting challenge in its own right, yet a proper understanding could open new avenues in materials design. Combining first-principles calculations and symmetry arguments, we study the effect of strain (or pressure) and dimensionality on ferroic coupling in the layered Sr-Ti-O perovskites.

Birol, Turan; Benedek, Nicole; Fennie, Craig

2010-03-01

121

First-principles studies of electrical resistivity of iron under pressure  

NASA Astrophysics Data System (ADS)

We investigate the temperature and pressure dependences of the electrical resistivity, thermal conductivity and thermal diffusivity for bcc and hcp Fe using the low-order variational approximation and theoretical transport spectral functions calculated from the first-principles linear response linear-muffin-tin-orbital method in the generalized gradient approximation. The calculated values for the electrical resistivity show a strong increase with temperature and decrease with pressure, and are in agreement with high-temperature shock data. We also discuss the behavior of the electrical resistivity for the {bcc \\to hcp} phase transition.

Sha, Xianwei; Cohen, R. E.

2011-02-01

122

First-principles study of leakage current through a Si/SiO2 interface  

NASA Astrophysics Data System (ADS)

The relationship between the presence of defects at the stacking structure of the Si/SiO2 interface and leakage current is theoretically studied by first-principles calculation. I found that the leakage current through the interface with dangling bonds is 530 times larger than that without any defects, which is expected to lead to dielectric breakdown. The direction of the dangling bonds is closely related to the performance of the oxide as an insulator. In addition, it is proved that the termination of the dangling bonds by hydrogen atoms is effective for reducing the leakage current.

Ono, Tomoya

2009-05-01

123

Investigation of structural stability and elastic properties of Zrh and Zrh2: A first principles study  

NASA Astrophysics Data System (ADS)

The electronic, structural and mechanical properties of ZrH and ZrH2 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 ZrH2 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.

Kanagaprabha, S.; Rajeswarapalanichamy, R.; Sudhapriyanga, G.; Murugan, A.; Santhosh, M.; Iyakutti, K.

2014-04-01

124

Spin Polarization Properties of Na Doped Meridianal Tris(8-Hydroxyquinoline) Aluminum Studied by First Principles Calculations  

NASA Astrophysics Data System (ADS)

We theoretically investigate the electronic structure and spin polarization properties of Na-doped meridianal tris(8-hydroxyquinoline) aluminum (Alq3) by first principles calculations. It is found that the spin density is distributed mainly in the Alq3 part in the Alq3:Na complex. Electron charge transfer takes place from the Na atom to the Alq3 molecule, which induces asymmetric changing of the molecule bond lengths, thus the spin density distribution becomes asymmetric. Spin polarization of the complex originates from the preferable filling of the spin-split nitrogen and carbon p-orbitals because of the different bond length changes of the Alq3 molecule upon Na doping.

Ren, Jun-Feng; Yuan, Xiao-Bo; Hu, Gui-Chao

2014-04-01

125

First principles calculation on the adsorption of water on lithium-montmorillonite (Li-MMT).  

PubMed

The interaction of water molecules and lithium-montmorillonite (Li-MMT) is theoretically investigated using density functional theory (DFT) based first principles calculation. The mechanism of water adsorption at two different water concentrations on Li-MMT as well as their structural and electronic properties are investigated. It is found that the adsorption stability in Li-MMT is higher in higher water concentration. It is also found that an adsorbed water molecule on Li-MMT causes the Li to protrude from the MMT surface, so it is expected that Li may be mobile on H(2)O/Li-MMT. PMID:23110845

Wungu, Triati Dewi Kencana; Agusta, Mohammad Kemal; Saputro, Adhitya Gandaryus; Dipojono, Hermawan Kresno; Kasai, Hideaki

2012-11-28

126

First principles calculation on the adsorption of water on lithium-montmorillonite (Li-MMT)  

NASA Astrophysics Data System (ADS)

The interaction of water molecules and lithium-montmorillonite (Li-MMT) is theoretically investigated using density functional theory (DFT) based first principles calculation. The mechanism of water adsorption at two different water concentrations on Li-MMT as well as their structural and electronic properties are investigated. It is found that the adsorption stability in Li-MMT is higher in higher water concentration. It is also found that an adsorbed water molecule on Li-MMT causes the Li to protrude from the MMT surface, so it is expected that Li may be mobile on H2O/Li-MMT.

Dewi Kencana Wungu, Triati; Kemal Agusta, Mohammad; Gandaryus Saputro, Adhitya; Kresno Dipojono, Hermawan; Kasai, Hideaki

2012-11-01

127

Structural and thermodynamic properties of MgB 2 from first-principles calculations  

NASA Astrophysics Data System (ADS)

A first-principles plane wave method with the relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter (HGH) scheme in the frame of local density approximation is performed to calculate the lattice parameters and the equation of states (EOS) of superconducting MgB 2. Our calculations show that the ratio c/ a of about 1.134 is the most stable structure for MgB 2, as is consistent with experiment and other theoretical results. Also, the isothermal and isobaric properties are discussed from energy-volume curves using a quasi-harmonic Debey model.

Guo, Hua-Zhong; Chen, Xiang-Rong; Cai, Ling-Cang; Zhu, Jun; Gao, Jie

2005-06-01

128

X-ray magnetic circular dichroism in UGe2: first-principles calculations  

NASA Astrophysics Data System (ADS)

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of UGe2 at the U N4,5, N2,3 and Ge K and L2,3 edges are investigated theoretically from first principles, using the fully relativistic spin-polarized Dirac linear muffin-tin orbital (LMTO) band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as the LSDA+U method. The origin of the XMCD spectra in the compound is examined.

Antonov, V. N.; Harmon, B. N.; Yaresko, A. N.

2007-05-01

129

Spin Crossover in Ferropericlase From First-Principles Molecular Dynamics Simulations  

NASA Astrophysics Data System (ADS)

Ferropericlase is believed to be the second-most abundant mineral of the lower mantle of the Earth. It is experimentally known that with increasing pressure, the iron ions in the mineral begin to collapse from a high-spin to low-spin state. This spin crossover looks certain to have geophysical effects, and hence a good theoretical understanding of the phenomenon is necessary. Using first-principles molecular dynamics simulations in conjunction with thermodynamic integration, we construct a phase diagram of the spin crossover as a function of pressure and temperature. In addition, we predict that the mineral loses its electrically insulating character within the lower mantle.

Holmstrom, E.; Stixrude, L. P.

2013-12-01

130

First-principles simulation of molecular dissociation-recombination equilibrium.  

PubMed

For the first time, the equilibrium composition of chemical dissociation-recombination reaction is simulated from first-principles, only. Furthermore, beyond the conventional ab initio Born-Oppenheimer quantum chemistry the effects from the thermal and quantum equilibrium dynamics of nuclei are consistently included, as well as, the nonadiabatic coupling between the electrons and the nuclei. This has been accomplished by the path integral Monte Carlo simulations for full NVT quantum statistics of the H(3)(+) ion. The molecular total energy, partition function, free energy, entropy, and heat capacity are evaluated in a large temperature range: from below room temperature to temperatures relevant for planetary atmospheric physics. Temperature and density dependent reaction balance of the molecular ion and its fragments above 4000 K is presented, and also the density dependence of thermal ionization above 10,000 K is demonstrated. PMID:21932896

Kylänpää, Ilkka; Rantala, Tapio T

2011-09-14

131

First-principles simulations of exciton diffusion in organic semiconductors  

NASA Astrophysics Data System (ADS)

Exciton diffusion is crucial for the performance of organic semiconductors in photovoltaic and solid state lighting applications. We propose a first-principles approach that can predict exciton dynamics in organic semiconductors. The method is based on time-dependent density functional theory to describe the energy and many-body wave functions of excitons. Nonadiabatic ab initio molecular dynamics is used to calculate phonon-assisted transition rates between localized exciton states. Using Monte Carlo simulations, we determine the exciton diffusion length, lifetime, diffusivity, and harvesting efficiency in poly(3-hexylthiophene) polymers at different temperatures, which agree very well with the experiments. We find that exciton diffusion is primarily determined by the density of states of low-energy excitons. A widely speculated diffusion mechanism, namely an initial downhill migration followed by thermally activated migration, is confirmed and elucidated by the simulations. Some general guidelines for designing more efficient organic solar cells are obtained from the simulations.

Zhang, Xu; Li, Zi; Lu, Gang

2011-12-01

132

Ferroelectric transitions at ferroelectric domain walls found from first principles.  

PubMed

We present a first-principles study of model domain walls (DWs) in prototypic ferroelectric PbTiO_{3}. At high temperature the DW structure is somewhat trivial, with atoms occupying high-symmetry positions. However, upon cooling the DW undergoes a symmetry-breaking transition characterized by a giant dielectric anomaly and the onset of a large and switchable polarization. Our results thus corroborate previous arguments for the occurrence of ferroic orders at structural DWs, providing a detailed atomistic picture of a temperature-driven DW-confined transformation. Beyond its relevance to the field of ferroelectrics, our results highlight the interest of these DWs in the broader areas of low-dimensional physics and phase transitions in strongly fluctuating systems. PMID:24996110

Wojde?, Jacek C; Iñiguez, Jorge

2014-06-20

133

First-principles methodology for quantum transport in multiterminal junctions.  

PubMed

We present a generalized approach for computing electron conductance and I-V characteristics in multiterminal junctions from first-principles. Within the framework of Keldysh theory, electron transmission is evaluated employing an O(N) method for electronic-structure calculations. The nonequilibrium Green function for the nonequilibrium electron density of the multiterminal junction is computed self-consistently by solving Poisson equation after applying a realistic bias. We illustrate the suitability of the method on two examples of four-terminal systems, a radialene molecule connected to carbon chains and two crossed-carbon chains brought together closer and closer. We describe charge density, potential profile, and transmission of electrons between any two terminals. Finally, we discuss the applicability of this technique to study complex electronic devices. PMID:19894925

Saha, Kamal K; Lu, Wenchang; Bernholc, J; Meunier, Vincent

2009-10-28

134

Hydrogen storage in LiH: A first principle study  

NASA Astrophysics Data System (ADS)

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.

Banger, Suman; Nayak, Vikas; Verma, U. P.

2014-04-01

135

First-principles computational discovery of materials for hydrogen storage  

NASA Astrophysics Data System (ADS)

Hydrogen-fuelled vehicles require a cost-effective, lightweight material with precisely targeted thermodynamics and fast kinetics of hydrogen release. Since none of the conventional metal hydrides satisfy the multitude of requirements for a practical H2 storage system, recent research efforts have turned to advanced multicomponent systems based on complex hydrides. We show that first-principles density-functional theory (DFT) calculations have become a valuable tool for understanding and predicting novel hydrogen storage materials and understanding the atomic-scale kinetics of hydrogen release. Recent studies have used DFT calculations to (i) predict crystal structures of new solid-state hydrides, (ii) determine phase diagrams and thermodynamically favoured reaction pathways in multinary hydrides, and (iii) study microscopic kinetics of diffusion, phase transformations, and hydrogen release.

Ozolins, V.; Akbarzadeh, A. R.; Gunaydin, H.; Michel, K.; Wolverton, C.; Majzoub, E. H.

2009-07-01

136

NMR shifts for polycyclic aromatic hydrocarbons from first-principles  

SciTech Connect

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.

Thonhauser, Timo; Ceresoli, Davide; Marzari, Nicola N.

2009-09-03

137

Understanding the State of the Inner Core from First Principles  

NASA Astrophysics Data System (ADS)

Earth's inner core plays a vital role in the dynamics of our planet and is itself strongly exposed to dynamic processes as evidenced by a complex pattern of elastic structure. To gain deeper insight into the nature of these processes we rely on a characterization of the physical properties of the inner core that are governed by the material physics of its main constituent, iron. We will discuss the thermal state and elasticity of the inner core, based on predictions of material properties of iron under inner core conditions, obtained from first principles computational methods. We will show that aggregate seismic properties of the inner core, including its high Poissons ratio, can be explained by temperature and compression effects on the elasticity of pure iron alone. We use single crystal anisotropy of hcp iron to develop a speculative model of texture in the inner core that can explain major aspects of inner core anisotropy.

Steinle-Neumann, G.; Stixrude, L.; Cohen, R. E.

2002-05-01

138

First-principles simulation of molecular dissociation-recombination equilibrium  

SciTech Connect

For the first time, the equilibrium composition of chemical dissociation-recombination reaction is simulated from first-principles, only. Furthermore, beyond the conventional ab initio Born-Oppenheimer quantum chemistry the effects from the thermal and quantum equilibrium dynamics of nuclei are consistently included, as well as, the nonadiabatic coupling between the electrons and the nuclei. This has been accomplished by the path integral Monte Carlo simulations for full NVT quantum statistics of the H{sub 3}{sup +} ion. The molecular total energy, partition function, free energy, entropy, and heat capacity are evaluated in a large temperature range: from below room temperature to temperatures relevant for planetary atmospheric physics. Temperature and density dependent reaction balance of the molecular ion and its fragments above 4000 K is presented, and also the density dependence of thermal ionization above 10 000 K is demonstrated.

Kylaenpaeae, Ilkka; Rantala, Tapio T. [Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere (Finland)

2011-09-14

139

Hydration of alkali ions from first principles molecular dynamics revisited.  

PubMed

Structural and dynamical properties of the hydration of Li(+), Na(+), and K(+) in liquid water at ambient conditions were studied by first principles molecular dynamics. Our simulations successfully captured the different hydration behavior shown by the three alkali ions as observed in experiments. The present analyses of the dependence of the self-diffusion coefficient and rotational correlation time of water on the ion concentration suggest that Li(+) (K(+)) is certainly categorized as a structure maker (breaker), whereas Na(+) acts as a weak structure breaker. An analysis of the relevant electronic structures, based on maximally localized Wannier functions, revealed that the dipole moment of H(2)O molecules in the first solvation shell of Na(+) and K(+) decreases by about 0.1 D compared to that in the bulk, due to a contraction of the oxygen lone pair orbital pointing toward the metal ion. PMID:17249878

Ikeda, Takashi; Boero, Mauro; Terakura, Kiyoyuki

2007-01-21

140

First-principle Studies of armchair graphene nanoribbons  

NASA Astrophysics Data System (ADS)

In this article, by using the first principle calculations based on the density functional theory, we present a detailed investigation of the energy band and density of states of armchair graphene nanoribbons (AGNRs) with bare and H-terminated edges. Based on the structural optimization results, we compute the energy band and density of states of considered nanoribbons. The results show that there is a direct band gap for bare and H-terminated edges AGNRs, and indicate AGNRs have semiconductor properties for both cases our calculated. There are localized states turns up at -2.520eV for the case of bare edges, after modification of hydrogen atoms, the localized states disappeared, the band gap is widened form 0.535eV for the bare edges to 0.722eV for H-terminated edges, at the same time, and the energy band degeneracy appeared.

Wang, N. M.; Zhao, G. J.; Liang, X. X.; Song, T. L.

2014-03-01

141

Ferroelectric Transitions at Ferroelectric Domain Walls Found from First Principles  

NASA Astrophysics Data System (ADS)

We present a first-principles study of model domain walls (DWs) in prototypic ferroelectric PbTiO3. At high temperature the DW structure is somewhat trivial, with atoms occupying high-symmetry positions. However, upon cooling the DW undergoes a symmetry-breaking transition characterized by a giant dielectric anomaly and the onset of a large and switchable polarization. Our results thus corroborate previous arguments for the occurrence of ferroic orders at structural DWs, providing a detailed atomistic picture of a temperature-driven DW-confined transformation. Beyond its relevance to the field of ferroelectrics, our results highlight the interest of these DWs in the broader areas of low-dimensional physics and phase transitions in strongly fluctuating systems.

Wojde?, Jacek C.; Íñiguez, Jorge

2014-06-01

142

Vibration–rotation transition dipoles from first principles  

NASA Astrophysics Data System (ADS)

The use of ab initio methods to calculate line positions and associated transition intensities for the infrared spectrum of small molecules has recently become common. The first principles calculation of transition dipoles, upon which the intensity is based, relies on three distinct steps: the quantum chemical calculation of the dipole moment surface at a grid of geometries, the accurate representation of this surface using an appropriate functional form and the wave functions used to represent the initial and final states, which in turn depend on the accuracy of the potential energy surface used to generate them. Each of these stages is discussed with a view to obtaining the highest possible accuracy. The prospect of computed transition intensities displacing measured ones as the primary source of such information is considered.

Tennyson, Jonathan

2014-04-01

143

Terahertz dynamics of ferroelectric vortices from first principles  

NASA Astrophysics Data System (ADS)

A first-principles-based effective Hamiltonian is used to reveal dynamics of vortices in ferroelectrics. In addition to the "usual" dielectric modes that are generated by the fluctuation of the electrical polarization, additional toroidic modes, resulting from the electric toroidal moment fluctuations, are also found in the THz regime. Such latter modes can have their own dynamics, with a resonant frequency that softens via a square-root law when the temperature approaches the critical temperature at which electric vortices form. These dynamics are also found to be governed by the fluctuation of the self-organized azimuthal component of individual electric dipoles. Toroidic modes thus behave as pendulums, in contrast to springs that represent polarization dynamics.

Gui, Zhigang; Bellaiche, L.

2014-02-01

144

First-principles studies of native defects in olivine phosphates  

NASA Astrophysics Data System (ADS)

Olivine phosphates LiMPO4 (M=Mn, Fe, Co, Ni) are promising candidates for rechargeable Li-ion battery electrodes because of their energy storage capacity and electrochemical and thermal stability. It is known that native defects have strong effects on the performance of olivine phosphates. Yet, the formation and migration of these defects are not fully understood, and we expect that once such understanding has been established, one can envisage a solution for improving the materials' performance. In this talk, we present our first-principles density-functional theory studies of native point defects and defect complexes in LiMPO4, and discuss the implications of these defects on the performance of the materials. Our results also provide guidelines for obtaining different native defects in experiments.

Hoang, Khang; Johannes, Michelle

2011-03-01

145

First-principles study of III-V semiconductor nanowires  

NASA Astrophysics Data System (ADS)

Semiconductor nanowires play an important role in nanotechnology and present challenges in fundamental physics as well. Physical properties of semiconductor nanowires are expected to be essentially different from those of bulk crystals. They can be tuned by a wide range of parameters such as diameters, aspect ratios, orientations, and chemical compositions. Previously, we have studied single-cystal Si wires of different orientations using the density functional theory in the local density approximation (LDA). Due to the importance of III-V binary semicondutors, III-V semicondutor nanowires have legitimately attracted much interest recently. In this work, we studied from first-principles the III-V nanowires of GaAs and GaN, specially focusing on the size-dependent properties as a function of chemical species and compositions.

Zhao, Xinyuan; Chou, Mei-Yin

2004-03-01

146

First principles study of oxygen vacancy defects in tantalum pentoxide  

NASA Astrophysics Data System (ADS)

First principles total energy calculations were performed to characterize oxygen vacancy defects in tantalum pentoxide (Ta2O5). A simplified version of the crystalline orthorhombic phase of Ta2O5 was used in this study. Results indicate that O vacancies in Ta2O5 can be broadly classified based on their location in the lattice. One type of vacancy that occupies the ``in-plane'' sites displays deep or midgap occupied states and shallow unoccupied states, while a second type occupying ``cap'' sites results in shallow occupied states. For a wide range of Fermi levels or chemical potentials, the neutral and +2 charged states of the in-plane type vacancy and the +2 charge state of the cap type vacancy are found to be most stable.

Ramprasad, R.

2003-11-01

147

First principles study of oxygen vacancy migration in tantalum pentoxide  

NASA Astrophysics Data System (ADS)

First principles total energy calculations were performed in order to determine oxygen vacancy migration energies in Ta2O5. A simplified version of the crystalline orthorhombic phase of Ta2O5 was used in this study. O vacancies in the chosen model of Ta2O5 can be broadly classified into `cap' and `in-plane' sites based on their location in the lattice. The cap type of vacancies display the largest barriers both for migration to a neighboring cap site or to a neighboring in-plane site, thus behaving as oxygen vacancy `sinks'. A lowering of the barriers to migration is generally seen when the vacancies are doubly positively charged. All calculations were performed within the local density approximation of density functional theory, and the elastic band method was used in the estimation of migration barriers.

Ramprasad, R.

2004-02-01

148

First principle study of manganese doped cadmium sulphide sheet  

NASA Astrophysics Data System (ADS)

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 ?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.

Kumar, Sanjeev; Kumar, Ashok; Ahluwalia, P. K.

2014-04-01

149

Integration (Multi-Variable Included) From First Principles  

NSDL National Science Digital Library

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

Spunde, Walter

2011-09-14

150

First principles studies of silicon as a negative electrode material  

NASA Astrophysics Data System (ADS)

Batteries with higher volumetric and specific energy capacities are needed. Silicon is a promising candidate to replace graphite as the negative electrode material in Li-ion batteries. Silicon alloys with lithium, meaning its structure changes significantly during lithiation. Unlike other lithium alloys, lithiated silicon is amorphous when created electrochemically at room temperature. However, when lithiated at 415°C, crystalline Li-Si phases are experimentally found. This thesis focused on the study of the Li-Si crystalline phases and the lithiation of amorphous LixSi using first-principles calculations. A novel protocol to model the lithiation of amorphous silicon was developed, yielding results in good agreement with experiment. This represents the first time the lithiation of an amorphous alloy material has been modeled using first-principles calculations. Density functional theory calculations yielded formation energies for the crystalline and amorphous structures, from which potential-composition curves were calculated and compared to experiment. Good agreement with experiment was found, providing validation of the calculation methods and proposed protocol. Charge transfer studies and calculations of electronic densities of states for crystalline and amorphous structures were also completed. These confirmed the understanding of Li-Si structures as Zintl phases and quantified the charge transferred from Li to Si atoms. Phonon studies were completed for the crystalline Li-Si phases and helped explain their stability as a function of temperature. The phonon studies revealed that the Li15Si4 phase is unstable with respect to the other crystalline phases at elevated temperature, in agreement with experiment. Finally, experimental thermal studies of lithiated Si were used to obtain activation energies of the various crystallization events that occur when heating lithiated Si.

Chevrier, Vincent L.

151

Fundamental limits on transparency: first-principles calculations of absorption  

NASA Astrophysics Data System (ADS)

Transparent conducting oxides (TCOs) are a technologically important class of materials with applications ranging from solar cells, displays, smart windows, and touch screens to light-emitting diodes. TCOs combine high conductivity, provided by a high concentration of electrons in the conduction band, with transparency in the visible region of the spectrum. The requirement of transparency is usually tied to the band gap being sufficiently large to prevent absorption of visible photons. This is a necessary but not sufficient condition: indeed, the high concentration of free carriers can also lead to optical absorption by excitation of electrons to higher conduction-band states. A fundamental understanding of the factors that limit transparency in TCOs is essential for further progress in materials and applications. The Drude theory is widely used, but it is phenomenological in nature and tends to work poorly at shorter wavelengths, where band-structure effects are important. First-principles calculations have been performed, but were limited to direct transitions; as we show in the present work, indirect transitions assisted by phonons or defects actually dominate. Our calculations are the first to address indirect free-carrier absorption in a TCO completely from first principles. We present results for SnO2 [1], but the methodology is general and is also being applied to ZnO and In2O3. The calculations provide not just quantitative results but also deeper insights in the mechanisms that govern absorption processes in different wavelength regimes, which is essential for engineering improved materials to be used in more efficient devices. For SnO2, we find that absorption is modest in the visible, and much stronger in the ultraviolet and infrared. [4pt] [1] H. Peelaers, E. Kioupakis, and C.G. Van de Walle, Appl. Phys. Lett. 100, 011914 (2012).

Peelaers, Hartwin

2013-03-01

152

First-principles studies for understanding diverse high-Tc  

NASA Astrophysics Data System (ADS)

In this talk, I survey results and insights gained from first-principles calculations on materials that exhibit superconducting behavior at temperatures higher than those characteristic of conventional BCS superconductors. These range from highly correlated cuprate Mott insulators as represented by the bismuth-strontium-calcium-copper-oxides (BSCCOs) to border-line itinerant-Mott systems such as the recently discovered 1111 and 122 pnictides. ultimate goal of our studies is to correlate Tc with specific material composition using detailed first-principles calculations in conjunction with many-body physics techniques via the critical step of constructing real-materials model Hamiltonians. By manipulating impurity doping, which plays a crucial role in the phase diagrams of high Tc materials, we hope to find guidance for designing candidate systems with Tc higher than ones currently known. BSCCO material, density functional calculations using a good generalized-gradient approximation (GGA) yield structural information that is correlated to the experimentally observed (STM) super-modulation and impurity peak in the high energy regime (˜1 eV), even though the Kohn-Sham bands from such functionals fail to have a band gap. For FeAs-based high-Tc systems, DFT band-structure calculations provide a very good starting point for constructing model Hamiltonians for studies of spin fluctuation and electron pairing mechanisms. Fermi sheets that have been constructed using Wannier transformed Kohn-Sham states have provided critical information for understanding this family of superconducting materials. Analysis of the details of magnetic ordering, density of states, and 2D vs. 3D features in both the 1111 and 122 materials have been valuable in understanding sometimes perplexing experimental findings. Effects of Co impurities have been studied and fully analyzed as well., I will discuss persistent challenges related to calculations on the structure of the non-magnetic state Ba1Fe2As2 system. Both further examination of the underlying physics and development of new approximate functionals are needed.

Cheng, Hai-Ping

2011-03-01

153

First principles multiplet calculations of the calcium L?,? x-ray absorption spectra of CaO and CaF?.  

PubMed

First principles calculations are performed for the interpretation of the L?,? x-ray absorption spectrum of calcium oxide and calcium fluoride. The first principles calculations are based on configuration interaction (CI) calculations using fully relativistic molecular spinors. The first principles results are compared to experimental data and also to calculations based on a semi-empirical crystal field multiplet model and also on a multichannel multiple scattering method. We show that the CI calculations show good agreement with experiment, both for bulk and for surface experiments. The remaining differences with experiment and between the theoretical models are discussed in detail. PMID:21427477

Miedema, P S; Ikeno, H; de Groot, F M F

2011-04-13

154

First principles multiplet calculations of the calcium L2, 3 x-ray absorption spectra of CaO and CaF2  

NASA Astrophysics Data System (ADS)

First principles calculations are performed for the interpretation of the L2, 3 x-ray absorption spectrum of calcium oxide and calcium fluoride. The first principles calculations are based on configuration interaction (CI) calculations using fully relativistic molecular spinors. The first principles results are compared to experimental data and also to calculations based on a semi-empirical crystal field multiplet model and also on a multichannel multiple scattering method. We show that the CI calculations show good agreement with experiment, both for bulk and for surface experiments. The remaining differences with experiment and between the theoretical models are discussed in detail.

Miedema, P. S.; Ikeno, H.; de Groot, F. M. F.

2011-04-01

155

First principles pseudopotential calculation of electron energy loss near edge structures of lattice imperfections.  

PubMed

Theoretical calculations of electron energy loss near edge structures (ELNES) of lattice imperfections, particularly a Ni(111)/ZrO?(111) heterointerface and an Al?O? stacking fault on the {1100} plane, are performed using a first principles pseudopotential method. The present calculation can qualitatively reproduce spectral features as well as chemical shifts in experiment by employing a special pseudopotential designed for the excited atom with a core-hole. From the calculation, spectral changes observed in O-K ELNES from a Ni/ZrO? interface can be attributable to interfacial oxygen-Ni interactions. In the O-K ELNES of Al?O? stacking faults, theoretical calculation suggests that the spectral feature reflects coordination environment and chemical bonding. Powerful combinations of ELNES with a pseudopotential method used to investigate the atomic and electronic structures of lattice imperfections are demonstrated. PMID:21803589

Mizoguchi, Teruyasu; Matsunaga, Katsuyuki; Tochigi, Eita; Ikuhara, Yuichi

2012-01-01

156

First-principles calculation of vibrational Raman spectra of tetrahedral amorphous carbon  

NASA Astrophysics Data System (ADS)

The nonresonant vibrational Raman spectra of tetrahedral amorphous carbon are calculated from first principles. The structural model was generated using Car-Parinello molecular dynamics, the vibrational modes are determined using the linear response approach and Raman tensors are calculated using the finite electric field method. Our theoretical visible and reduced Raman spectra show an overall good agreement with experimental spectra, and better than previous calculated results. The analysis in terms of atomic vibrations shows that the Raman spectrum mainly comes from sp 2 contribution, G peak is due to the stretching vibration of any pair of sp 2 atoms and only a small sp 3 contribution can be noticed. The differences between peak intensities of reduced theoretical and experimental results mainly come from defects and the high sp 3 content in our simulated structure.

Niu, Li; Zhu, Jiaqi; Gao, Wei; Liu, Aiping; Han, Xiao; Du, Shanyi

2008-10-01

157

EDITORIAL: Challenges for first-principles based properties of defects in semiconductors and oxides Challenges for first-principles based properties of defects in semiconductors and oxides  

NASA Astrophysics Data System (ADS)

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

2009-12-01

158

First Principles Studies of ABO3 Perovskite Surfaces and Nanostructures  

NASA Astrophysics Data System (ADS)

Perovskite-type complex oxides, with general formula ABO 3, constitute one of the most prominent classes of metal oxides which finds key applications in diverse technological fields. In recent years, properties of perovskites at reduced dimensions have aroused considerable interest. However, a complete atomic-level understanding of various phenomena is yet to emerge. To fully exploit the materials opportunities provided by nano-structured perovskites, it is important to characterize and understand their bulk and near-surface electronic structure along with the electric, magnetic, elastic and chemical properties of these materials in the nano-regime, where surface and interface effects naturally play a dominant role. In this thesis, state-of-the-art first principles computations are employed to systematically study properties of one- and two-dimensional perovskite systems which are of direct technological significance. Specifically, our bifocal study targets (1) polarization behavior and dielectric response of ABO3 ferroelectric nanowires, and (2) oxygen chemistry relevant for catalytic properties of ABO3 surfaces. In the first strand, we identify presence of novel closure or vortex-like polarization domains in PbTIO3 and BaTiO3 ferroelectric nanowires and explore ways to control the polarization configurations by means of strain and surface chemistry in these prototypical model systems. The intrinsic tendency towards vortex polarization at reduced dimensions and the underlying driving forces are discussed and previously unknown strain induced phase transitions are identified. Furthermore, to compute the dielectric permittivity of nanostructures, a new multiscale model is developed and applied to the PbTiO3 nanowires with conventional and vortex-like polarization configurations. The second part of the work undertaken in this thesis is comprised of a number of ab initio surface studies, targeted to investigate the effects of surface terminations, prevailing chemical environment and processing conditions on the surface relaxations, local electronic structure and chemical reactivity. By combining our first principles computations with an in-house developed kMC simulation approach, we describe the thermodynamics, steady-state kinetics and the long-time and large-length scale behavior of the catalytically active (001) MnO2-terminated LaMnO3 surface in contact with an oxygen reservoir, as a function of temperature and partial pressure of oxygen. The results obtained are in excellent agreement with available experimental data in the literature.

Pilania, Ghanshyam

159

First-principles modelling of magnesium titanium hydrides  

NASA Astrophysics Data System (ADS)

Mixing Mg with Ti leads to a hydride MgxTi(1 - x)H2 with markedly improved (de)hydrogenation properties for x\\lesssim 0.8 , as compared to MgH2. Optically thin films of MgxTi(1 - x)H2 have a black appearance, which is remarkable for a hydride material. In this paper we study the structure and stability of MgxTi(1 - x)H2, x = 0-1 by first-principles calculations at the level of density functional theory. We give evidence for a fluorite to rutile phase transition at a critical composition xc = 0.8-0.9, which correlates with the experimentally observed sharp decrease in (de)hydrogenation rates at this composition. The densities of states of MgxTi(1 - x)H2 have a peak at the Fermi level, composed of Ti d states. Disorder in the positions of the Ti atoms easily destroys the metallic plasma, however, which suppresses the optical reflection. Interband transitions result in a featureless optical absorption over a large energy range, causing the black appearance of MgxTi(1 - x)H2.

Er, Süleyman; van Setten, Michiel J.; de Wijs, Gilles A.; Brocks, Geert

2010-02-01

160

First-principles simulations of exciton diffusion in organic semiconductors  

NASA Astrophysics Data System (ADS)

Exciton diffusion is of great importance to the performance of organic optoelectronic devices, including organic photovoltaics and solid-state lighting. The ability to control exciton diffusion in organic semiconductors is crucial to the design of efficient optoelectronic devices. However, such ability can only be achieved through a fundamental understanding of exciton diffusion mechanism. We have proposed a first-principles based frame work that can predict exciton dynamics in organic semiconductors.The framework is based on time-dependent density functional theory to provide the energy and many-body wave functions of excitons. Nonadiabatic ab initio molecular dynamics is used to calculate phonon-assisted transition rates between localized exciton states. Using Monte Carlo simulations, we determine exciton diffusion length, lifetime, diffusivity, and harvesting efficiency in poly(3-hexylthiophene) polymers at different temperatures, and the results agree very well with corresponding experimental values. We find that exciton diffusion is primarily determined by the density of states of low-energy excitons; a widely speculated diffusion mechanism has been confirmed and elucidatedby the simulations. Some general guidelines for designing more efficient organic solar cells can be gleaned from the simulation results

Zhang, Xu; Li, Zi; Lu, Gang

2013-03-01

161

Equilibrium Se isotope fractionation parameters: A first-principles study  

NASA Astrophysics Data System (ADS)

Several important equilibrium Se isotope fractionation parameters are investigated by first-principles calculations, involving dominant inorganic and organic Se-bearing species in gaseous, aqueous and condensed phases. Because anharmonic effects are found to be negligible for Se isotope fractionation calculation, the Bigeleisen-Mayer equation method is used without corrections beyond harmonic approximation. All calculations are made at B3LYP/6-311 + G(d,p) level, with a frequency scaling factor of 1.05. Solvation effects are carefully evaluated by the explicit solvent model (i.e. the "water droplet" method). A number of conformers are used for aqueous complexes in order to reduce the possible error coming from different configurations. Redox state is found to be an important factor controlling equilibrium Se isotope fractionations. Our results suggest a trend of heavy Se isotopes enrichment as SeO 42- > SeO 32- > HSeO 3- > SeO 2 > selenoamino acids > alkylselenides > Se(0) or H 2Se > HSe -. The Se(- II) species regardless of organic and inorganic forms can enrich extremely light Se isotopes comparing with other species. Equilibrium Se isotope fractionation factors provided in this study suggest Se isotopes can be used as a tracer of redox conditions and also useful to study Se cycling.

Li, Xuefang; Liu, Yun

2011-04-01

162

First-principles study of hydrogen storage materials  

NASA Astrophysics Data System (ADS)

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.

Ma, Zhu

163

First-principles calculations of uranium diffusion in uranium dioxide  

NASA Astrophysics Data System (ADS)

The present work reports first-principles DFT +U calculations of uranium self-diffusion in uranium dioxide (UO2), with a focus on comparing calculated activation energies to those determined from experiments. To calculate activation energies, we initially formulate a point defect model for UO2±x that is valid for small deviations from stoichiometry. We investigate five migration mechanisms and calculate the corresponding migration barriers using both the LDA +U and GGA +U approximations. These energy barriers are calculated using the occupation matrix control scheme that allows one to avoid the metastable states that exist in the DFT +U approximation. The lowest migration barrier is obtained for a vacancy mechanism along the <110> direction. This mechanism involves significant contribution from the oxygen sublattice, with several oxygen atoms being displaced from their original position. The <110> vacancy diffusion mechanism is predicted to have lower activation energy than any of the interstitial mechanisms and comparison to experimental data for stoichiometric UO2 also confirms this mechanism.

Dorado, Boris; Andersson, David A.; Stanek, Christopher R.; Bertolus, Marjorie; Uberuaga, Blas P.; Martin, Guillaume; Freyss, Michel; Garcia, Philippe

2012-07-01

164

Gypsum under pressure: A first-principles study  

NASA Astrophysics Data System (ADS)

We investigate by means of first-principles methods the structural response of gypsum (CaSO4?2H2O) to pressures within and above the stability range of gypsum-I (P?4GPa) . Structural and vibrational properties calculated for gypsum-I are in excellent agreement with experimental data. Compression within gypsum-I takes place predominantly through a reduction in the volume of the CaO8 polyhedra and through a distortion of the hydrogen bonds. The distance between CaSO4 layers becomes increasingly incompressible, indicating a mechanical limit to the packing of water molecules between the layers. We find that a structure with collapsed interlayer distances becomes more stable than gypsum-I above about 5 GPa. The collapse is concomitant with a rearrangement of the hydrogen-bond network of the water molecules. Comparison of the vibrational spectra calculated for this structure with experimental data taken above 5 GPa supports the validity of our model for the high-pressure phase of gypsum.

Giacomazzi, Luigi; Scandolo, Sandro

2010-02-01

165

First-principles pressure-temperature phase diagrams in metals  

SciTech Connect

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.

Moriarty, J.A.

1993-07-01

166

First-principles study of zinc oxide honeycomb structures  

NASA Astrophysics Data System (ADS)

We present a first-principles study of the atomic, electronic, and magnetic properties of two-dimensional (2D), single and bilayer ZnO in honeycomb structure and its armchair and zigzag nanoribbons. In order to reveal the dimensionality effects, our study includes also bulk ZnO in wurtzite, zincblende, and hexagonal structures. The stability of 2D ZnO, its nanoribbons and flakes are analyzed by phonon frequency, as well as by finite temperature ab initio molecular-dynamics calculations. 2D ZnO in honeycomb structure and its armchair nanoribbons are nonmagnetic semiconductors but acquire net magnetic moment upon the creation of zinc-vacancy defect. Zigzag ZnO nanoribbons are ferromagnetic metals with spins localized at the oxygen atoms at the edges and have high spin polarization at the Fermi level. However, they change to nonmagnetic metal upon termination of their edges with hydrogen atoms. From the phonon calculations, the fourth acoustical mode specified as twisting mode is also revealed for armchair nanoribbon. Under tensile stress the nanoribbons are deformed elastically maintaining honeycomblike structure but yield at high strains. Beyond yielding point honeycomblike structure undergo a structural change and deform plastically by forming large polygons. The variation in the electronic and magnetic properties of these nanoribbons have been examined under strain. It appears that plastically deformed nanoribbons may offer a new class of materials with diverse properties.

Topsakal, M.; Cahangirov, S.; Bekaroglu, E.; Ciraci, S.

2009-12-01

167

First-principles calculations of mobilities in novel MOSFETs  

NASA Astrophysics Data System (ADS)

Nanoscale metal-oxide-semiconductor field-effect transistors (MOSFETs) incorporating novel materials demonstrate unusual electron transport behavior. Straining the silicon lattice results in significant increases in electron and hole mobility. However, mobility calculations using standard approximations have difficulty explaining this increase. MOSFETs using novel gate dielectrics (e.g. hafnium oxide) have mobilities that are much lower than MOSFETs using silicon dioxide as the dielectric. ``Interface quality'' has been invoked as a likely cause of this difference, but few attempts have been made to tie the mobility decrease to scattering mechanisms associated with the novel dielectric structure. In this talk, we report results of mobility calculations in MOSFETs with a strained-Si channel and with alternate gate dielectrics. The calculations employed a recently developed first-principles method based on atomic-scale interface models.[1] Changes in the local environment of atomic-scale interface roughness defects are shown to potentially account for the increase in mobility under strain. Interstitial Hf defects near the silicon-oxide interface can act as traps and are shown to impact the mobility in MOSFETs with hafnium oxide gate dielectrics. [1] M. H. Evans, X.-G. Zhang, J. D. Joannopoulos, and S. T. Pantelides, Phys. Rev. Lett., v. 95, p. 106802 (2005).

Evans, Matthew; Pantelides, Sokrates

2006-03-01

168

First principles calculation of finite temperature magnetism in Ni  

NASA Astrophysics Data System (ADS)

We harnesses the computational power of massively parallel computers to calculate finite temperature magnetic properties by combining classical Monte-Carlo calculations with our first principles multiple scattering electronic structure code (LSMS) for constrained magnetic states. Our previous calculations of Fe and Fe3C [J. Appl. Phys. 109, 07E138 (2011)] only considered fluctuations in the local moment directions. Recent advances, both in the understanding of the Wang-Landau method used in our calculations [Phys. Rev. E 84, 065702(R) (2011)] and more powerful computing resources have enabled us to investigate Ni where the fluctuation in the magnitude of the local magnetic moments is of importance equal to their directional fluctuations. Here we will present our recent results for Ni that axpands our method to an even wider class of 3d element based ferromagnets. This research was sponsored by the Offices of Basic Energy Science (M.E. and D.M.N) and the Office of Advanced Computing Research (J.Y. and Y.W.L) of the US Department of Energy. This research used resources of the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory, which is supported by the Office of Science of the Department of Energy under contract DE-AC05-00OR22725.

Eisenbach, Markus; Yin, Junqi; Nicholson, Don M.; Li, Ying Wai

2013-03-01

169

Solubility of nonelectrolytes: a first-principles computational approach.  

PubMed

Using a combination of classical molecular dynamics and symmetry adapted intermolecular perturbation theory, we develop a high-accuracy computational method for examining the solubility energetics of nonelectrolytes. This approach is used to accurately compute the cohesive energy density and Hildebrand solubility parameters of 26 molecular liquids. The energy decomposition of symmetry adapted perturbation theory is then utilized to develop multicomponent Hansen-like solubility parameters. These parameters are shown to reproduce the solvent categorizations (nonpolar, polar aprotic, or polar protic) of all molecular liquids studied while lending quantitative rigor to these qualitative categorizations via the introduction of simple, easily computable parameters. Notably, we find that by monitoring the first-order exchange energy contribution to the total interaction energy, one can rigorously determine the hydrogen bonding character of a molecular liquid. Finally, this method is applied to compute explicitly the Flory interaction parameter and the free energy of mixing for two different small molecule mixtures, reproducing the known miscibilities. This methodology represents an important step toward the prediction of molecular solubility from first principles. PMID:24773531

Jackson, Nicholas E; Chen, Lin X; Ratner, Mark A

2014-05-15

170

First-Principles Investigation of Ag-Doped Gold Nanoclusters  

PubMed Central

Gold nanoclusters have the tunable optical absorption property, and are promising for cancer cell imaging, photothermal therapy and radiotherapy. First-principle is a very powerful tool for design of novel materials. In the present work, structural properties, band gap engineering and tunable optical properties of Ag-doped gold clusters have been calculated using density functional theory. The electronic structure of a stable Au20 cluster can be modulated by incorporating Ag, and the HOMO–LUMO gap of Au20?nAgn clusters is modulated due to the incorporation of Ag electronic states in the HOMO and LUMO. Furthermore, the results of the imaginary part of the dielectric function indicate that the optical transition of gold clusters is concentration-dependent and the optical transition between HOMO and LUMO shifts to the low energy range as the Ag atom increases. These calculated results are helpful for the design of gold cluster-based biomaterials, and will be of interest in the fields of radiation medicine, biophysics and nanoscience.

Zhang, Xiao-Dong; Guo, Mei-Li; Wu, Di; Liu, Pei-Xun; Sun, Yuan-Ming; Zhang, Liang-An; She, Yi; Liu, Qing-Fen; Fan, Fei-Yue

2011-01-01

171

First Principles Thermoelasticity of Tantalum at High Pressures  

SciTech Connect

The thermoelastic properties of bcc tantalum have been investigated over a broad range of temperatures (up to 12000 K) and pressures (up to 10 Mbar) using first-principles methods that account for cold, electron-thermal, and ion-thermal contributions. Specifically, we have combined ab initio all electron electronic-structure calculations for the cold and electron-thermal contributions to the elastic moduli with phonon contributions for the ion-thermal part calculated using model generalized pseudopotential theory (MGPT). For the latter, a summation of terms over the Brillouin zone is performed within the quasi-harmonic approximation, where each term is composed of a strain derivative of the phonon frequency at a particular k-point. At ambient pressure, the resulting temperature dependence of the elastic moduli is in excellent agreement with ultrasonic measurements. The experimentally observed anomalous behavior of C44 at low temperatures is shown to originate from the electron-thermal contribution. At higher temperatures, the dominant contribution to the temperature dependence of the elastic moduli comes from thermal expansion. Also, the pressure dependence of the moduli compares well with recent diamond and cell measurements up to 105 GPa. The calculated longitudinal and bulk sound velocities at higher pressure and temperature agree well with data obtained from shock experiments. Additionally, the temperature dependence of the Steinberg-Guinan model is examined for ambient pressure.

Orlikowski, D.A.; Soderlind, P.; Moriarty, J.

2002-06-21

172

Physical Properties of III-Antiminodes — a First Principles Study  

NASA Astrophysics Data System (ADS)

A comprehensive first principles study of III-Antimonide binary compounds is hardly found in literature. We report a broad study of structural and electronic properties of boron antimonide (BSb), aluminium antimonide (AlSb), gallium antimonide (GaSb) and indium antimonide (InSb) in zincblende phase based on density functional theory (DFT). Our calculations are based on Full-Potential Linearized Augmented Plane wave plus local orbitals (FP-L(APW+lo)) method. Different forms of exchange-correlation energy functional and corresponding potential are employed for structural and electronic properties. Our computed results for lattice parameters, bulk moduli, their pressure derivatives, and cohesive energy are consistent with the available experimental data. Boron antimonide is found to be the hardest compound of this group. For band structure calculations, in addition to LDA and GGA, we used GGA-EV, an approximation employed by Engel and Vosko. The band gap results with GGA-EV are of significant improvement over the earlier work.

Ahmed, Rashid; Fazal-E-Aleem; Javad Hashemifar, S.; Rashid, Haris; Akbarzadeh, H.

2009-09-01

173

First-principles thermodynamic modeling of lanthanum chromate perovskites  

NASA Astrophysics Data System (ADS)

Tendencies toward local atomic ordering in (A,A')(B,B')O3-? mixed composition perovskites are modeled to explore their influence on thermodynamic, transport, and electronic properties. In particular, dopants and defects within lanthanum chromate perovskites are studied under various simulated redox environments. (La1-x,Srx)(Cr1-y,Fey)O3-? (LSCF) and (La1-x,Srx)(Cr1-y,Ruy)O3-? (LSCR) are modeled using a cluster expansion statistical thermodynamics method built upon a density functional theory database of structural energies. The cluster expansions are utilized in lattice Monte Carlo simulations to compute the ordering of Sr and Fe(Ru) dopant and oxygen vacancies (Vac). Reduction processes are modeled via the introduction of oxygen vacancies, effectively forcing excess electronic charge onto remaining atoms. LSCR shows increasingly extended Ru-Vac associates and short-range Ru-Ru and Ru-Vac interactions upon reduction; LSCF shows long-range Fe-Fe and Fe-Vac interaction ordering, inhibiting mobility. First principles density functional calculations suggest that Ru-Vac associates significantly decrease the activation energy of Ru-Cr swaps in reduced LSCR. These results are discussed in view of experimentally observed extrusion of metallic Ru from LSCR nanoparticles under reducing conditions at elevated temperature.

Dalach, P.; Ellis, D. E.; van de Walle, A.

2012-01-01

174

Coarse graining approach to First principles modeling of structural materials  

SciTech Connect

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.

Odbadrakh, Khorgolkhuu [ORNL; Nicholson, Don M [ORNL; Rusanu, Aurelian [ORNL; Samolyuk, German D [ORNL; Wang, Yang [Pittsburgh Supercomputing Center; Stoller, Roger E [ORNL; Zhang, X.-G. [Oak Ridge National Laboratory (ORNL); Stocks, George Malcolm [ORNL

2013-01-01

175

Mechanical properties of graphyne monolayers: a first-principles study.  

PubMed

We investigated the mechanical properties of graphyne monolayers using first-principles calculations based on the Density Functional Theory. Graphyne has a relatively low in-plane Young's modulus (162 N m(-1)) and a large Poisson ratio (0.429) compared to graphene. It can sustain large nonlinear elastic deformations up to an ultimate strain of 0.2 followed by strain softening until failure. The single bond is more vulnerable to rupture than the triple bond and aromatic bond, although it has a shorter bond length (0.19 Å shorter) than the aromatic bond. A rigorous continuum description of the elastic response is formulated by expanding the elastic strain energy density in a Taylor series in strain truncated after the fifth-order term. We obtained a total of fourteen nonzero independent elastic constants which are components of tensors up to the tenth order. Pressure effects on the second-order elastic constants, in-plane Young's modulus, and Poisson ratio are predicted. This study implies that graphyne-based surface acoustic wave sensors and waveguides may be synthesized by introducing precisely controlled local strains on graphyne monolayers. PMID:22941420

Peng, Qing; Ji, Wei; De, Suvranu

2012-10-14

176

Properties of Multiferroic Bismuth Iron Oxide from First Principles  

NASA Astrophysics Data System (ADS)

In this dissertation, a first-principle-based approach is developed to study magnetoelectric effect in multiferoic materials. Such approach has a significant predictive power and might serve as a guide to new experimental works. As we will discuss in the course of this work, it also gives an important insight to the underlying physics behind the experimentally observed phenomena. We start by applying our method to investigate properties of a generic multiferroic material. We observe how magnetic susceptibility of such materials evolves with temperature and compare this evolution with the characteristic behavior of magnetic susceptibility for pure magnetic systems. Then we focus our attention to particular multiferroic - BiFeO3 - and reproduce its magnetic states with all of their essential features. Those magnetic states include (i) antiferromagnetic state, (ii) state with weak ferromagnetism resulting from canting of magnetic moments, and (iii) cycloidal magnetic structure. All of those magnetic states were also studied under external electric and magnetic fields. Under such electric fields magnetic order parameters of the systems undergo interesting transformations and sometimes take unexpected path. Finally, we study the material under strain and explore possibilities of favoring one magnetic state over another and even "creating" states that can be stable only under the strain.

Rahmedov, Dovran

177

First principles calculation of the activity of cytochrome P450  

NASA Astrophysics Data System (ADS)

The cytochrome P450 superfamily of enzymes is of enormous interest in the biological sciences due to the wide range of endogenous and xenobiotic compounds which it metabolises, including many drugs. We describe the use of first principles quantum mechanical modeling techniques, based on density functional theory, to determine the outcome of interactions between an enzyme and a number of compounds. Specifically, we calculate the spin state of an Fe3+ ion present in a haem moiety at the active site of these enzymes. The spin state of this ion indicates if the catalytic reaction will proceed. The computational results obtained compare favorably with experimental data. Only the principle components of the active site of the enzyme are included in the computational models, demonstrating that only a small fragment of the protein needs to be included in the models in order to accurately reproduce this aspect of the enzymes' function. These results open the way for further investigation of this superfamily of enzymes using the methods detailed in this paper.

Segall, M. D.; Payne, M. C.; Ellis, S. W.; Tucker, G. T.; Boyes, R. N.

1998-04-01

178

Kondo conductance in an atomic nanocontact from first principles  

NASA Astrophysics Data System (ADS)

The electrical conductance of atomic metal contacts represents a powerful tool for detecting nanomagnetism. Conductance reflects magnetism through anomalies at zero bias-generally with Fano line shapes-owing to the Kondo screening of the magnetic impurity bridging the contact. A full atomic-level understanding of this nutshell many-body system is of the greatest importance, especially in view of our increasing need to control nanocurrents by means of magnetism. Disappointingly, at present, zero-bias conductance anomalies are not calculable from atomistic scratch. Here, we demonstrate a working route connecting approximately but quantitatively density functional theory (DFT) and numerical renormalization group (NRG) approaches and leading to a first-principles conductance calculation for a nanocontact, exemplified by a Ni impurity in a Au nanowire. A Fano-like conductance line shape is obtained microscopically, and shown to be controlled by the impurity s-level position. We also find a relationship between conductance anomaly and geometry, and uncover the possibility of opposite antiferromagnetic and ferromagnetic Kondo screening-the latter exhibiting a totally different and unexplored zero-bias anomaly. The present matching method between DFT and NRG should permit the quantitative understanding and exploration of this larger variety of Kondo phenomena at more general magnetic nanocontacts.

Lucignano, Procolo; Mazzarello, Riccardo; Smogunov, Alexander; Fabrizio, Michele; Tosatti, Erio

2009-07-01

179

First-principles study of liquid and amorphous metals  

NASA Astrophysics Data System (ADS)

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.

Ganesh, Panchapakesan

180

Thin film bulk acoustic wave resonators tuning from first principles  

NASA Astrophysics Data System (ADS)

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.

Kvasov, Alexander; Tagantsev, Alexander K.

2013-05-01

181

First-principles Raman Spectra of Lead Titanate with Pressure  

NASA Astrophysics Data System (ADS)

PbTiO3 displays[1,2] a morphotropic phase boundary (MPB) under pressure at which electromechanical properties are maximal. Previously only complex solid-solutions were thought to exhibit such a boundary. To aid in the experimental study of the MPB region, we compute Raman scattering spectra of different phases of PbTiO3 with pressure using a DFT based first-principles approach and Density Functional Perturbation Theory (DFPT) [3]. The computed intensities and shifts with pressure agree very well with the experimental data measured on powder samples. Computations further allow comparison of Raman spectra and shifts in energetically competing phases raising the possibility of using calculations for experimental calibration of Raman spectra at any pressure. The results substantiate previous claims of a low-temperature monoclinic phase at the MPB at approximately 10 GPa in PbTiO3 as well as refute the possibility of an I4cm phase at higher pressures as suggested by other groups [4]. [1] Z. Wu and R. E. Cohen, Phys. Rev. Lett. 95, 037601 (2005), [2] M. Ahart et.al., Nature 451, 545 (2008), [3] P. Hermet et.al., J. Phys.:Condens. Matter 21, 215901 (2009) [4] P.E. Janolin et.al., Phys. Rev. Lett. 101, 237601 (2008).

Schad, A.; Ganesh, P.; Cohen, R. E.; Ahart, M.

2010-03-01

182

"Postural first" principle when balance is challenged in elderly people.  

PubMed

Human cognitive processing limits can lead to difficulties in performing two tasks simultaneously. This study aimed to evaluate the effect of cognitive load on both simple and complex postural tasks. Postural control was evaluated in 128 noninstitutionalized elderly people (mean age = 73.6 ± 5.6 years) using a force platform on a firm support in control condition (CC) and mental counting condition (MCC) with eyes open (EO) and eyes closed (EC). Then, the same tests were performed on a foam support. Sway path traveled and area covered by the center of foot pressure were recorded, low values indicating efficient balance. On firm support, sway path was higher in MCC than in CC both in EO and EC conditions (?p < 0.001). On foam support, sway path was higher in CC than in MCC in EC condition (?p < 0.001), area being higher in CC than in MCC both in EO (?p < 0.05) and EC (p < 0.001) conditions. The results indicate that cognitive load alters balance control in a simple postural task (i.e. on firm support), which is highlighted by an increase of energetic expenditure (i.e. increase of the sway path covered) to balance. Awareness may not be increased and the attentional demand may be shared between balance and mental task. Conversely, cognitive load does not perturb the realization of a new complex postural task. This result showed that postural control is prioritized ("postural first" principle) when seriously challenged. PMID:24205810

Lion, Alexis; Spada, Rosario S; Bosser, Gilles; Gauchard, Gérome C; Anello, Guido; Bosco, Paolo; Calabrese, Santa; Iero, Antonella; Stella, Giuseppe; Elia, Maurizio; Perrin, Philippe P

2014-08-01

183

First principles study of graphene nanoribbons and nanorectangles  

NASA Astrophysics Data System (ADS)

We have studied the finite size effect on the electronic structure of graphene nanoribbons (GNRs) using first principles density functional techniques. In particular, we have computed the energy gap dependence on the width and length for zero-dimensional nanorectangles for both the armchair and zigzag ribbons; and compared to the one-dimensional (infinite length) cases. One-dimensional armchair ribbons are expected to be metallic if the number of carbon atoms across the ribbon is N = 3M-1, and non-metallic N 3M-1, where M is an integer. In addition to quantum confinement along the width of the ribbon for metallic widths, an additional finite size effect emerges along the length of ribbons only for non-metallic armchair ribbons. The origin of additional quantum confinement in these structures is explained based on the energy states near the Fermi energy. The differences between zero- and one-dimensional electronic structure properties are considered with the addition of passivating groups and their effect on the electronic properties of graphenes and their impact on nanoelectronics devices are discussed.

Shemella, Philip; Chen, Li; Zhou, Yu; Zhang, Yiming; Subbulakshmi, Sreekala; Ajayan, Pulickel; Nayak, Saroj

2008-03-01

184

Selenium adsorption on Mo(110): A first-principles investigation  

NASA Astrophysics Data System (ADS)

Selenium adsorption on molybdenum surfaces is a relevant process in the production of thin-film solar cells, in particular as far as the formation of the layered compound MoSe2 is concerned. In this paper we investigate the energetics of Se adsorption on the (110) surface of molybdenum using first-principles calculations in the two limiting cases of low and high coverage, and we establish a comparison with the more extensively investigated case of sulfur adsorption at submonolayer coverage. The studied system provides the opportunity for testing the most crucial approximations, namely, the choice of the exchange-correlation functional and the pseudopotential generation. We find that semicore states of molybdenum have an influence on calculated surface energies and, to a lesser extent, on adsorption energies. We compare some more or less popular semilocal exchange-correlation functionals, including one recently proposed as an improvement for quasi-two-dimensional systems. The results show that the preferred adsorption site changes with coverage and suggest a strong variation of the adsorption energy with coverage.

Roma, Guido; Chiodo, Letizia

2013-06-01

185

First-principles theory of capacitive and electrochemical energy storage  

NASA Astrophysics Data System (ADS)

Recently there has been much interest in development of new electrochemical capacitors to meet high-power and high-energy applications. Pseudo-capacitors using fast surface redox reactions can store electrical energy of 10 to 100 times larger than supercapacitors and still exhibit fast and reversible charge-discharge responses in contrast to batteries. Yet, energy storage mechanisms in super- and pseudo-capacitors have not been fully understood at the level of electrons. Here we have performed first-principles calculations for electrical double layers of a TiO2 (101) electrode and solvated lithium ions on the surface, with the ethylene carbonates (EC) as solvent molecules. As Li ions are desolvated from Li-EC4 to Li-EC3 and bare Li ions, the capacitance gets larger due to the reduced distance between the Li ions and the electrode. When Li ions are intercalated into the subsurface of the TiO2 electrode as supposed in pseudocapacitors, the electrostatic energy due to charge separation is reduced for a given stored charge, but the electrochemical reaction starts to occur causing a large increase in the capacitance.

Kang, Joongoo; Kim, Yong-Hyun

2009-03-01

186

First principles study of lithium insertion in bulk silicon  

NASA Astrophysics Data System (ADS)

Si is an important anode material for the next generation of Li ion batteries. Here the energetics and dynamics of Li atoms in bulk Si have been studied at different Li concentrations on the basis of first principles calculations. It is found that Li prefers to occupy an interstitial site as a shallow donor rather than a substitutional site. The most stable position is the tetrahedral (Td) site. The diffusion of a Li atom in the Si lattice is through a Td-Hex-Td trajectory, where the Hex site is the hexagonal transition site with an energy barrier of 0.58 eV. We have also systematically studied the local structural transition of a LixSi alloy with x varying from 0 to 0.25. At low doping concentration (x = 0-0.125), Li atoms prefer to be separated from each other, resulting in a homogeneous doping distribution. Starting from x = 0.125, Li atoms tend to form clusters induced by a lattice distortion with frequent breaking and reforming of Si-Si bonds. When x >= 0.1875, Li atoms will break some Si-Si bonds permanently, which results in dangling bonds. These dangling bonds create negatively charged zones, which is the main driving force for Li atom clustering at high doping concentration.

Wan, Wenhui; Zhang, Qianfan; Cui, Yi; Wang, Enge

2010-10-01

187

First principles modeling of donor materials for organic solar cells: where theory complements experiment  

NASA Astrophysics Data System (ADS)

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).

Zhugayevych, Andriy; Tretiak, Sergei; Bazan, Guillermo

2013-03-01

188

The nonlinear optical susceptibility and electro-optic tensor of ferroelectrics: first-principle study  

NASA Astrophysics Data System (ADS)

The nonlinear optical properties of some ABO3 materials (BaTiO3, KNbO3, LiTaO3 and LiNbO3) are studied by density functional theory (DFT) in the local density approximation (LDA) expressions based on first-principle calculations. Our goals are to give the details of the calculations for linear and nonlinear optical properties, including the linear electro-optic (EO) tensor for some ABO3 structures with oxygen octahedral structures using first-principles methods. These results can then be used in the study of the physics of ferroelectrics, specifically, we present calculations of the second harmonic generation response coefficient X {/ijk (2)} (-2 ?, ?, ?) over a large frequency range for ABO3 crystals. The electronic linear EO susceptibility X {/ijk (2)} ( -?, ?,0) is also evaluated below the band gap. These results are based on a series of the LDA calculations using DFT. Results for X {/ijk (2)} ( -?, ?,0) are in agreement with experiments below the band gap. The results are compared with the theoretical calculations and the available experimental data.

Cabuk, Suleyman

2012-02-01

189

Waltzing of a helium pair in tungsten: Migration barrier and trajectory revealed from first-principles  

NASA Astrophysics Data System (ADS)

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.

Niu, J. G.; Zhan, Q.; Geng, W. T.

2014-06-01

190

First-principles investigations of homogeneous lattice-distortive strain and shuffles in Ni2MnGa  

Microsoft Academic Search

A series of first-principles calculations were performed for ferromagnetic Ni2MnGa using density functional theory and PAW potentials. Theoretically, a tetragonal crystal structure homogeneous lattice-distortive strain is stabilized around c\\/a = 0.94 with respect to the L21 structure when, in addition, modulation shuffles with a period of five atomic planes are taken into account. This is in agreement with the observed

A. T. Zayak; P. Entel; J. Enkovaara; A. Ayuela; R. M. Nieminen

2003-01-01

191

First-principles study of the structural, elastic, and electronic properties of the cubic perovskite BaHfO$_3$  

Microsoft Academic Search

First principles study of structural, elastic, and electronic properties of the cubic perovskitetype BaHfO$_3$ has been performed using the plane wave ultrasoft pseudo-potential method based on density functional theory with revised Perdew-Burke-Ernzerhof exchange-correlation functional of the generalized gradient approximation (GGA-RPBE). The calculated equilibrium lattice constant of this compound is in good agreement with the available experimental and theoretical data reported

Hongsheng Zhao; Aimin Chang; Yunlan Wang

2008-01-01

192

Electronic structure of the thermoelectric materials Bi2Te3 and Sb2Te3 from first-principles calculations  

Microsoft Academic Search

The electronic structures of Bi2Te3 and Sb2Te3 crystals were calculated using the first-principles full-potential linearized augmented plane-wave method. We studied not only the unrelaxed crystals, which have the experimental lattice parameters and scaled atom coordinates, but also the relaxed crystals, which have the lattice parameters and scaled atom coordinates determined from theoretical structure optimizations. We found that Bi2Te3 has six

Guofeng Wang; Tahir Cagin

2007-01-01

193

First-principles thermodynamic framework for the evaluation of thermochemical H2O - or CO2 -splitting materials  

Microsoft Academic Search

We present an analysis of the equilibrium thermodynamics of two-step metal oxide-based water and carbon dioxide-splitting cycles. Within this theoretical framework, we propose a first-principles computational approach based on density-functional theory (DFT) for evaluating new materials for these cycles. Our treatment of redox-based gas-splitting chemistry is completely general so that the thermodynamic conclusions herein hold for all materials used for

B. Meredig; C. Wolverton

2009-01-01

194

Cobalt (hydro)oxide electrodes under electrochemical conditions: a first principle study  

NASA Astrophysics Data System (ADS)

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.

Chen, Jia; Selloni, Annabella

2013-03-01

195

First-principles study of III-V electrode interfaces for photoelectrochemical hydrogen production  

NASA Astrophysics Data System (ADS)

Photoelectrochemical (PEC) cells promise clean, sustainable production of hydrogen fuel using water and sunlight. However, combining solar conversion efficiency with durability in electrolyte solution has proven difficult, in part because the complex chemistry active at the electrode-electrolyte interface remains poorly understood. We use first-principles molecular dynamics simulations and model density-functional calculations to study the structure, stability, and chemical activity of GaP/InP semiconductor electrodes in contact with water. We find that a local bond-topological model is able to capture much of the basic surface chemistry. Interpretation of our results points to the particular importance of surface-adsorbed oxygen in determining the available reaction pathways for photocorrosion and water dissociation. Electronic signatures of the local bond topologies are compared to data from X-ray absorption and emission spectroscopy for insight into actual electrode structure.

Wood, Brandon; Ogitsu, Tadashi; Choi, Wooni; Schwegler, Eric

2012-02-01

196

Experimental and first-principles study of ferromagnetism in Mn-doped zinc stannate nanowires  

SciTech Connect

Room temperature ferromagnetism was observed in Mn-doped zinc stannate (ZTO:Mn) nanowires, which were prepared by chemical vapor transport. Structural and magnetic properties and Mn chemical states of ZTO:Mn nanowires were investigated by X-ray diffraction, superconducting quantum interference device (SQUID) magnetometry and X-ray photoelectron spectroscopy. Manganese predominantly existed as Mn{sup 2+} and substituted for Zn (Mn{sub Zn}) in ZTO:Mn. This conclusion was supported by first-principles calculations. Mn{sub Zn} in ZTO:Mn had a lower formation energy than that of Mn substituted for Sn (Mn{sub Sn}). The nearest neighbor Mn{sub Zn} in ZTO stabilized ferromagnetic coupling. This observation supported the experimental results.

Deng Rui; Zhou Hang; Qin Jieming; Wan Yuchun; Jiang Dayong; Liang Qingcheng [School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022 (China); Li Yongfeng [Key Laboratory of Physics and Technology for Advanced Batteries, College of Physics, Jilin University, Changchun 130012 (China); Wu, Tom [Physical Sciences and Engineering Division, Solar and Photovoltaics Engineering Research Center, King Abdullah University of Science and Technology, Thuwal 23955-6900 (Saudi Arabia); Yao Bin [State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012 (China); Liu Lei [State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, No.3888 Dongnanhu Road, Changchun 130033 (China)

2013-07-21

197

Electron field emission in nanostructures: A first-principles study  

NASA Astrophysics Data System (ADS)

The objective of this work was to study electron field emission from several nanostructures using a first-principles framework. The systems studied were carbon nanowires, graphene nanoribbons, and nanotubes of varying composition. These particular structures were chosen because they have recently been identified as showing novel physical phenomena, as well as having tremendous industrial applications. We examined the field emission under a variety of conditions, including laser illumination and the presence of adsorbates. The goal was to explore how these conditions affect the field emission performance. In addition to the calculations, this dissertation has presented computational developments by the author that allowed these demanding calculations to be performed. There are many possible choices for basis when performing an electronic structure calculation. Examples are plane waves, atomic orbitals, and real-space grids. The best choice of basis depends on the structure of the system being analyzed and the physical processes involved (e.g., laser illumination). For this reason, it was important to conduct rigorous tests of basis set performance, in terms of accuracy and computational efficiency. There are no existing benchmark calculations for field emission, but transport calculations for nanostructures are similar, and so provide a useful reference for evaluating the performance of various basis sets. Based on the results, for the purposes of studying a non-periodic nanostructure under field emission conditions, we decided to use a real-space grid basis which incorporates the Lagrange function approach. Once a basis was chosen, in this case a real-space grid, the issue of boundary conditions arose. The problem is that with a non-periodic system, field emitted electron density can experience non-physical reflections from the boundaries of the calculation volume, leading to inaccuracies. To prevent this issue, we used complex absorbing potentials (CAPs) to absorb electron density before it could reach the boundaries and reflect. The CAPs were zero in the region of the emitting structure and in regions were measurements were made. Still, we wanted to make sure that complex potentials were an accurate and efficient solution to the boundary problem. To evaluate this, we once again turned to benchmarks from transport calculations. The results showed CAPs to be an extremely accurate and efficient computational tool, and were incorporated into all of this dissertation's calculations. Our calculations show that adsorbate atoms significantly increase the field emission current of carbon nanotubes. Adsorbate atoms also cause strong differences and nonlinearity in the Fowler-Nordheim plot. The source of the enhanced current is electronic states introduced by the adsorbate atom. We have investigated the emission of electrons from nanostructures induced by short intense laser pulses in the presence of a weaker uniform static field. Based on the results of our simulations, two important qualitative features of this process have been determined: (1) a significant enhancement of the emission when a laser pulse is applied, and (2) the field emission current has a peak of some duration and the position of this peak correlates with the time of the pulse arrival. These two features suggest the possibility of using short laser pulses for making few-electron emitters of nanoscale size [212]. Such emitters could have many desirable properties, especially very high spatial and time resolutions. The field emission from nanotubes of various composition has been studied. The calculations predict that the GaN, SiC, and Si nanotubes are particularly good field emitters. The highest-current nanotube, Si, is predicted to produce a current an order of magnitude higher than BN or C nanotubes. The calculations predict that carbon nanotubes with various adsorbates can be used as spin-polarized current sources. These are the first first-principles calculations to show spin-polarized field emission for carbon nanotubes with iron adsorbates. Also, there i

Driscoll, Joseph Andrew

198

First principles explanation of the positive seebeck coefficient of lithium.  

PubMed

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

Xu, Bin; Verstraete, Matthieu J

2014-05-16

199

A digitally reconstructed radiograph algorithm calculated from first principles  

SciTech Connect

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.

Staub, David; Murphy, Martin J. [Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia 23298 (United States)

2013-01-15

200

Risk reduction and the privatization option: First principles  

SciTech Connect

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.

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

201

First-Principles Investigation of C60-Pd Interface  

NASA Astrophysics Data System (ADS)

Conductivity and hybridization of C60-Pd nano-system have been investigated using density functional calculations. From analysis of geometry, energetics and electronic structures, the interaction of C60 mono-layer and Pd clusters gives rise to electronic charge transfer at the interface and facilitates the dissociation and uptake of hydrogen, which lead to hydrogen storage. The first-principles studies are carried out by self-consistent plane-wave method. The interaction between ions and electrons is described by projector-augmented wave (PAW) approach. In our calculations, the C60 monolayer is doped by the Pdn atoms on h-BN with n = 1-4 and 15, but it also forms a metal-C60 nano-array with the Pd clusters. Charge transfer occurs at the interface, from the Pd atoms towards the C60 monolayer. This electronic property strongly depends on the nature and number of metal atoms. A large amount of charge transfer between the Pd atoms and the C60 monolayer indicates a strong interaction under the ionic effect, in contrast with the interaction of the C60 monolayer and a metallic surface. The h-BN surface merely gains 0.1 electrons via C60, proving that h-BN is an insulating material. We also find that Pd is a good catalyst for dissociation and storage of hydrogen on the C60 molecules. Hydrogen is sufficiently dissociated in the presence of the Pd atoms/clusters, which assists in bonding of the individual H atoms to C60. Dehydrogenation of C60Hx is also discussed in energetics.

Li, Lan; Cheng, Hai-Ping

2008-03-01

202

Oxidation of Al doped Au clusters: a first principles study.  

PubMed

Using first principles method we report the oxidation of Al doped Au clusters. This work is divided into two parts: (i) the equilibrium structures and stability of Al doped Au(n-1) clusters (n=2-7,21) and (ii) the interaction of O(2) with stable clusters. The calculations are performed using the plane wave pseudopotential approach under the density functional theory and generalized gradient approximation for the exchange and correlation functional. The optimized geometries of Au(n-1)Al clusters indicate that the substitution of Au by Al results an early onset of three-dimensional structures from tetramer onwards. This is different from the results of transition metal doped Au clusters, where the planar conformation of Au clusters retains up to heptamer. The stability of Au(n-1)Al clusters has been analyzed based on the binding energy, second difference in energy, and the energy gaps between the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. Based on the energetics, the Au(3)Al and Au(5)Al clusters are found to have extraordinary stability. The oxidation mechanism of Al doped Au clusters have been studied by the interaction of O(2) with Al, Au, AuAl, Au(3)Al, and Au(20)Al clusters. It is found that the oxidation of Au(n-1)Al clusters undergoes via dissociative mechanism, albeit significant charge transfer from Al to Au. Moreover, the O(2) molecule prefers to attach at the Al site rather than at the Au site. PMID:19548729

Rajesh, Chinagandham; Majumder, Chiranjib

2009-06-21

203

Evolutionary approach for determining first-principles model Hamiltonians  

NASA Astrophysics Data System (ADS)

The ability to perform accurate solid-state calculations based completely on first principles (for relatively small unit cells) has made it possible to develop model Hamiltonians that can be rapidly ``searched'' for optimal target properties---i.e., true materials-by-design. Recent applications include ferroelectric properties and band-gap engineering. The most difficult step in ``training'' such model Hamiltonians is making choices for the number and types of parameters in the model that insure the predictive power of the model. Based on an evolutionary approach, we have developed an algorithm for selecting the types and number of terms in a Cluster Expansion model for a binary alloy. This approach removes much of the tedium of constructing the model and robustly finds the best possible set of parameters. The approach is general and can be applied to a wide variety of other models as well. I illustrate the success of the new approach first on systems where the best parameter set is known analytically, and second, as applied to several recent ``real-world'' examples, including (1) the role of long-period-superlattices in the Cu-Pd system, (2) predicting configuration-dependent bulk-moduli in transition- metal carbides and nitrides, (3) predicting optimal superlattice stacking/orientations to engineer desired band-gaps in MgO-ZnO wide gap alloys. Gus L. W. Hart, V. Blum, M. J. Walorski, and A. Zunger, Nature Materials 4 391 (2005) V. Blum, Gus L. W. Hart, M. J. Walorski, and A. Zunger, Phys. Rev. B 72, 165111 (2005).

Hart, Gus L. W.

2006-03-01

204

A digitally reconstructed radiograph algorithm calculated from first principles  

PubMed Central

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.

Staub, David; Murphy, Martin J.

2013-01-01

205

First-principles modeling of titanate/ruthenate superlattices  

NASA Astrophysics Data System (ADS)

The possibility to create highly confined two-dimensional electron gases (2DEG) at oxide interfaces has generated much excitement during the last few years. The most widely studied system is the 2DEG formed at the LaO/TiO2 polar interface between LaAlO3 and SrTiO3, where the polar catastrophe at the interface has been invoked as the driving force. More recently, partial or complete delta doping of the Sr or Ti cations at a single layer of a SrTiO3 matrix has also been used to generate 2DEG. Following this recipe, we report first principles characterization of the structural and electronic properties of (SrTiO3)5/(SrRuO3)1 superlattices, where all the Ti of a given layer have been replaced by Ru. We show that the system exhibits a spin-polarized two-dimensional electron gas extremely confined to the 4d orbitals of Ru in the SrRuO3 layer, a fact that is independent of the level of correlation included in the simulations. For hybrid functionals or LDA+U, every interface in the superlattice behaves as minority-spin half-metal ferromagnet, with a magnetic moment of ? = 2.0 ?B/SrRuO3 unit. The shape of the electronic density of states, half metallicity and magnetism are explained in terms of a simplified tight-binding model, considering only the t2g orbitals plus (i) the bi-dimensionality of the system, and (ii) strong electron correlations. Possible applications are discussed, from their eventual role in thermoelectric applications to the possible tuning of ferromagnetic properties of the 2DEG with the polarization of the dielectric. Work done in collaboration with P. Garc'ia, M. Verissimo-Alves, D. I. Bilc, and Ph. Ghosez.

Junquera, Javier

2013-03-01

206

Oxidation of Al doped Au clusters: A first principles study  

SciTech Connect

Using first principles method we report the oxidation of Al doped Au clusters. This work is divided into two parts: (i) the equilibrium structures and stability of Al doped Au{sub n-1} clusters (n=2-7,21) and (ii) the interaction of O{sub 2} with stable clusters. The calculations are performed using the plane wave pseudopotential approach under the density functional theory and generalized gradient approximation for the exchange and correlation functional. The optimized geometries of Au{sub n-1}Al clusters indicate that the substitution of Au by Al results an early onset of three-dimensional structures from tetramer onwards. This is different from the results of transition metal doped Au clusters, where the planar conformation of Au clusters retains up to heptamer. The stability of Au{sub n-1}Al clusters has been analyzed based on the binding energy, second difference in energy, and the energy gaps between the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. Based on the energetics, the Au{sub 3}Al and Au{sub 5}Al clusters are found to have extraordinary stability. The oxidation mechanism of Al doped Au clusters have been studied by the interaction of O{sub 2} with Al, Au, AuAl, Au{sub 3}Al, and Au{sub 20}Al clusters. It is found that the oxidation of Au{sub n-1}Al clusters undergoes via dissociative mechanism, albeit significant charge transfer from Al to Au. Moreover, the O{sub 2} molecule prefers to attach at the Al site rather than at the Au site.

Rajesh, Chinagandham [RMC, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085 (India); Majumder, Chiranjib [Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085 (India)

2009-06-21

207

First principles study of III-V diluted magnetic semiconductors  

NASA Astrophysics Data System (ADS)

As a good candidate for spintronic applications, diluted magnetic semiconductors (DMS) have been widely investigated in the past several years. First principles calculations are used to study the magnetic exchange interactions and Curie temperature (Tc) in III-V based diluted magnetic semiconductors and delta-doped layers. The local spin density approximation (LSDA) is combined with a linear-response technique to map the magnetic energy onto a Heisenberg hamiltonian, but no significant further approximations are made. The effect of disorder inherent in the DMS system on the ferromagnetism in these materials is studied. Large dispersion in the pairwise exchange interactions is shown. The dispersion strongly reduces Curie temperature of these materials. Clustering due to the annealing process further decreases Tc. With all the factors taken into account, Tc is reasonably predicted by the local spin-density approximation in manganese(Mn) doped gallium arsenide (GaAs) ((GaMn)As). Based on the result of (GaMn)As, two kinds of ternary alloy systems, manganese and chromium (Cr) co-doped GaAs ((Ga,Mn,Cr)As) and Mn and phosphorus (P) co-doped GaAs ((GaMn)(AsP)) are predicted to have higher Tc by double-exchange and Ruderman Kittel-Kasuya-Yosida (RKKY) models. LSDA calculations are used to test the prediction. The results show that neither co-doping Mn with Cr, nor alloying As with P improves Tc. Another approach to increase Tc is to develop delta-doped (GaMn)As, which is confirmed by LSDA calculations. By studying the exchange interactions of the high Mn concentration systems, this high Tc case is explained by considering the ferromagnetic (FM) and anti-ferromagnetic (AFM) contribution of bands with different symmetries and the crystallographic dependence of exchange interactions.

Xu, Jialei

208

First Principles Explanation of the Positive Seebeck Coefficient of Lithium  

NASA Astrophysics Data System (ADS)

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.

Xu, Bin; Verstraete, Matthieu J.

2014-05-01

209

Structural, electronic, and dynamical properties of Pca21-TiO2 by first principles  

NASA Astrophysics Data System (ADS)

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.

Abbasnejad, M.; Mohammadizadeh, M. R.; Maezono, R.

2012-03-01

210

Electronic structure, structural stability, mechanical and superconducting properties of group VB nitrides: A first principles study  

NASA Astrophysics Data System (ADS)

The electronic, structural, mechanical and superconducting properties of group VB mononitrides are investigated by means of first principles calculation based on density functional theory with generalized gradient approximation. The calculated ground state properties are in good agreement with previous experimental and theoretical results. Among the three crystallographic structures that have been investigated, the hexagonal WC phase is found to more stable than the cubic ones. Under high pressure, a series of structural phase transition from WC ? NaCl ? CsCl phase is also predicted in VN, NbN and TaN. The calculated elastic constants indicate that all the three nitrides are mechanically stable at ambient pressure. The estimated Zener ratio and linear compressibility coefficients Kc/Ka reveals that these materials exhibit elastic anisotropy. The estimated superconducting transition temperature (Tc) values as a function of pressure for VN, NbN and TaN are 35.5, 37.5 and 30.5 K respectively.

Asvini Meenaatci, A. T.; Rajeswarapalanichamy, R.; Iyakutti, K.

2013-05-01

211

First-Principle Investigation of Electric Structures and Thermodynamic Properties for Zirconium Nitride Under High Pressure  

NASA Astrophysics Data System (ADS)

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.

Tang, Cui-Ming; Chen, Xiao-Xu; Wang, Jun; Hu, Yan-Fei; Wang, Hong-Yan

2013-06-01

212

Stability of graphitic-like zinc oxide layers under carriers doping: a first-principles study  

NASA Astrophysics Data System (ADS)

Although theoretical works have demonstrated that (0001) polar films of wurtzite (WZ) ZnO automatically transform into graphitic-like (GP) structures, the experimental realization of GP ZnO is limited to a thickness of several atomic layers. Here, using first-principles calculations, we demonstrated that the stability of GP ZnO is closely related to the concentration of near-free carriers. Our results show that the doped carriers, originating from the rich oxygen vacancies, can effectively screen the polar field, and stabilize the WZ structure. Thus, in order to obtain GP ZnO layers with much thicker films, it is necessary to reduce the near-free carrier concentration.

Kan, Erjun; Deng, Kaiming; Wu, Fang

2013-11-01

213

Spin crossover in ferropericlase from first-principles molecular dynamics simulations  

NASA Astrophysics Data System (ADS)

Ferropericlase is the second-most abundant mineral of the lower mantle of the Earth. It is experimentally known that with increasing pressure, the iron ions in the mineral begin to collapse from a high-spin to low-spin state. This spin crossover alters various properties of the material, and hence a good theoretical understanding of the phenomenon is necessary. Using first-principles molecular dynamics simulations in conjunction with thermodynamic integration, we construct a phase diagram of the spin crossover as a function of pressure and temperature. In addition, we present the thermal equation of state of the mineral up to 140 GPa and 4000 K, and predict that the electrical conductivity of ferropericlase reaches semi-metallic values within the lower mantle.

Holmstrom, Eero; Stixrude, Lars

2014-05-01

214

Experimental and first principle studies on electronic structure of BaTiO3  

NASA Astrophysics Data System (ADS)

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.

Sagdeo, Archna; Ghosh, Haranath; Chakrabarti, Aparna; Kamal, C.; Ganguli, Tapas; Phase, D. M.; Deb, S. K.

2014-04-01

215

Determination of ground-state and low-energy structures of perovskite superlattices from first principles  

NASA Astrophysics Data System (ADS)

In the development of first-principles high-throughput searches for materials with desirable functional properties, there is a clear need for an efficient method to determine the ground-state and low-energy alternative structures of superlattices. A method based on a simple strategy—to generate starting structures based on low-energy structures of the constituent compounds, which are then optimized via structural relaxation calculations—is proposed. This "stacking method" is demonstrated on the 2:2 PbTiO3/SrTiO3 superlattice, which has been the subject of recent experimental and theoretical interest. Considerations relevant to wider use of the method are discussed.

Zhou, Yuanjun; Rabe, Karin M.

2014-06-01

216

Hydrogen and oxygen adsorption on ZnO nanowires: A first-principles study  

NASA Astrophysics Data System (ADS)

We employ first-principles calculations to investigate the structural stability and electronic properties of zinc oxide (ZnO) nanowires adsorbed with different chemical functional groups. The nanowires with one hydrogen monolayer on the surfaces adsorbed with both O and Zn atoms maintain their bulklike geometries, whereas the surface relaxation is found to be significant for the bare and partially adsorbed nanowires. While a half monolayer coverage of hydrogen on an oxygen-adsorbed surface induces metallic behavior, the adsorption of a full monolayer removes the states from the band gap to render a system with a well-defined band gap, revising previous theoretical predictions of metallicity in the latter. On the other hand, when all surface atoms are saturated, either with hydrogen atoms only or with both OH and H groups, the semiconducting behavior is recovered. Our results open up the possibility of tailoring the electronic properties by controlling the surface adsorption sites.

Xu, Hu; Fan, Wei; Rosa, A. L.; Zhang, R. Q.; Frauenheim, Th.

2009-02-01

217

Formaldehyde molecule adsorbed on doped graphene: A first-principles study  

NASA Astrophysics Data System (ADS)

Adsorption of formaldehyde (H2CO) on B-, N-, Si-, Al-, Cr-, Mn-, and Au-doped graphene was theoretically studied using first-principles approach based on density functional theory in order to exploit their potential applications as H2CO gas sensors. The electronic and magnetic properties of the graphene-molecule adsorption adducts are strongly dependent on the dopants. H2CO molecule is adsorbed weakly on B- and N-doped graphene; in general, strong chemisorption is observed on Si-, Al-, Cr-, Mn-, and Au-doped graphene. The most stable adsorption geometries, adsorption energies, magnetic moments, charge transfers, and density of states of these systems are thoroughly discussed. This work reveals that the sensitivity of graphene-based chemical gas sensors for H2CO can be drastically improved by introducing appropriate dopant. Al and Mn are found to be the best choices among all the dopants.

Liu, Xu-Ying; Zhang, Jian-Min

2014-02-01

218

Chlorine molecule adsorbed on graphene and doped graphene: A first-principle study  

NASA Astrophysics Data System (ADS)

Adsorption of molecular chlorine (Cl2) on intrinsic and B-, Si-, Cr-, Cu-, Fe-, Mn-, Ni-, Ti- and Au-doped graphene was theoretically studied using first-principle approach based on density functional theory in order to develop their potential applications as Cl2 gas sensors. The structural, electronic and magnetic properties of the graphene-molecule adsorption adducts are strongly dependent on the dopants. Cl2 molecule is adsorbed weakly on intrinsic and B-doped graphene; in general, strong chemisorption is observed on Si-, Cr-, Cu-, Fe-, Mn-, Ni-, Ti- and Au-doped graphene. The most stable adsorption geometries, energies, magnetic moments, charge transfers and density of states of these systems are thoroughly discussed. This work reveals that the sensitivity of graphene-based chemical gas sensors for Cl2 can be drastically improved by introducing appropriate dopant, and Ti as well as Au is the best choice among all the dopants.

Liu, Xu-Ying; Zhang, Jian-Min; Xu, Ke-Wei

2014-03-01

219

First principles study of properties of the oxidized Cu(100) and Cu(110)  

NASA Astrophysics Data System (ADS)

Copper based catalysts are of importance to a number of industrial processes including the synthesis of methanol, the reduction and decomposition of nitrogen oxides, and treatment of waste water. In copper catalysis surface oxidation and oxidic overlayers are believed to play a crucial role. In this work using density functional theory (DFT) within the generalized gradient approximation (GGA) we have studied the stability and associated electronic properties of the oxidized Cu(100) and Cu(110) surfaces. Especially, we have focused on studies of changes in the interlayer spacing, electron work function, binding energy, and density of states with oxygen coverage. We have examined the cases of various oxygen coverages of the non-reconstructed, missing row reconstructed Cu(100), and added row reconstructed Cu (110) surfaces. The first-principles calculations in this work have been performed using DMOl3 code. The obtained theoretical results have been compared with available experimental data.

Olenga, Antoine

220

Sulfur dioxide adsorbed on graphene and heteroatom-doped graphene: a first-principles study  

NASA Astrophysics Data System (ADS)

The adsorption of sulfur dioxide (SO2) on intrinsic graphene and heteroatom-doped (B, N, Al, Si, Cr, Mn, Ag, Au, and Pt) graphene samples was theoretically studied using first-principles approach based on density functional theory to exploit their potential applications as SO2 gas sensors. The structural and electronic properties of the graphene-molecule adsorption adducts are strongly dependent on the dopants. SO2 molecule is adsorbed weakly on intrinsic graphene, and B-, N-doped graphene; in general, strong chemisorption is observed on Al-, Si-, Cr-, Mn-, Ag-, Au-, and Pt-doped graphene. The adsorption mechanisms are discussed from charge transfers and density of states. This work reveals that the sensitivity of graphene-based chemical gas sensors for SO2 can be drastically improved by introducing appropriate dopant, and Cr, as well as Mn, may be the best choices among all the dopants.

Shao, Li; Chen, Guangde; Ye, Honggang; Wu, Yelong; Qiao, Zhijuan; Zhu, Youzhang; Niu, Haibo

2013-02-01

221

First-principles calculations on thermodynamic properties of BaTiO3 rhombohedral phase.  

PubMed

The calculations based on the linear combination of atomic orbitals have been performed for the low-temperature phase of BaTiO(3) crystal. Structural and electronic properties, as well as phonon frequencies were obtained using hybrid PBE0 exchange-correlation functional. The calculated frequencies and total energies at different volumes have been used to determine the equation of state and thermal contribution to the Helmholtz free energy within the quasiharmonic approximation. For the first time, the bulk modulus, volume thermal expansion coefficient, heat capacity, and Grüneisen parameters in BaTiO(3) rhombohedral phase have been estimated at zero pressure and temperatures form 0 to 200 K, based on the results of first-principles calculations. Empirical equation has been proposed to reproduce the temperature dependence of the calculated quantities. The agreement between the theoretical and experimental thermodynamic properties was found to be satisfactory. PMID:22514059

Bandura, Andrei V; Evarestov, Robert A

2012-07-01

222

Formation and annealing behaviors of qubit centers in 4H-SiC from first principles  

NASA Astrophysics Data System (ADS)

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.

Wang, Xiaopeng; Zhao, Mingwen; Bu, Hongxia; Zhang, Hongyu; He, Xiujie; Wang, Aizhu

2013-11-01

223

First-principles calculations of elastic constants of c-BN  

NASA Astrophysics Data System (ADS)

The lattice parameters, elastic constants, bulk modulus and its pressure derivative of c-BN are calculated using a first-principles plane wave method with the relativistic analytic pseudopotential of the Hartwigen, Goedecker and Hutter (HGH)-type and the pseudopotential of Troullier-Martins (TM)-type in the frame of local density approximation, and the dependences of bulk modulus on temperature and on pressure are investigated by the quasi-harmonic Debye model. The Debye temperature and the thermal expansion coefficient of c-BN are also calculated. The results obtained are well consistent with the available experimental data and other theoretical results. At zero pressure, the thermal expansion coefficients are about 6.0×10 -6 K -1 at T=1724 K and 6.5×10 -6 K -1 at T=1972 K, respectively, consistent with the experimental data.

Hao, Yan-Jun; Chen, Xiang-Rong; Cui, Hong-Ling; Bai, Yu-Lin

2006-06-01

224

Thermal conductivity of Si nanowires: A first-principles analysis of the role of defects  

NASA Astrophysics Data System (ADS)

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.

Kang, By.; Estreicher, S. K.

2014-04-01

225

First principles DFT investigation of yttrium-doped graphene: Electronic structure and hydrogen storage  

NASA Astrophysics Data System (ADS)

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.

Desnavi, Sameerah; Chakraborty, Brahmananda; Ramaniah, Lavanya M.

2014-04-01

226

Strain effect on electronic structures of graphene nanoribbons: A first-principles study.  

PubMed

We report a first-principles study on the electronic structures of deformed graphene nanoribbons (GNRs). Our theoretical results show that the electronic properties of zigzag GNRs are not sensitive to uniaxial strain, while the energy gap modification of armchair GNRs (AGNRs) as a function of uniaxial strain displays a nonmonotonic relationship with a zigzag pattern. The subband spacings and spatial distributions of the AGNRs can be tuned by applying an external strain. Scanning tunneling microscopy dI/dV maps can be used to characterize the nature of the strain states, compressive or tensile, of AGNRs. In addition, we find that the nearest neighbor hopping integrals between pi-orbitals of carbon atoms are responsible for energy gap modification under uniaxial strain based on our tight binding approximation simulations. PMID:19044789

Sun, Lian; Li, Qunxiang; Ren, Hao; Su, Haibin; Shi, Q W; Yang, Jinlong

2008-08-21

227

Electronic and elastic properties of Al4Ce binary compound under pressure via first-principles  

NASA Astrophysics Data System (ADS)

In this paper, the effects of pressure on the structural, electronic and mechanical properties of tetragonal Al4Ce phase have been analyzed by means of first-principles method based on the density functional theory within generalized gradient approximation (GGA). The calculated equilibrium lattice parameters under zero pressure are in good agreement with the previous experimental and other theoretical data. The obtained total density of state (TDOS) and partial density of states (PDOS) of Al4Ce at varying pressures indicate that this compound exhibits favorable metallic behavior. In addition, the isotropic bulk modulus B, shear modulus G, Young modulus E and Poisson' ratio ? of Al4Ce at different pressures are investigated by using the Voigt–Reuss–Hill averaging scheme. The results demonstrate that Al4Ce phase is ductile according to the analysis of BH/GH and has good mechanical stability. Finally, the Debye temperatures (?D), which is also obtained from the elastic constants, increases with increasing pressure.

Fu, Li; Zhao, Yuhong; Yang, Ling; Duan, Yaping; Ge, Kun; Han, Peide

2014-05-01

228

First-principles study of B1 to B2 phase transition in PbS  

NASA Astrophysics Data System (ADS)

The high pressure structural phase transition in PbS has been studied by means of first-principles total energy calculations which are based on linear combination of atomic orbitals (LCAO) method within local density approximation (LDA). In the present study, the exchange scheme of Becke and correlation functional of von-Barth-Hedin (VBH) are employed. It is observed that more stable phase for PbS is NaCl type (B1) and PbS transforms to the CsCl type (B2) structure under high pressure (22.8 GPa). The calculated value of transition pressure (Pt) from B1 to B2 structure is found in good agreement with the earlier experimental and theoretical investigations.

Bhambhani, P.; Munjal, N.; Sharma, G.; Vyas, V.; Sharma, B. K.

2012-07-01

229

Pressure-induced phase transition for ScVO4: A first-principles study  

NASA Astrophysics Data System (ADS)

We theoretically investigated the structural stability and electronic properties of ScVO4 by the first-principles pseudopotential method. The tetragonal zircon-type and scheelite-type structures, LaTaO4-type structure of ScVO4 have been considered. The calculations indicate that the LaTaO4-type phase is not stable in the pressure 0-100 GPa, and the structural phase transformation from zircon to scheelite-type structure occurs at 5.4 GPa. The band structure shows that zircon-type structure at zero pressure and scheelite-type structure at transition pressure have direct gaps of 2.58 eV and 2.35 eV, respectively. The detailed volume changes during the phase transition were analyzed.

Sheng, Shu-Fang

2013-10-01

230

First-principles calculations of conductivity in transparent semiconducting oxides  

NASA Astrophysics Data System (ADS)

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.

Varley, Joel Basile

231

ABINIT: First-principles approach to material and nanosystem properties  

NASA Astrophysics Data System (ADS)

ABINIT [ http://www.abinit.org] allows one to study, from first-principles, systems made of electrons and nuclei (e.g. periodic solids, molecules, nanostructures, etc.), on the basis of Density-Functional Theory (DFT) and Many-Body Perturbation Theory. Beyond the computation of the total energy, charge density and electronic structure of such systems, ABINIT also implements many dynamical, dielectric, thermodynamical, mechanical, or electronic properties, at different levels of approximation. The present paper provides an exhaustive account of the capabilities of ABINIT. It should be helpful to scientists that are not familiarized with ABINIT, as well as to already regular users. First, we give a broad overview of ABINIT, including the list of the capabilities and how to access them. Then, we present in more details the recent, advanced, developments of ABINIT, with adequate references to the underlying theory, as well as the relevant input variables, tests and, if available, ABINIT tutorials. Program summaryProgram title: ABINIT Catalogue identifier: AEEU_v1_0 Distribution format: tar.gz Journal reference: Comput. Phys. Comm. Programming language: Fortran95, PERL scripts, Python scripts Computer: All systems with a Fortran95 compiler Operating system: All systems with a Fortran95 compiler Has the code been vectorized or parallelized?: Sequential, or parallel with proven speed-up up to one thousand processors. RAM: Ranges from a few Mbytes to several hundred Gbytes, depending on the input file. Classification: 7.3, 7.8 External routines: (all optional) BigDFT [1], ETSF IO [2], libxc [3], NetCDF [4], MPI [5], Wannier90 [6] Nature of problem: This package has the purpose of computing accurately material and nanostructure properties: electronic structure, bond lengths, bond angles, primitive cell size, cohesive energy, dielectric properties, vibrational properties, elastic properties, optical properties, magnetic properties, non-linear couplings, electronic and vibrational lifetimes, etc. Solution method: Software application based on Density-Functional Theory and Many-Body Perturbation Theory, pseudopotentials, with planewaves, Projector-Augmented Waves (PAW) or wavelets as basis functions. Running time: From less than one second for the simplest tests, to several weeks. The vast majority of the >600 provided tests run in less than 30 seconds. References:[1] http://inac.cea.fr/LSim/BigDFT. [2] http://etsf.eu/index.php?page=standardization. [3] http://www.tddft.org/programs/octopus/wiki/index.php/Libxc. [4] http://www.unidata.ucar.edu/software/netcdf. [5] http://en.wikipedia.org/wiki/MessagePassingInterface. [6] http://www.wannier.org.

Gonze, X.; Amadon, B.; Anglade, P.-M.; Beuken, J.-M.; Bottin, F.; Boulanger, P.; Bruneval, F.; Caliste, D.; Caracas, R.; Côté, M.; Deutsch, T.; Genovese, L.; Ghosez, Ph.; Giantomassi, M.; Goedecker, S.; Hamann, D. R.; Hermet, P.; Jollet, F.; Jomard, G.; Leroux, S.; Mancini, M.; Mazevet, S.; Oliveira, M. J. T.; Onida, G.; Pouillon, Y.; Rangel, T.; Rignanese, G.-M.; Sangalli, D.; Shaltaf, R.; Torrent, M.; Verstraete, M. J.; Zerah, G.; Zwanziger, J. W.

2009-12-01

232

Electron Exchange and Conduction in Nontronite from First-Principles  

SciTech Connect

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.

Alexandrov, Vitali Y.; Neumann, Anke; Scherer, Michelle; Rosso, Kevin M.

2013-01-11

233

Doping of rhenium disulfide monolayers: a systematic first principles study.  

PubMed

The absence of a direct-to-indirect band gap transition in ReS2 when going from the monolayer to bulk makes it special among the other semiconducting transition metal dichalcogenides. The functionalization of this promising layered material emerges as a necessity for the next generation technological applications. Here, the structural, electronic, and magnetic properties of substitutionally doped ReS2 monolayers at either the S or Re site were systematically studied by using first principles density functional calculations. We found that substitutional doping of ReS2 depends sensitively on the growth conditions of ReS2. Among the large number of non-metallic atoms, namely H, B, C, Se, Te, F, Br, Cl, As, P, and N, we identified the most promising candidates for n-type and p-type doping of ReS2. While Cl is an ideal candidate for n-type doping, P appears to be the most promising candidate for p-type doping of the ReS2 monolayer. We also investigated the doping of ReS2 with metal atoms, namely Mo, W, Ti, V, Cr, Co, Fe, Mn, Ni, Cu, Nb, Zn, Ru, Os and Pt. Mo, Nb, Ti, and V atoms are found to be easily incorporated in a single layer of ReS2 as substitutional impurities at the Re site for all growth conditions considered in this work. Tuning chemical potentials of dopant atoms energetically makes it possible to dope ReS2 with Fe, Co, Cr, Mn, W, Ru, and Os at the Re site. We observe a robust trend for the magnetic moments when substituting a Re atom with metal atoms such that depending on the electronic configuration of dopant atoms, the net magnetic moment of the doped ReS2 becomes either 0 or 1 ?B. Among the metallic dopants, Mo is the best candidate for p-type doping of ReS2 owing to its favorable energetics and promising electronic properties. PMID:25001566

Cak?r, Deniz; Sahin, Hasan; Peeters, François M

2014-07-16

234

First Principles Modeling of Bimolecular Reactions with Diffusion  

NASA Astrophysics Data System (ADS)

We consider three approaches to modeling A + B ? C irreversible reactions in natural media: 1) a discretized diffusion-reaction equation (DRE), 2) a particle tracking (PT) scheme in which reaction occurs if and only if an A and B particle pair are within a fixed distance, r (the "reaction radius"), and 3) a PT scheme using an alternative to the fixed reaction radius: a collocation probability distribution derived directly from first principles. Each approach has advantages. In some cases a discretized DRE may be the most computationally efficient method. For PT simulations, robust codes exist based on use of a fixed reaction radius. And finally, collocation probabilities may be derived directly from the Fick's Law constant, D, which is a well-established property for most species. In each approach, a single parameter governs the 'promiscuity' of the reaction (i.e. the thermodynamic favorability of reaction, predicated on the particles being locally well mixed). For the DRE, fixed-reaction-radius PT, and collocation-based PT, these parameters are, respectively: a second-order decay rate, r, and D. We established a number of new results enhancing these approaches and relating them to each other (and to nature). In particular, a thought experiment concerning a simple system in which the predictions of each approach can be computed analytically was used to derive formulas establishing a universal one-to-one correspondence among each of the governing parameters. We thus showed the conditions for equivalence of the three approaches, and grounded both the DRE approach and the fixed-radius PT approach in the Fick's Law D. We further showed that the existing collocation-based PT theory is based on a probability distribution that is only correct for infinitesimally small times, but which can be modified to be accurate for larger times by means of continuous time random walk analysis and first-passage probability distributions. Finally, we employed a novel mathematical approach to adapt this into a workable collocation-based particle tracking technique.

Hansen, S. K.; Scher, H.; Berkowitz, B.

2013-12-01

235

Group theoretical selection rules for electron-impact spectroscopy.  

NASA Technical Reports Server (NTRS)

Simple group theoretical principles are applied to the consideration of electron scattering off atoms and diatomic and polyatomic molecules. This approach is also used to estimate relative strengths of electron-impact-induced transitions of diatomic and polyatomic molecules.

Goddard, W. A., III; Cartwright, D. C.; Trajmar, S.; Huestis, D. L.

1971-01-01

236

First principle optoelectronic studies of visible light sensitive CZT  

NASA Astrophysics Data System (ADS)

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.

Khan, Imad; Aliabad, H. A. Rahnamaye; Ahmad, Waqar; Ali, Zahid; Ahmad, Iftikhar

2013-11-01

237

Theoretical analysis of anharmonic coupling and cascading Raman signals observed with femtosecond stimulated Raman spectroscopy  

Microsoft Academic Search

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

Randy D. Mehlenbacher; Brendon Lyons; Kristina C. Wilson; Yong Du; David W. McCamant

2009-01-01

238

Structural, electronic, and optical properties of the diindenoperylene molecule from first-principles density-functional theory  

SciTech Connect

The structure, energetics, electronic, and optical properties of the organic molecule, diindenoperylene (DIP), are investigated by first-principles density-functional and time-dependent density-functional theory. The photoabsorption cross section, computed on the optimized geometry within the linear response theory, gives results in good agreement with experimental data, with minor differences ascribed to vibrational levels and to solvent effects. Ab initio dynamical simulations of the molecular triplet excited state show that DIP is stable against distortions, at least on the picosecond time scale. The theoretical approach, involving a combination of first-principles techniques, is shown to be able to describe in detail the response properties of low-dimensional organic semiconductor systems, currently important in nanotechnological applications.

Ramaniah, Lavanya M.; Boero, Mauro [High Pressure Physics Division, Physics Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400085 (India); Center for Computational Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577 (Japan)

2006-10-15

239

First-principles models of equilibrium tellurium isotope fractionation  

NASA Astrophysics Data System (ADS)

In this study, equilibrium mass-dependent isotopic fractionation among representative Te-bearing species is estimated with first-principles thermodynamic calculations. Tellurium is a group 16 element (along with O, S, and Se) with eight stable isotopes ranging in mass from 120Te to 130Te, and six commonly-occurring oxidation states: -II, -I, 0, +II, +IV, and +VI. In its reduced form, Te(-II), tellurium has a unique crystal-chemical role as a bond partner for gold and silver in epithermal and orogenic gold deposits, which likely form when oxidized Te species (e.g., H2TeO3, TeO32-) or perhaps polytellurides (e.g., Te22-) interact with precious metals in hydrothermal solution. Te(IV) is the most common oxidation state at the Earth's surface, including surface outcrops of telluride ore deposits, where tellurite and tellurate minerals form by oxidation. In the ocean, dissolved tellurium tends to be scavenged by particulate matter. Te(VI) is more abundant than Te(IV) in the ocean water (1), even though it is thought to be less stable thermodynamically. This variety of valence states in natural systems and range of isotopic masses suggest that tellurium could exhibit geochemically useful isotope abundance variations. Tellurium isotope fractionations were determined for representative molecules and crystals of varying complexity and chemistry. Gas-phase calculations are combined with supermolecular cluster models of aqueous and solid species. These in turn are compared with plane-wave density functional theory calculations with periodic boundary conditions. In general, heavyTe/lightTe is predicted to be higher for more oxidized species, and lower for reduced species, with 130Te/125Te fractionations as large as 4‰ at 100?C between coexisting Te(IV) and Te(-II) or Te(0) compounds. This is a much larger fractionation than has been observed in naturally occurring redox pairs (i.e., Te (0) vs. Te(IV) species) so far, suggesting that disequilibrium processes may control oxidation and reduction. Se- and S-isotope redox systems also show kinetically-controlled isotopic disequilibrium, and may serve as useful analogues. Among Te(-II) species, fractionations are smaller (< 1‰ at 100?C). (Au,Ag)Te2 minerals (calaverite, krennerite) and (Au,Ag)2Te minerals (petzite, hessite) are expected to have similar isotopic compositions to vapor-phase H2Te, and there appears to be little discernable effect from Ag,Au solid solution. Altaite (PbTe) will have somewhat lower 130Te/125Te. Calculated fractionation factors for gas-phase species like H2Te and TeF6, based on hybrid density functional theory (B3LYP), agree well with earlier estimates (2). For telluride-bearing crystals, cluster models are in qualitative agreement with periodic boundary condition calculations, especially when larger basis sets (e.g., aug-cc-PVDZ-PP) are used; however, cluster models tend to predict higher 130Te/125Te for reasons that are not yet clear. References: 1. Hein et al. (2003) GCA 67:6. 2. Smithers et al. (1968) Can. J. Chem. 46:4.

Haghnegahdar, M. A.; Schauble, E. A.; Fornadel, A. P.; Spry, P. G.

2013-12-01

240

Modeling of compositionally graded barium strontium titanate from first principles  

NASA Astrophysics Data System (ADS)

Barium Strontium Titanate (BaxSr1-xTiO 3 or BST) is a Perovskite alloy of interest for both technological and intellectual reasons. Its ferroelectric and piezoelectric properties make it useful in a variety of electric components such as transducers and actuators, and BST in particular is a material of interest for the development of a ferroelectric RAM for computers.(1) The inclusion of SrTiO3, an incipient ferroelectric, and the fact that the properties of a BST system depend strongly on its relative composition of BaTiO3 (BT) and SrTiO3 (ST), make also this a material of high interest. (2) Compositionally graded systems are of further interest (see e.g., Refs. (3), (4), (5) and references therein), partly because their compositional grading leads to a built-in polarization gradient. Due to this, these systems could act as transcapacitors, devices which act as charge amplifiers in much the same way that transistors act as current amplifiers.(3), (4) Here, compositionally graded BST systems were modeled using a first-principles derived effective Hamiltonian method within Monte-Carlo simulation. (6) The graded systems under consideration had an average Ba composition of 70%. These systems were modeled under stress-free conditions, as well as, under epitaxial strain due to a SrTiO3 substrate. Both the degree of grading and the thickness of the layers were varied. The investigation revealed that graded BST systems behaved differently from bulk BST systems in several ways. First, some graded BST systems possessed both monodomain states qualitatively similar to those found in bulk systems (except that the polarization exhibited a "wave" behavior inside the graded systems), and also states with domain striping. Where this occurred, the monodomain state was lower in energy, and was therefore the ground-state, but the striped domain state was found to be metastable, representing a local energy minimum. Analyzing unstrained compositionally graded systems layer by layer revealed that, for small layer thicknesses, the material responded rather homogeneously. However, for large layer thicknesses, each compositionally distinct block responded quite independently, and responded like its equivalent bulk system. This led to some overall systems possessing phases that do not exist in BST bulks (such as monoclinic phases), and to the apparent merging of two phase transitions in unstrained systems. When compositionally graded BST systems were modeled under a compressive epitaxial strain, only the z-component of the polarization (that is the component along the growth direction) was found to increase from zero below a single critical temperature. As in unstrained systems, some strained systems were found to have monodomain and striped domain states, with the monodomain representing the ground state and the striped domains representing a local minimum. Development of a BST thin-film code was also undertaken. Initial simulations of BST thin-films were performed using this code, on both disordered and graded systems. These results verified that the new BST thin-film code was functional, as well as revealed interesting phenomena related to compositional grading in two-dimensional materials.

Walizer, Laura Elizabeth

241

Shock Hugoniot of osmium up to 800 GPa from first principles calculations.  

PubMed

First principles total energy calculations on hcp, ? (a three atom simple hexagonal), ? (bcc) and fcc phases of osmium have been performed as a function of hydrostatic compression employing the FP-LAPW method. The comparison of total energies of these phases up to a maximum compression V/V(0) = 0.58 (pressure?700 GPa) shows that the hcp structure remains stable up to this compression. The 300 K isotherm is determined after adding finite temperature thermal contributions to the total energy calculated as a function of volume at 0 K. From the theoretically determined isotherm, we have derived the shock Hugoniot of this metal and determined the shock parameters C(0) and s to be 4.48 km s(-1) and 1.32, respectively. Employing the theoretically calculated Gruneisen parameter in the differential form of the Lindemann melting rule, we have determined the variation of melting point of the osmium with pressure. The theoretically derived melting curve and the temperature rise along the Hugoniot predict the shock melting of osmium at ?447 GPa with a corresponding temperature of ?9203 K. PMID:21693986

Joshi, K D; Gupta, Satish C; Banerjee, S

2009-10-14

242

Photoexcitation and Photochemical Stability of Organic Photovoltaic Materials from First Principles  

NASA Astrophysics Data System (ADS)

The development of high efficiency organic photovoltaics (OPV) has recently become enabled by the synthesis of new conjugated polymers with low band gap that allow light absorption over a broader range of the spectrum. Stability of these new polymers, a key requirement for commercialization, has not yet received sufficient attention. Here, we report first-principles theoretical modeling of photo-induced degradation of OPV polymers carried out using ab-initio density functional theory (DFT). We report photooxidation routes and reaction products for reactive species including superoxide oxygen anions and hydroxyl groups interacting with the standard workhorse OPV polymer, poly(3-hexyl-thiophene) (P3HT). We discuss theoretical issues and challenges affecting the modeling such reactions in OPV polymers. We also discuss the application of theoretical methods to low-band-gap polymers, and in particular, the effect of the chemical substitution on the photoexcitation properties of these new polymers. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Deparment of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Sai, Na; Leung, Kevin

2013-03-01

243

First-principles modeling of the infrared spectrum of kaolinite  

Microsoft Academic Search

The theoretical infrared spectrum of kaolinite (Al2Si2O5(OH)4, triclinic) was computed using ab initio quantum mechanical calculations. Calculations were performed using the Density Functional Theory and the generalized gradient approximation. The low-frequency dielectric tensor of kaolin- ite was determined as a function of the light frequency using linear response theory. The IR spec- trum was then calculated using a model that

ETIENNE BALAN; A. MARCO SAITTA; FRANCESCO MAURI; GEORGES CALAS

244

Theoretical Modeling for the X-ray Spectroscopy of Iron-bearing MgSiO3 under High Pressure  

NASA Astrophysics Data System (ADS)

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).

Wang, X.; Tsuchiya, T.

2012-12-01

245

First-principles elastic properties of (alpha)-Pu  

SciTech Connect

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.

Soderlind, P; Klepeis, J E

2008-11-04

246

First principles Investigations of the Conductance of Stretched Molecules  

NASA Astrophysics Data System (ADS)

A novel experimental setup developed at Arizona State University examines the molecular conductance across a variety of gap lengths by lowering a gold-plated AFM tip into a monolayer deposited on a gold substrate [1]. Theoretical investigations into these systems have revealed interesting trends in the conductance of these molecules as they are stretched. We investigate this system using a variety of theoretical models, such as DFT and Hartree-Fock calculations of the Hamiltonian (and a variety of basis sets), which is implemented into a Landauer formula based rapid transfer matrix method with charge self-consistency [2]. Here we solve a self-consistent potential, which obviates the need to parameterize the voltage. Conduction across the molecule occurs in multiple channels; gold states couple with varying strengths to the orbitals of the molecule. We will report the effects of strain across the molecule, and distortion of the molecule, on the conductive nature of the coupling. * Work supported by the Office of Naval Research [1] B. Xu and N. J. Tao, Science 301, 1221 (2003). [2] T. Usuki, M. Saito, M. Takatsu, R.A. Kiehl, and N. Yokoyama, Phys. Rev. B 52, 8244 (1995).

Speyer, Gil; Akis, Richard; Ferry, David K.; Li, Jun; Sankey, Otto F.

2004-03-01

247

First-Principles Investigation of Li Intercalation Kinetics in Phospho-Olivines  

NASA Astrophysics Data System (ADS)

This thesis focuses broadly on characterizing and understanding the Li intercalation mechanism in phospho-olivines, namely LiFePO 4 and Li(Fe,Mn)PO4, using first-principles calculations. Currently Li-ion battery technology is critically relied upon for the operation of electrified vehicles, but further improvements mainly in cathode performance are required to ensure widespread adoption, which in itself requires learning from existing commercial cathode chemistries. LiFePO4 is presently used in commercial Li-ion batteries, known for its rapid charge and discharge capability but with underwhelming energy density. This motivates the three central research efforts presented herein. First, we investigate the modified phase diagram and electrochemical properties of mixed olivines, such as Li(Fe,Mn)PO4, which offer improved theoretical energy density over LiFePO4 (due to the higher redox voltage associated with Mn2+/Mn3+). The Lix(Fe1-yMny)PO4 phase diagram is constructed by Monte Carlo simulation on a cluster expansion Hamiltonian parametrized by first-principles determined energies. Deviations from the equilibrium phase behavior and voltages of pure LiFePO4 and LiMnPO 4 are analyzed and discussed to good agreement with experimental observations. Second, we address why LiFePO4 exhibits superior rate performance strictly when the active particle size is brought down to the nano-scale. By considering the presence of immobile point defects residing in the 1D Li diffusion path, specifically by calculating from first principles both defect formation energies and Li migration barriers in the vicinity of likely defects, the Li diffusivity is recalculated and is found to strongly vary with particle size. At small particle sizes, the contribution from defects is small, and fast 1D Li diffusion is accessible. However, at larger particle sizes (microm scale and above) the contribution from defects is much larger. Not only is Li transport impeded, but it is also less anisotropic in agreement with experiments on large LiFePO4 single crystals. Third, we investigate why LiFePO4 can be charged and discharged rapidly despite having to undergo a first-order phase transition. Conventional wisdom dictates that a system with strong equilibrium Li segregation behavior requires both nucleation and growth in the charge and discharge process, which should impede the overall kinetics. Rather, through first-principles calculations, we determine the minimal energy required to access a non-equilibrium transformation path entirely through the solid solution. Not only does this transformation mechanism require little driving force, but it also rationalizes how a kinetically favorable but nonequilibrium path is responsible for the extremely high rate performance associated with this material. The consequences of a rapid non-equilibrium single-particle transformation mechanism on (dis)charging a multi-particle assembly, as is the case in porous electrodes, are discussed and compared to experimental observations. (Copies available exclusively from MIT Libraries, libraries.mit.edu/docs - docs mit.edu)

Malik, Rahul

248

Atomistic simulation of point defects and dislocations in bbc transition metals from first principles  

SciTech Connect

Using multi-ion interatomic potentials derived from first-principles generalized pseudopotential theory, we have been studying point defects and dislocations in bcc transition metals, with molybdenum (Mo) as a prototype. For point defects in Mo, the calculated vacancy formation and activation energies are in excellent agreement with experimental results. The energetics of six self-interstitial configurations in Mo have also been investigated. The <110> split dumb-bell is found to have the lowest formation energy, as is experimentally observed, but the corresponding migration energy is calculated to be 3--15 times higher than previous theoretical estimates. The atomic structure and energetics of <111> screw dislocations in Mo are now being investigated. We have found that the ``easy`` core configuration has a lower formation energy than the ``hard`` one, consistent with previous theoretical studies. The former has a distinctive 3-fold symmetry with a spread out of the dislocation core along the <112> directions, an effect which is driven by the strong angular forces present in these metals.

Xu, W; Moriarty, J.A.

1996-01-19

249

A first-principles measure for the twinnability of FCC metals  

NASA Astrophysics Data System (ADS)

Twinnability is the property describing the ease with which a metal plastically deforms by twinning relative to deforming by dislocation-mediated slip. In this paper a theoretical measure for twinnability in face-centered-cubic (fcc) metals is obtained through homogenization of a recently introduced criterion for deformation twinning (DT) at a crack tip in a single crystal. The DT criterion quantifies the competition between slip and twinning at the crack tip as a function of crack orientation and applied loading. The twinnability of bulk material is obtained by constructing a representative volume element of the material as a polycrystal containing a distribution of microcracks and integrating the DT criterion over all possible grain and microcrack orientations. The resulting integral expression depends weakly on Poisson's ratio and significantly on three interfacial energies: the stacking-fault energy, the unstable-stacking energy and the unstable-twinning energy. All these four quantities can be computed from first principles. The weak dependence on Poisson's ratio is exploited to derive a simple and accurate closed-form approximation for twinnability which clarifies its dependence on the remaining material parameters. To validate the new measure, the twinnability of eight pure fcc metals is computed using parameters obtained from quantum-mechanical tight-binding calculations. The ranking of these materials according to their theoretical twinnability agrees with the available experimental evidence, including the low incidence of DT in Al, and predicts that Pd should twin as easily as Cu.

Tadmor, E. B.; Bernstein, N.

2004-11-01

250

First-principles calculations of elastic and electronic properties of NbB(2) under pressure.  

PubMed

The structural parameters, elastic constants and electronic structure of NbB(2) under pressure are investigated by using first-principles plane-wave pseudopotential density functional theory within the generalized gradient approximation (GGA). The obtained results are in agreement with the available theoretical data. It is found that the elastic constants and the Debye temperature of NbB(2) increase monotonically and the anisotropies weaken with pressure. The band structure and density of states (DOS) of NbB(2) under pressure are also presented. It is the ? hole that determines the superconductivity in NbB(2), and the features of the ? bands are unchanged after applying pressure except for a shift of position. The density of states (DOS) at the Fermi level decreases with increasing pressure, in conjunction with Bardeen-Cooper-Schrieffer (BCS) theory, which can predict T(c) decreasing with pressure, in agreement with the trend of the theoretical T(c) versus pressure. PMID:21813982

Li, Xiao-Feng; Ji, Guang-Fu; Zhao, Feng; Chen, Xiang-Rong; Alfè, Dario

2009-01-14

251

First principles investigation of L-alanine in terahertz region.  

PubMed

Terahertz absorption spectrum (0.5-4.0 THz) of L-alanine in the solid phase was measured by terahertz time-domain spectroscopy at room temperature. Simulations utilizing gaseous-state and solid-state theory were performed to determine the origins of the observed vibrational features. Our calculations showed that the measured features in solid-state materials could be well understood by considering the crystal packing interactions in a solid-state density functional theory calculation. Furthermore, intermolecular vibrations of L-alanine are found to be the dominating contributions to these measured spectral features in the range of 0.5-4.0 THz, except that located at 3.11 THz. PMID:23729906

Zheng, Zhuan-Ping; Fan, Wen-Hui

2012-06-01

252

Elastic Properties of Monoclinic at Finite Temperatures Via First Principles  

NASA Astrophysics Data System (ADS)

The structural and elastic properties of orthorhombic as a function of temperature are investigated by the generalized gradient approximation (GGA) correction scheme in the framework of density functional theory (DFT) and the quasi-harmonic Debye model. The thirteen independent elastic constants of at temperatures to 3200 K are theoretically investigated for the first time. It is found that with increasing temperature, all elastic constants change, especially change rapidly in the temperature range of 1400 K to 1600 K and 2200 K to 2600 K, respectively. We also obtain the bulk modulus , shear modulus , Young's moduli , as well as Poisson's ratio of at high temperatures. Our work suggests that it is very important to predict the melting properties of materials via the elastic constants at temperatures.

Cheng, Yan; Zhang, Tian; Qi, Yuan-Yuan

2014-01-01

253

First-Principles Study of Casimir Repulsion in Metamaterials  

SciTech Connect

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.

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

254

First principles studies of interface dielectric properties of polymer\\/metal-oxide nanocomposites  

Microsoft Academic Search

This thesis is devoted to studying interface dielectric properties of polymer nanocomposites from first principles. We aim to understand at atomic scale the role of interface effects and the dielectric finite size effects of nanoparticles in determining the effective dielectric properties of polymer nanocomposites. To study surface effects from first principles, we first investigate the two common methods, namely dipole

Liping Yu

2009-01-01

255

Thermodynamics and elastic properties of Ta from first-principles calculations  

NASA Astrophysics Data System (ADS)

Within the framework of the quasiharmonic approximation, the thermodynamics and elastic properties of Ta, including phonon density of states (DOS), equation of state, linear thermal expansion coefficient, entropy, enthalpy, heat capacity, elastic constants, bulk modulus, shear modulus, Young's modulus, microhardness, and sound velocity, are studied using the first-principles projector-augmented wave method. The vibrational contribution to Helmholtz free energy is evaluated from the first-principles phonon DOS and the Debye model. The thermal electronic contribution to Helmholtz free energy is estimated from the integration over the electronic DOS. By comparing the experimental results with the calculation results from the first-principles and the Debye model, it is found that the thermodynamic properties of Ta are depicted well by the first-principles. The elastic properties of Ta from the first-principles are consistent with the available experimental data.

Li, Qiang; Huang, Duo-Hui; Cao, Qi-Long; Wang, Fan-Hou; Cai, Ling-Cang; Zhang, Xiu-Lu; Jing, Fu-Qian

2012-12-01

256

Transmission resonance Raman spectroscopy: experimental results versus theoretical model calculations.  

PubMed

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

Gonzálvez, Alicia G; González Ureña, Ángel

2012-10-01

257

Theoretical studies of aluminum and aluminide alloys using CALPHAD and first-principles approach  

Microsoft Academic Search

Heat-treatable aluminum alloys have been widely used in the automobile and aerospace industries as structural materials due to their light weight and high strength. To study the age-hardening process in heat-treatable aluminum alloys, the Gibbs energies of the strengthening metastable phases, e.g. theta ' and theta\\

Chao Jiang

2004-01-01

258

Nitrate isotope fractionations during biological nitrate reduction: Insights from first principles theoretical modeling  

NASA Astrophysics Data System (ADS)

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.

Guo, W.; Granger, J.; Sigman, D. M.

2010-12-01

259

First principle study on structural, mechanical and electronic properties of REAg (RE-Y, La, Pr and Er) intermetallic compounds  

NASA Astrophysics Data System (ADS)

The structural, electronic and mechanical properties of binary B2 - type CsCl structured intermetallic compounds of Ag (ReAg, Re= Y, La, Pr and Er) have been studies systematically by means of first principles density functional theory within generalized gradient approximation. Ground state properties such as lattice constant (a0), bulk modulus (B) and its pressure derivative (B') are obtained. The present results are in good agreement with the experimental and other theoretical calculation available. Amongst all the Intermetallics is found ErAg to be most ductile due to the presence of strong metallic bonding.

Sahu, A.; Makode, Chandrabhan; Pataiya, J.; Sanyal, Sankar P.

2013-02-01

260

First-principles calculation of optical absorption spectra in conjugated polymers: Role of electron-hole interaction  

SciTech Connect

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).

Rohlfing, Michael; Tiago, M.L.; Louie, Steven G.

2000-03-20

261

Ferroelectric behaviour in solid croconic acid using neutron scattering and first-principles density functional theory  

NASA Astrophysics Data System (ADS)

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.

Mukhopadhyay, S.; Gutmann, M. J.; Jura, M.; Jochym, D. B.; Jimenez-Ruiz, M.; Sturniolo, S.; Refson, K.; Fernandez-Alonso, F.

2013-12-01

262

Electronic and optical properties of Au-doped Cu2O: A first principles investigation  

NASA Astrophysics Data System (ADS)

The Cu2O and Au-doped Cu2O films are prepared on MgO (001) substrates by pulsed laser deposition. The X-ray photoelectron spectroscopy proves that the films are of Au-doped Cu2O. The optical absorption edge decreases by 1.6% after Au doping. The electronic and optical properties of pure and Au-doped cuprite Cu2O films are investigated by the first principles. The calculated results indicate that Cu2O is a direct band-gap semiconductor. The scissors operation of 1.64 eV has been carried out. After correcting, the band gaps for pure and Au doped Cu2O are about 2.17 eV and 2.02 eV, respectively, decreasing by 6.9%. All of the optical spectra are closely related to the dielectric function. The optical spectrum red shift corresponding to the decreasing of the band gap, and the additional absorption, are observed in the visible region for Au doped Cu2O film. The experimental results are generally in agreement with the calculated results. These results indicate that Au doping could become one of the more important factors influencing the photovoltaic activity of Cu2O film.

Jiang, Zhong-Qian; Yao, Gang; An, Xin-You; Fu, Ya-Jun; Cao, Lin-Hong; Wu, Wei-Dong; Wang, Xue-Min

2014-05-01

263

First-principles calculations for XAS of infinite-layer iron oxides  

NASA Astrophysics Data System (ADS)

The oxygen defect perovskite SrFeO3-x shows various properties such as the giant magnetoresistance effect and the thermoelectric effect. It had been believed that the oxygen content in SrFeO3-x changes up to x=0.5. Recently, Tsujimoto et al. have succeeded in synthesizing the infinite-layer iron oxide SrFeO2. SrFeO2 has a square-planar oxygen coordination, while the iron oxides usually have the tetrahedral and octahedral coordination. CaFeO2 has also infinite layer structure and the same magnetic ordering as SrFeO2. However, it is suggested that the oxygen coordination of CaFeO2 is different from that of SrFeO2. In order to investigate the electronic structure of iron in (Ca, Sr)FeO2, the x-ray absorption spectroscopy (XAS) spectrum has been measured. In this work, we perform the calculation for XAS spectrum near the Fe-K edge of (Ca, Sr)FeO2 using the first-principles calculations. We compare the results with the experiment and discuss the electronic structure of iron in (Ca, Sr)FeO2.

Kodera, Mitsuru; Shishidou, Tatsuya; Oguchi, Tamio

2011-03-01

264

Hydration thermodynamics of pyrochlore structured oxides from TG and first principles calculations.  

PubMed

In this contribution we investigate trends in the defect chemistry and hydration thermodynamics of rare-earth pyrochlore structured oxides, RE(2)X(2)O(7) (RE = La-Lu and X = Ti, Sn, Zr and Ce). First principles density functional theory (DFT) calculations have been performed to elucidate trends in the general defect chemistry and hydration enthalpy for the above-mentioned series. Further, to justify the use of such theoretical methods, the hydration properties of selected compositions were studied by means of thermogravimetric measurements. Both DFT calculations and TG measurements indicate that the hydration enthalpy becomes less exothermic with decreasing radii of RE ions within the RE(2)X(2)O(7) series (X = Ti, Sn, Zr and Ce), while it is less dependent on the X site ion. The observed hydration trends are discussed in connection with trends in the stability of both protons and oxygen vacancies and changes in the electronic density of states and bonding environment through the series. Finally, the findings are discussed with respect to existing correlations for other binary and ternary oxides. PMID:23001186

Bjørheim, Tor S; Besikiotis, Vasileios; Haugsrud, Reidar

2012-11-21

265

Electrical conductivity in oxygen-deficient phases of tantalum pentoxide from first-principles calculations  

NASA Astrophysics Data System (ADS)

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.

Bondi, Robert J.; Desjarlais, Michael P.; Thompson, Aidan P.; Brennecka, Geoff L.; Marinella, Matthew J.

2013-11-01

266

First-principles solubilities of alkali and alkaline-earth metals in Mg-B alloys  

NASA Astrophysics Data System (ADS)

In this article, we present a comprehensive theoretical study of solubilities of alkali (Li, Na, K, Rb, Cs) and alkaline-earth (Be, Ca, Sr, Ba) metals in the boron-rich Mg-B system. The study is based on first-principles calculations of solutes formation energies in MgB2 , MgB4 , MgB7 alloys and subsequent statistical-thermodynamical evaluation of solubilities. The advantage of the approach consists in considering all the known phase boundaries in the ternary phase diagram. Substitutional Na, Ca, and Li demonstrate the largest solubilities, and Na has the highest (0.5%-1% in MgB7 at T=650-1000K ). All the considered interstitials have negligible solubilities. The solubility of Be in MgB7 cannot be determined because the corresponding low-solubility formation energy is negative indicating the existence of an unknown ternary ground state. We have performed a high-throughput search of ground states in binary Mg-B, Mg-A , and B-A systems, and we construct the ternary phase diagrams of Mg-B-A alloys based on the stable binary phases. Despite its high-temperature observations, we find that Sr9Mg38 is not a low-temperature equilibrium structure. We also determine two possible ground states CaB4 and RbB4 , not yet observed experimentally.

Chepulskii, Roman V.; Curtarolo, Stefano

2009-04-01

267

First-principles calculations on Mg/Al2CO interfaces  

NASA Astrophysics Data System (ADS)

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.

Wang, F.; Li, K.; Zhou, N. G.

2013-11-01

268

First principle study of elastic and thermodynamic properties of FeB4 under high pressure  

NASA Astrophysics Data System (ADS)

The elastic properties, elastic anisotropy, and thermodynamic properties of the lately synthesized orthorhombic FeB4 at high pressures are investigated using first-principles density functional calculations. The calculated equilibrium parameters are in good agreement with the available experimental and theoretical data. The obtained normalized volume dependence of high pressure is consistent with the previous experimental data investigated using high-pressure synchrotron x-ray diffraction. The complete elastic tensors and crystal anisotropies of the FeB4 are also determined in the pressure range of 0-100 GPa. By the elastic stability criteria and vibrational frequencies, it is predicted that the orthorhombic FeB4 is stable up to 100 GPa. In addition, the calculated B/G ratio reveals that FeB4 possesses brittle nature in the range of pressure from 0 to 100 GPa. The calculated elastic anisotropic factors suggest that FeB4 is elastically anisotropic. By using quasi-harmonic Debye model, the compressibility, bulk modulus, the coefficient of thermal expansion, the heat capacity, and the Grüneisen parameter of FeB4 are successfully obtained in the present work.

Zhang, Xinyu; Qin, Jiaqian; Ning, Jinliang; Sun, Xiaowei; Li, Xinting; Ma, Mingzhen; Liu, Riping

2013-11-01

269

Rare-earth vs. heavy metal pigments and their colors from first principles  

PubMed Central

Many inorganic pigments contain heavy metals hazardous to health and environment. Much attention has been devoted to the quest for nontoxic alternatives based on rare-earth elements. However, the computation of colors from first principles is a challenge to electronic structure methods, especially for materials with localized f-orbitals. Here, starting from atomic positions only, we compute the colors of the red pigment cerium fluorosulfide as well as mercury sulfide (classic vermilion). Our methodology uses many-body theories to compute the optical absorption combined with an intermediate length-scale modelization to assess how coloration depends on film thickness, pigment concentration, and granularity. We introduce a quantitative criterion for the performance of a pigment. While for mercury sulfide, this criterion is satisfied because of large transition matrix elements between wide bands, cerium fluorosulfide presents an alternative paradigm: the bright red color is shown to stem from the combined effect of the quasi-2D and the localized nature of states. Our work shows the power of modern computational methods, with implications for the theoretical design of materials with specific optical properties.

Tomczak, Jan M.; Pourovskii, Leonid V.; Vaugier, Loig; Georges, Antoine; Biermann, Silke

2013-01-01

270

Rare-earth vs. heavy metal pigments and their colors from first principles.  

PubMed

Many inorganic pigments contain heavy metals hazardous to health and environment. Much attention has been devoted to the quest for nontoxic alternatives based on rare-earth elements. However, the computation of colors from first principles is a challenge to electronic structure methods, especially for materials with localized f-orbitals. Here, starting from atomic positions only, we compute the colors of the red pigment cerium fluorosulfide as well as mercury sulfide (classic vermilion). Our methodology uses many-body theories to compute the optical absorption combined with an intermediate length-scale modelization to assess how coloration depends on film thickness, pigment concentration, and granularity. We introduce a quantitative criterion for the performance of a pigment. While for mercury sulfide, this criterion is satisfied because of large transition matrix elements between wide bands, cerium fluorosulfide presents an alternative paradigm: the bright red color is shown to stem from the combined effect of the quasi-2D and the localized nature of states. Our work shows the power of modern computational methods, with implications for the theoretical design of materials with specific optical properties. PMID:23302689

Tomczak, Jan M; Pourovskii, Leonid V; Vaugier, Loig; Georges, Antoine; Biermann, Silke

2013-01-15

271

Water solubility in calcium aluminosilicate glasses investigated by first principles techniques  

SciTech Connect

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

Bouyer, Frederic, E-mail: frederic.bouyer@cea.f [Laboratoire d'etude du Comportement a Long Terme, Commissariat a l'Energie Atomique et aux Energies Alternatives, Centre de Marcoule, SECM - LCLT, BP 17171, 30207 Bagnols-sur-Ceze Cedex (France); Geneste, Gregory [CEA, DAM, DIF, F-91297 Arpajon (France); Ispas, Simona; Kob, Walter [Laboratoire des Colloides, Verres et Nanomateriaux, UMR CNRS-UM2 5587, Universite Montpellier II, Place Eugene Bataillon, 34095 Montpellier Cedex 5 (France); Ganster, Patrick [Centre SMS, UMR CNRS 5146, Ecole des Mines de Saint-Etienne, 158 cours Fauriel, 42023 Saint-Etienne Cedex 2 (France)

2010-12-15

272

Oxygen K-edge XANES of germanates investigated using first-principles calculations  

SciTech Connect

O K-edge x-ray absorption near-edge structure (XANES) spectra of {alpha}-quartz-type and rutile-type GeO{sub 2} polymorphs and of K{sub 2}Ge{sub 8}O{sub 17} have been analyzed using first-principles plane-wave pseudopotential calculations. XANES spectra have been calculated using supercell including core-hole effects and good agreement with experiment has been obtained. In the the case of GeO{sub 2} polymorphs, local density of empty states has been performed and peaks in the experimental spectra can be assigned to transitions involving hybridization of the O p orbitals with the Ge s, Ge p, Ge sp, and Ge d orbitals. Furthermore, peak positions in the theoretical spectra appear to be correlated with changes in the Ge-O-Ge angle as well as indirectly with the Ge coordination geometry. Analysis of O K-edge XANES spectra for individual O sites in K{sub 2}Ge{sub 8}O{sub 17} shows that oxygens shared between two fivefold Ge atoms or one fourfold and one fivefold Ge atom exhibit subtle shifts to lower energy of the peaks, which have been previously observed in alkali germanate glasses at and above the germanate anomaly.

Cabaret, Delphine; Mauri, Francesco; Henderson, Grant S. [Institut de Mineralogie et de Physique des Milieux Condenses, UMR CNRS 7590, Universite Pierre et Marie Curie, Paris 6, 140 rue de Lourmel, F-75015 Paris (France); Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1 (Canada)

2007-05-01

273

First-principles study of high-pressure behavior of solid beta-HMX.  

PubMed

A first-principles plane-wave method with an ultrasoft pseudopotential scheme in the framework of the generalized gradient approximation (GGA) was used to calculate the lattice parameters, bulk modulus and its pressure derivative, energy band structures, density of states, phonon density of states, thermodynamic properties, and absorption spectra of solid beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (beta-HMX). The current study is focused on the thermodynamics and electronic properties that were not reported previously. The bulk modulus and its pressure derivative are also consistent with experimental data and other theoretical results. From the results for the band gaps and density of states, it was found that beta-HMX has the tendency to become a semiconductor with increasing pressure. As the temperature increases, the heat capacity, enthalpy, product of temperature and entropy, and Debye temperature all increase, whereas the free energy decreases. The optical absorption coefficients shift to higher frequencies/energies with increasing pressure. The present study leads to a better understanding of how energetic materials respond to compression. PMID:20028003

Cui, Hong-Ling; Ji, Guang-Fu; Chen, Xiang-Rong; Zhu, Wei-Hua; Zhao, Feng; Wen, Ya; Wei, Dong-Qing

2010-01-21

274

First-principles simulations on bonding pathways of chemical transformations under hydrostatic compression  

NASA Astrophysics Data System (ADS)

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.

Hu, Anguang; Zhang, Fan

2012-02-01

275

Exciton diffusion in disordered small molecules for organic photovoltaics: insights from first-principles simulations.  

PubMed

Exciton diffusion in small molecules 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione [DPP(TBFu)2] is studied using first-principles simulations. We have examined dependence of exciton diffusion on structure disorder, temperature and exciton energy. We find that exciton diffusion length and diffusivity increase with structural order, temperature and the initial exciton energy. Compared to conjugated polymer poly(3-hexylthiophene) (P3HT), DPP(TBFu)2 small molecules exhibit a much higher exciton diffusivity, but a shorter lifetime. The exciton diffusion length in DPP(TBFu)2 is 50% longer than that in P3HT, yielding a higher exciton harvesting efficiency; the physical origin behind these differences is discussed. The time evolutions of exciton energy, electron-hole distance, and exciton localization are explored, and the widely speculated exciton diffusion mechanism is confirmed theoretically. The connection between exciton diffusion and carrier mobilities is also studied. Finally we point out the possibility to estimate exciton diffusivity by measuring carrier mobilities under AC electric fields. PMID:24759042

Li, Z; Zhang, X; Lu, G

2014-05-01

276

Ordered structures in rhenium binary alloys from first-principles calculations.  

PubMed

Rhenium is an important alloying agent in catalytic materials and superalloys, but the experimental and computational data on its binary alloys are sparse. Only 6 out of 28 Re transition-metal systems are reported as compound-forming. Fifteen are reported as phase-separating, and seven have high-temperature disordered ? or ? phases. Comprehensive high-throughput first-principles calculations predict stable ordered structures in 20 of those 28 systems. In the known compound-forming systems, they reproduce all the known compounds and predict a few unreported ones. These results indicate the need for an extensive revision of our current understanding of Re alloys through a combination of theoretical predictions and experimental validations. The following systems are investigated: AgRe(?), AuRe(?), CdRe(?), CoRe, CrRe(?), CuRe(?), FeRe, HfRe, HgRe(?), IrRe, MnRe, MoRe, NbRe, NiRe, OsRe, PdRe, PtRe, ReRh, ReRu, ReSc, ReTa, ReTc, ReTi, ReV, ReW(?), ReY, ReZn(?), and ReZr ((?) = systems in which the ab initio method predicts that no compounds are stable). PMID:21142072

Levy, Ohad; Jahnátek, Michal; Chepulskii, Roman V; Hart, Gus L W; Curtarolo, Stefano

2011-01-12

277

First-principles studies of hydrogen interaction with ultrathin Mg and Mg-based alloy films  

NASA Astrophysics Data System (ADS)

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.

Yoon, Mina; Weitering, Hanno H.; Zhang, Zhenyu

2011-01-01

278

First-principles calculation on β-SiC(111)/α-WC(0001) interface  

NASA Astrophysics Data System (ADS)

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.

Jin, Na; Yang, Yanqing; Li, Jian; Luo, Xian; Huang, Bin; Sun, Qing; Guo, Pengfei

2014-06-01

279

First principles DFT study of dye-sensitized CdS quantum dots  

NASA Astrophysics Data System (ADS)

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.

Jain, Kalpna; Kishor, Shyam; Singh, Kh. S.; Josefesson, Ida; Odelius, Michael; Ramaniah, Lavanya M.

2014-04-01

280

Accurate first-principles calculation of the rare earth crystal field  

NASA Astrophysics Data System (ADS)

Rare earth (RE) doped wide band-gap semiconductors play an important role in solid state lighting. Many aspects of the performance of these materials are characterized and determined by the f-electron crystal field (CF). However, CF effects are usually rather small for f electron: the CF splitting is at the order of 0.1 eV, compared to several eV for d-electrons. Therefore accurate theoretical description of RE crystal field is challenging. We present a first-principles method of CF calculation based on an improved LDA+U method. By careful cancellation of errors, the method can reach relatively high accuracy for the CF parameters. As a demonstration we calculate the experimentally well-characterized RE:LaF3 system, which has low point-group symmetry and a large number of CF parameters, representing a stringent test of theory. The predicted CF excitation energies of Ce:LaF3 agree within about 10 meV with experiment, and within several meV if the errors in the free-ion parameters are excluded. Work is underway to apply the method to other materials for solid-state lighting and laser applications.

Zhou, Fei; Ozolins, Vidvuds

2012-02-01

281

Characteristics of Co islets on Cu(111) from first principles calculations  

NASA Astrophysics Data System (ADS)

Through first principles electronic calculations, based on the spin-polarized density functional theory using the generalized gradient approximation and the ultrasoft pseudopotential method in the plane wave representation, we have examined the structure and magnetic properties of Co monomer, dimer and several n-mers on Cu(111). We find that the monomer has slight preference for the fcc site as compared to the hcp (about 0.02eV) while there is no such preference in the case of the Co dimer. The dimer bond length is found to be about 2.15?. For the 6 atoms cluster, we find that it prefers to be antiferromagnetic and absolute magnetic moment of each Co atom is about 0.07-0.08?B. The monomer is non-magnetic while a high magnetic moment of 1.94?B per Co atom is found in the case of dimer. We discuss our results in the context of recent experimental and theoretical findings [1,2] [3pt] [1] S. Borisova et al, Phys. Rev. B 78, 075428 (2008) [0pt] [2] O. Mironets et al, Phys. Rev. Lett. 100, 096103 (2008)

Le, Duy; Raman, Talat

2009-03-01

282

First-principles investigation on high-pressure structural evolution of MnTiO3  

NASA Astrophysics Data System (ADS)

First-principle calculations using density-functional theory with linearized augmented plane wave method and projector-augmented method have been performed for the high-pressure MnTiO3 polymorphs and their possible dissociation products. Theoretical results demonstrate that ilmenite-type MnTiO3 transforms into perovskite phase at 27 GPa and 0 K. The lithium niobate phase of MnTiO3 is confirmed to be metastable according to its higher Gibbs free energy compared with that of ilmenite at ambient conditions. In ilmenite and lithium niobate phases, MnO6 octahedra become more distorted while TiO6 octahedra become more regular with increasing pressure. In orthorhombic perovskite phase, the structural distortion deviated from the ideal cubic perovskite is enhanced at higher pressure. Based on the non-spin-polarized calculations, perovskite phase MnTiO3 is predicted to dissociate into Fm3¯m-MnO+P21/c-MnTi2O5 at 29 GPa.

Zhu, Feng; Wu, Xiang; Qin, Shan

2012-06-01

283

First principles calculations for optical absorption spectra and polarizabilities of atomic clusters  

NASA Astrophysics Data System (ADS)

Electronic and structural calculations for atomic clusters present formidable challenges for traditional theoretical methods. While the computational framework for ground state properties of clusters is relatively well established, calculations for excited states remain notoriously difficult. This thesis presents rigorous, first principles calculations for polarizabilities and absorption spectra of atomic clusters. Our work employs a density-functional pseudopotential approach for solving the Kohn-Sham equations in real space. The polarizabilities are computed using a finite field method. The finite field method is based on ground state calculations for a perturbed system and is less computationally demanding than techniques calculating electronic excitations explicitly. To compute excitation energies and optical absorption spectra, we implement the time-dependent local density approximation (TDLDA). The TDLDA formalism incorporates electronic screening and relevant correlation effects for electronic excitations. As such, the TDLDA method represents a fully ab initio formalism for excited states that avoids many of the controversies associated with empirical or semi-empirical methods. Compared to other ab initio techniques for excited states, the TDLDA method requires considerably less computational effort and can be applied to much larger systems. We find the calculated polarizabilities, excitation energies, photoabsorption spectra, and optical absorption gaps for clusters to be in good agreement with experiment.

Vasiliev, Igor Valerievich

284

Efficient first-principles method for calculating the circular dichroism of nanostructures  

NASA Astrophysics Data System (ADS)

A first-principles method is developed to study the natural optical activity of nanostructures, making large-scale calculations of electronic circular dichroism feasible. Expressions to calculate circular dichroism using density-functional theory for finite and periodic systems are obtained and implemented within the SIESTA program package. To show the versatility and applicability of the method, the circular dichroism of the high fullerenes C76 and C78 is investigated. The results for these fullerenes show good consistency with previous semiempirical calculations, and a very good agreement with experiments. The method is generalized to treat periodic structures such as nanotubes, and the circular dichroism of the carbon single-wall nanotube (4,2) is studied, and the spectrum is interpreted in terms of its electronic density of states. It is found that the calculated circular dichroism spectra can be used to discriminate among different nanostructures through optical activity experiments. It is concluded that this methodology provides theoretical support for the quantification, understanding, and prediction of chirality and its measurement in nanostructures. It is expected that this information would be useful to motivate further experimental studies and the interpretation of natural optical activity in terms of the electronic circular dichroism in nanostructures.

Hidalgo, Francisco; Sánchez-Castillo, A.; Noguez, Cecilia

2009-02-01

285

Pressure effect on elastic, lattice dynamic and superconducting behaviour of yttrium sulfide: A first principle study  

NASA Astrophysics Data System (ADS)

First principles calculations have been carried out to analyze structural, elastic, and dynamic stability of yttrium sulphide (YS) under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of compression suggests the B1 ? B2 transition at ˜49 GPa (the same transition occurs at ˜48 GPa at 300 K). Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state. The single crystal elastic constants derived from the energy strain method agree well with the experimental values. The activation barrier between B1 and B2 phases calculated at transition point is ˜17/mRy/f.u. Our lattice dynamic calculations show that at ambient condition, the B1 phase is lattice dynamically stable, and frequencies of phonon modes in different high symmetry directions of Brillouin zone agrees well with experimental values. The B2 phase also is dynamical stable at ambient condition as well as at ˜49 GPa, supporting our static lattice calculation. The effect of temperature on volume and bulk modulus of the YS in B1 phase has also been examined. The superconducting temperature of ˜2.78 K determine at zero pressure agrees well with experimental data. The effect of pressure is found to suppress the superconducting nature of this material.

Sahoo, B. D.; Joshi, K. D.; Gupta, Satish C.

2014-03-01

286

Exciton diffusion in disordered small molecules for organic photovoltaics: insights from first-principles simulations  

NASA Astrophysics Data System (ADS)

Exciton diffusion in small molecules 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione [DPP(TBFu)2] is studied using first-principles simulations. We have examined dependence of exciton diffusion on structure disorder, temperature and exciton energy. We find that exciton diffusion length and diffusivity increase with structural order, temperature and the initial exciton energy. Compared to conjugated polymer poly(3-hexylthiophene) (P3HT), DPP(TBFu)2 small molecules exhibit a much higher exciton diffusivity, but a shorter lifetime. The exciton diffusion length in DPP(TBFu)2 is 50% longer than that in P3HT, yielding a higher exciton harvesting efficiency; the physical origin behind these differences is discussed. The time evolutions of exciton energy, electron-hole distance, and exciton localization are explored, and the widely speculated exciton diffusion mechanism is confirmed theoretically. The connection between exciton diffusion and carrier mobilities is also studied. Finally we point out the possibility to estimate exciton diffusivity by measuring carrier mobilities under AC electric fields.

Li, Z.; Zhang, X.; Lu, G.

2014-05-01

287

High pressure phase transformation in yttrium sulfide(YS): A first principle study  

NASA Astrophysics Data System (ADS)

First principles calculations have been carried out to analyze structural, elastic and dynamic stability, of YS under hydrostatic compression. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of compression suggests the B1?B2 transition at ˜ 49 GPa. Various physical quantities such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus have been derived from the theoretically determined equation of state. The single crystal elastic constants derived from the energy strain method agree well with the experimental values. The activation barrier between B1 and B2 phases calculated at transition point is ˜ 17/mRy/formula unit. Our lattice dynamic calculations show that at ambient condition, the B1 phase is lattice dynamically stable and frequencies of phonon modes in different high symmetry directions of Brillouin zone agrees well with experimental values. The B2 phase also is dynamical stable at ambient condition as well as at ˜ 49 GPa, supporting our static lattice calculation.

Sahoo, B. D.; Joshi, K. D.; Gupta, Satish C.

2014-04-01

288

First-principles prediction of insertion potentials in Li-Mn oxides for secondary Li batteries  

SciTech Connect

First-principles methods have started to be widely used in materials science for the prediction of properties of metals, alloys, and compounds. In this study, the authors demonstrate how first-principles methods can be used to predict the average open-circuit voltage that can be obtained from a lithium battery with spinel or layered manganese oxides used as the cathode. For this purpose the authors combine a basic thermodynamical model with the ab initio pseudopotential method. The good agreement between the computed and experimental average output potentials suggests that first-principles methods can be an important tool to design novel battery materials.

Aydinol, M.K.; Ceder, G. [Massachusetts Inst. of Tech., Cambridge, MA (United States). Dept. of Materials Science and Engineering

1997-11-01

289

Computational study of graphene growth on copper by first-principles and kinetic Monte Carlo calculations.  

PubMed

In this work the growth of a graphene monolayer on copper substrate, as typically achieved via chemical vapor deposition of propene (C3H6), was investigated by first-principles and kinetic Monte Carlo calculations. A comparison between calculated C1s core-level binding energies and electron spectroscopy measurements showed that graphene nucleates from isolated carbon atoms adsorbed on surface defects or sub-superficial layers upon hydrocarbon fragmentation. In this respect, ab initio nudged elastic band simulations yield the energetic barriers characterizing the diffusion of elemental carbon on the Cu(111) surface and atomic carbon uptake by the growing graphene film. Our calculations highlight a strong interaction between the growing film edges and the copper substrate, indicative of the importance of the grain boundaries in the epitaxy process. Furthermore, we used activation energies to compute the reaction rates for the different mechanisms occurring at the carbon-copper interface via harmonic transition state theory. Finally, we simulated the long-time system growth evolution through a kinetic Monte Carlo approach for different temperatures and coverage. Our ab initio and Monte Carlo simulations of the out-of-equilibrium system point towards a growth model strikingly different from that of standard film growth. Graphene growth on copper turns out to be a catalytic, thermally-activated process that nucleates from carbon monomers, proceeds by adsorption of carbon atoms, and is not self-limiting. Furthermore, graphene growth seems to be more effective at carbon supersaturation of the surface-a clear fingerprint of a large activation barrier for C attachment. Our growth model and computational results are in good agreement with recent X-ray photoelectron spectroscopy experimental measurements. PMID:24939464

Taioli, Simone

2014-07-01

290

Lattice Dynamics of Beryllium from a First-Principles Nonlocal Pseudopotential Approach.  

National Technical Information Service (NTIS)

The lattice dynamics of beryllium, a metal with hexagonal close-packed structure and two atoms per unit cell, is investigated within the framework of Harrison's first-principles pseudopotential theory, using (1) the Slater approximation for the conduction...

P. H. Cutler W. F. King

1970-01-01

291

First Principles Pseudopotential Calculations of Electronic and Atomic Properties of Solid and Liquid Alkali Metals.  

National Technical Information Service (NTIS)

First principles fully nonlocal pseudopotentials are constructed for the alkali metals Li, Na, and K. The orthogonalization hole contribution to the pseudopotential is treated exactly and comparison with approximate treatments show significant differences...

R. S. Day

1978-01-01

292

The Lattice Dynamics of Zinc from a Generalized First Principles Pseudopotential Approach.  

National Technical Information Service (NTIS)

The elastic shear constants, and phonon dispersion curves in three symmetry directions, are calculated for zinc and compared to experimental values. Harrison's generalized first principle pseudopotential theory in the second order, nonlocal form is used t...

J. K. G. Panitz

1975-01-01

293

First Principle Pseudopotential Calculation of the Elastic Shear Constants of Magnesium.  

National Technical Information Service (NTIS)

A formalism is developed for the calculation of the elastic shear constants of hexagonal close-packed metals from Harrison's first principle non-local pseudopotential theory. The modification of the dielectric function to include conduction-electron excha...

P. H. Cutler W. F. King

1970-01-01

294

First-principles Calculations of Twin-boundary and Stacking-fault Energies in Magnesium.  

National Technical Information Service (NTIS)

The interfacial energies of twin boundaries and stacking faults in metal magnesium have been calculated using first-principles supercell approach. Four types of twin boundaries and two types of stacking faults are investigated, namely, those due to the mi...

L. Chen S. N. Mathaudhu Y. Wang Z. Liu

2010-01-01

295

First-principles pseudopotential total-energy calculations for elemental, compound and alloy semiconductors  

Microsoft Academic Search

The first-principles pseudopotential total-energy calculation within the local density functional formalism has been performed for several types of semiconductors: Si, GaAs, AlAs, and Al(x)Ga(1-x)As. The calculated lattice constant, bulk modulus, cohesive energy, and overall energy-band structure are in excellent agreement with the experimental data. Both the hard core and the soft core pseudopotentials are constructed from the first principles, and

Atsushi Oshiyama; Mineo Saito

1987-01-01

296

First-principles study of hydrogen ordering in beta-YH2+x  

Microsoft Academic Search

The phase stability is studied for the beta-phase YH2+x system based on first-principles total energy calculations. Our study predicts that the D022, ``40'', and D1a structures are stable near x=0.25, 0.5, and 0.8, respectively. Using the effective cluster interactions obtained from the first-principles total-energy data, the phase diagram for the D022 and ``40'' ordered phases is calculated by the cluster

Sheng N. Sun; Yan Wang; M. Y. Chou

1994-01-01

297

Impact of strain on the surface properties of transition metal carbide films: First-principles study  

Microsoft Academic Search

The effect of in-plane lattice strain on the atomic and electronic properties of low-index transition metal (M=Ti, Nb, and Ta) carbide surfaces is studied by first-principles molecular dynamics calculations using a pseudopotential plane-wave technique. The most stable cubic rock-salt phase is considered for carbides. The first-principle study of various [(001), (110), and metal-terminated (111)] carbide surfaces reveals that both compressive

D. I. Bazhanov; I. V. Mutigullin; A. A. Knizhnik; B. V. Potapkin; A. A. Bagaturyants; L. R. C. Fonseca; M. W. Stoker

2010-01-01

298

A first principles calculations and experimental study of the ground- and excited-state properties of ladder oligo(p-aniline)s  

NASA Astrophysics Data System (ADS)

The molecular structure of three ladder oligo(p-aniline)s, 5,11-diethyl-6,12-dimethylindolo[3,2-b]carbazole (DIMER 2P), 14-ethyl-5,8-dihydro-diindolo[3,2-b:2',3'-h]carbazole (TRIMER 2P), and 5,8,14-triethyl-diindolo[3,2-b:2',3'-h]carbazole (TRIMER 3P) were investigated by first principles calculations at the Hartree-Fock (HF/6-31G*) and density functional theory (DFT/B3LYP/6-31G*) levels. It is found that the agreement between theoretical and x-ray geometrical parameters is good and rather similar for both theoretical methods. The nature and the energy of the first two singlet-singlet electronic transitions have been obtained by Zerner intermediate neglect of differential overlap/spectroscopy semiempirical calculations performed on the HF/6-31G* and DFT/B3LYP/6-31G* optimized geometries, as well as time-dependent density functional theory (TDDFT) calculations performed on the DFT/B3LYP/6-31G* optimized structures. For all the compounds and for all the theoretical approaches, it is observed that the S1<--S0 electronic transition (??*) is weakly allowed and polarized along the short axis (y) of the molecule. On the other hand, the S2<--S0 electronic transition of each oligomer possesses a much larger oscillator strength and is polarized along the long (x) molecular axis. It is found that TDDFT calculations provide the best overall agreement between the energies and the corresponding optical transitions obtained from the absorption bands (0-0 peaks) measured in dichloromethane as well as providing a good evaluation of the bathochromic shifts caused by the increase in the conjugation length or by the presence of extra alkyl chains on the nitrogen atoms in TRIMER 3P compared to TRIMER 2P.

Belletête, Michel; Durocher, Gilles; Hamel, Sébastien; Côté, Michel; Wakim, Salem; Leclerc, Mario

2005-03-01

299

Edge reconstructions of hexagonal boron nitride nanoribbons: A first-principles study  

NASA Astrophysics Data System (ADS)

The edge reconstructions of hexagonal boron nitride nanoribbons (BNNRs) and their stabilities have been investigated by the first-principles calculations of both their binding and edge energies. It is found from our calculations that the binding energy we have used is a reliable and useful quantity for judging the stabilities of different edge reconstructions of the hetero-elemental BNNRs instead of the conventional edge energy one, especially for those BNNR's edges with unequal number of B and N atoms. In addition, other four main results have been obtained: (1) the armchair BNNR is the most stable edge structure and the characteristic ac-48 edge reconstruction for the BNNRs is predicted to be the second most stable edge in all the discussed BNNR edge structures. But, its zigzag edge is less stable. (2) The zigzag-like ac-56 type reconstructions are more stable than the pristine zigzag structures, which is different from that of the graphene nanoribbon (GNR), being less stable than the zigzag GNR. (3) The stabilities of BNNR's ac-677 and ac-678 type edge reconstructions lie between its ac-56-B and zz-57-B edges. (4) The zz-57-B(-N) edge reconstruction lowers the edge energy by a small quantity, which is also different from that of GNR. Moreover, the zz-57-B edge structure is more stable than the zz-57-N one, indicating that B-rich edge is easier to be reconstructed while the N-rich edge is more stable. Our theoretical calculations suggest the many possible reconstructed edge structures for the bare BNNRs, which is important for BNNR's application in future nanoelectronics and spintronics.

Hu, Ting; Han, Yang; Dong, Jinming

2013-12-01

300

First-principles thermoelasticity of transition metals at high pressure: Tantalum prototype in the quasiharmonic limit  

NASA Astrophysics Data System (ADS)

The thermoelastic properties of tantalum have been investigated over its theoretical high-pressure bcc solid phase (up to 26000K at 10Mbar ) using an advanced first-principles approach that accurately accounts for cold, electron-thermal, and ion-thermal contributions in materials where anharmonic effects are small. Specifically, we have combined ab initio full-potential linear-muffin-tin-orbital electronic-structure calculations for the cold and electron-thermal contributions to the elastic moduli with phonon contributions for the ion-thermal part calculated using model generalized pseudopotential theory. For the latter, a summation of terms over the Brillouin zone is performed within the quasiharmonic approximation, where each term is composed of a strain derivative of the phonon frequency at a particular k point. At ambient pressure, the resulting temperature dependence of the Ta elastic moduli is in excellent agreement with ultrasonic measurements. The experimentally observed anomalous behavior of C44 at low temperatures is shown to originate from the electron-thermal contribution. At higher temperatures, the main contribution to the temperature dependence of the elastic moduli comes from thermal expansion, but inclusion of the electron- and ion-thermal contributions is essential to obtain quantitative agreement with experiment. In addition, the pressure dependence of the moduli at ambient temperature compares well with recent diamond-anvil-cell measurements to 1.05Mbar . Moreover, the calculated longitudinal and bulk sound velocities in polycrystalline Ta at higher pressure and temperature in the vicinity of shock melting (˜3Mbar) agree well with data obtained from shock experiments. However, at high temperatures along the melt curve above 1Mbar , the B' shear modulus becomes negative, indicating the onset of unexpectedly strong anharmonic effects. Finally, the assumed temperature dependence of the Steinberg-Guinan strength model obtained from scaling with the bulk shear modulus is examined at ambient pressure.

Orlikowski, Daniel; Söderlind, Per; Moriarty, John A.

2006-08-01

301

Nonlinear Socio-Ecological Dynamics and First Principles ofCollective Choice Behavior of ``Homo Socialis"  

NASA Astrophysics Data System (ADS)

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.

Sonis, M.

302

Composition-dependent bonding and hardness of γ-aluminum oxynitride: A first-principles investigation  

NASA Astrophysics Data System (ADS)

Spinel phase aluminum oxynitride solid solution (?-alon, with formula of Al(8+x)/3O4-xNx) exists in the narrow Al2O3-rich region of Al2O3-AlN systems. The first-principles calculations were developed to investigate the composition-dependent bonding and hardness of ?-alon. Six supercell model for Al(8+x)/3O4-xNx (x = 0, 0.25, 0.44, 0.63, 0.81, and 1) was constructed to perform our calculations with high accuracy. It was found that the lattice constant increases with increasing composition of nitrogen in ?-alon. The bond lengths of AlIV-O, AlVI-O, AlIV-N, and AlVI-N all increase with the expansion of crystal structure. The well-known Mulliken overlap populations were calculated to estimate the bonding and hardness. As the content of nitrogen substitute increases, the Al-N bonds present more covalent characteristic, while the Al-O bonds present more ionic characteristic. The AlIV-N is the hardest bond in ?-alon. The theoretical hardness of ?-alon could be slightly enhanced from 17.16 GPa to 17.97 GPa by increasing content of nitrogen in full solubility range. The contribution ratio, CH?, was proposed to quantify the contribution of bonds to hardness of ?-alon. The Al-O bonds are found to contribute more to the hardness. The Al-N bonds are the main influencing factor to enhance the hardness of ?-alon. These calculated results provide the basis for understanding the composition-dependent bonding and hardness of ?-alon.

Tu, Bingtian; Wang, Hao; Liu, Xiao; Khan, Shahzad A.; Wang, Weimin; Fu, Zhengyi

2014-06-01

303

First-principles thermodynamic framework for the evaluation of thermochemical H2O - or CO2 -splitting materials  

NASA Astrophysics Data System (ADS)

We present an analysis of the equilibrium thermodynamics of two-step metal oxide-based water and carbon dioxide-splitting cycles. Within this theoretical framework, we propose a first-principles computational approach based on density-functional theory (DFT) for evaluating new materials for these cycles. Our treatment of redox-based gas-splitting chemistry is completely general so that the thermodynamic conclusions herein hold for all materials used for such a process and could easily be generalized to any gas as well. We determine the temperature and pressure regimes in which the thermal reduction (TR) and gas-splitting (GS) steps of these cycles are thermodynamically favorable in terms of the enthalpy and entropy of oxide reduction, which represents a useful materials design goal. We show that several driving forces, including low TR pressure and a large positive solid-state entropy of reduction of the oxide, have the potential to enable future, more promising two-step gas-splitting cycles. Finally, we demonstrate a practical computational methodology for efficiently screening new materials for gas-splitting applications and find that first-principles DFT calculations can provide very accurate predictions of high-temperature thermodynamic properties relevant to gas splitting.

Meredig, B.; Wolverton, C.

2009-12-01

304

First-Principles-Based Development of Kinetic Mechanisms in Chemically Active Light-Emitting Nonthermal Plasmas and Gases  

SciTech Connect

Recent progress in several related research areas such as first-principles electronic-structure calculations of atoms and diatomic molecules, theory of elementary processes, kinetics, and numerical engineering, and also continued exponential growth in computational resources enhanced by recent advances in massively parallel computing have opened the possibility of directly designing kinetics mechanisms to describe chemical processes and light emission in such complex media as nonequilibrium plasmas and reacting gases. It is important that plasma and combustion kinetics can be described in the framework of this direct approach to a sufficiently high accuracy, which makes it an independent predictive research tool complementary to experimental techniques. This paper demonstrates the capabilities of the first-principles based approach to develop kinetic mechanisms. Two examples are discussed in detail: (1) the mechanism of hydrocarbon fuel combustion at high temperatures and (2) light emission in non-thermal glow discharge plasma of metal halides; special attention is paid to a comparison of the results obtained at every level of system description with the appropriate experimental data. In house software tools that can be used in such multilevel theoretical works are discussed as well.

Astapenko, Valerie; Bagatur'yants, Alexander; Chernishova, Irina; Deminsky, Maxim; Eletskii, Alexander; Knizhnik, Andrei; Potapkin, Boris; Rykova, Elena; Umanskii, Stanislaw; Zaitsevskii, Andrei; Safonov, Andrei [Kinetics Technologies, Moscow (Russian Federation); Kirillov, Igor; Strelkova, Marina; Sukhanov, Leonid [RRC Kurchatov Institute, Moscow (Russian Federation); Cotzas, George M.; Dean, Anthony; Michael, J. Darryl; Midha, Vikas; Smith, David J.; Sommerer, Timothy J. [GE Global Research, Niskayuna, New York (United States)] (and others)

2007-04-06

305

Thermal physics of the lead chalcogenides PbS, PbSe, and PbTe from first principles  

NASA Astrophysics Data System (ADS)

The lead chalcogenides represent an important family of functional materials, in particular due to the benchmark high-temperature thermoelectric performance of PbTe. A number of recent investigations, experimental and theoretical, have aimed to gather insight into their unique lattice dynamics and electronic structure. However, the majority of first-principles modeling has been performed at fixed temperatures, and there has been no comprehensive and systematic computational study of the effect of temperature on the material properties. We report a comparative lattice-dynamics study of the temperature dependence of the properties of PbS, PbSe, and PbTe, focusing particularly on those relevant to thermoelectric performance, viz. phonon frequencies, lattice thermal conductivity, and electronic band structure. Calculations are performed within the quasiharmonic approximation, with the inclusion of phonon-phonon interactions from many-body perturbation theory, which are used to compute phonon lifetimes and predict the lattice thermal conductivity. The results are critically compared against experimental data and other calculations, and add insight to ongoing research on the PbX compounds in relation to the off-centering of Pb at high temperatures, which is shown to be related to phonon softening. The agreement with experiment suggests that this method could serve as a straightforward, powerful, and generally applicable means of investigating the temperature dependence of material properties from first principles.

Skelton, Jonathan M.; Parker, Stephen C.; Togo, Atsushi; Tanaka, Isao; Walsh, Aron

2014-05-01

306

First Principles Density Functional Investigation of Supported Tungsten Cluster (Wn; n = 1 TO 6) on Anchored Graphite (0001) Surface  

NASA Astrophysics Data System (ADS)

Size-selected Wn clusters can be deposited firmly on a graphite (0001) surface using a novel technique, where the positive ions (of the same metal atom species) embedded on the graphite surface by ion implantation, act as anchors. The size selected metal clusters can then soft land on this anchored surface m [Hayakawa et al., 2009]. We have carried out a systematic theoretical study of the adsorption of Wn (n = 1-6) clusters on anchored graphite (0001) surface, using state-of-art spin-polarized density functional approach. In our first-principles calculations, the graphite (0001) surface has been suitably modeled as a slab separated by large vacuum layers. Wn clusters bond on clean graphite (0001) surface with a rather weak Van-der-Waals interaction. However, on the anchored graphite (0001) surface, the Wn clusters get absorbed at the defect site with a much larger adsorption energy. We report here the results of our first-principles investigation of this supported Wn cluster system, along with their reactivity trend as a function of the cluster size (n).

Barman, Sonali; Das, G. P.; Kawazoe, Y.

2013-12-01

307

First-principles calculation of pKa for cocaine, nicotine, neurotransmitters, and anilines in aqueous solution.  

PubMed

The absolute pKa values of 24 representative amine compounds, including cocaine, nicotine, 10 neurotransmitters, and 12 anilines, in aqueous solution were calculated by performing first-principles electronic structure calculations that account for the solvent effects using four different solvation models, i.e., the surface and volume polarization for electrostatic interaction (SVPE) model, the standard polarizable continuum model (PCM), the integral equation formalism for the polarizable continuum model (IEFPCM), and the conductor-like screening solvation model (COSMO). Within the examined computational methods, the calculations using the SVPE model lead to the absolute pKa values with the smallest root-mean-square-deviation (rmsd) value (1.18). When the SVPE model was replaced by the PCM, IEFPCM, and COSMO, the rmsd value of the calculated absolute pKa values became 3.21, 2.72, and 3.08, respectively. All types of calculated pKa values linearly correlate with the experimental pKa values very well. With the empirical corrections using the linear correlation relationships, the theoretical pKa values are much closer to the corresponding experimental data and the rmsd values become 0.51-0.83. The smallest rmsd value (0.51) is also associated with the SVPE model. All of the results suggest that the first-principles electronic structure calculations using the SVPE model are a reliable approach to the pKa prediction for the amine compounds. PMID:17691837

Lu, Haiting; Chen, Xi; Zhan, Chang-Guo

2007-09-01

308

Investigation of Ferroelectric Phase Transition in DMAAS Crystals: Neutron Diffraction, Neutron Spectroscopy, Theoretical Model  

Microsoft Academic Search

The aim of the present work is the comparative analysis of data of neutron diffraction investigation of DMAAS ((CH3)2NH2Al(SO4)2·6H2O) single crystals in paraelectric and ferroelectric phases at 293 and 135 K, neutron spectroscopy investigation of polycrystalline samples of DMAAS in a wide temperature range above and below the ferroelectric phase transition point and theoretical calculations with the purpose of ascertainment

V. Yu. Kazimirov; V. A. Sarin; M. B. Smirnov; L. A. Shuvalov

2004-01-01

309

First-Principles Simulation of Amide and Aromatic Side Chain Ultraviolet Spectroscopy of a Cyclic Dipeptide  

PubMed Central

By combining time-dependent density functional theory (TDDFT) and molecular dynamics (MD) simulations, we calculate the ultraviolet absorption and circular dichroism (CD) of a cyclic dipeptide, cyclo(L-Pro-D-Tyr), in the 185?300 nm region. The absorption is dominated by the phenol chromophore of tyrosine. The CD spectrum shows both phenol and amide units transitions. A crude coherent two dimensional ultraviolet spectrum (2DUV) calculated by neglecting the two-excitation states shows a cross peak between two transitions of the phenol in the tyrosine side chain. Additional cross peaks between the side chain and the backbone are observed when using a chirality-induced pulse polarization configuration.

Li, Zhenyu; Mukamel, Shaul

2008-01-01

310

Investigation of Halogenated Pyrimidines by X-ray Photoemission Spectroscopy and Theoretical DFT Methods  

NASA Astrophysics Data System (ADS)

The inner shell ionization of pyrimidine and some halogenated pyrimidines has been investigated experimentally by X-ray photoemission spectroscopy (XPS) and theoretically by density functional theory (DFT) methods. The selected targets-5-Br-pyrimidine, 2-Br-pyrimidine, 2-Cl-pyrimidine, and 5-Br-2-Cl-pyrimidine-allowed the study of the effect of the functionalization of the pyrimidine ring by different halogen atoms bound to the same molecular site, or by the same halogen atom bound to different molecular sites. The theoretical investigation of the inductive and resonance effects in the C(1s) ionization confirms the soundness of the resonance model for a qualitative description of the properties of an aromatic system. Moreover, the combination of the experimental results and the theoretical analysis provides a detailed description of the effects of the halogen atom on the screening of a C(1s) hole in the aromatic pyrimidine ring.

Bolognesi, P.; Mattioli, G.; O'Keeffe, P.; Feyer, V.; Plekan, O.; Ovcharenko, Y.; Prince, K. C.; Coreno, M.; Amore Bonapasta, A.; Avaldi, L.

2009-11-01

311

Interfacial structures and energetics of basal twins in alpha-Al2O3: First-principles density-functional and empirical calculations  

Microsoft Academic Search

Interfacial energetics, local ionic coordinations, and electronic states at basal twin interfaces in alpha-Al2O3 (corundum) were investigated by means of first-principles density-functional and empirical shell-model calculations. Three different metastable twin configurations were determined by both theoretical approaches, which exhibit mirror, twofold-rotation, and glide-mirror symmetries, respectively, with respect to the basal interface planes. In all three metastable configurations important changes in

A. G. Marinopoulos; S. Nufer; C. Elsässer

2001-01-01

312

Connecting microscopic structure and macroscopic mechanical properties of structural materials from first-principles  

NASA Astrophysics Data System (ADS)

Computer simulation plays a critical role in connecting microscopic structure and macroscopic mechanical properties of structural material, which is a key factor that needs to be considered in design of such kind of material. Via the quantum mechanics first-principles calculations, one can gain structure, elastic constant, energetics, and stress of selected material system, based on which one is able to predict the mechanical properties or provide useful insights for the mechanical properties of the materials. This can be done either directly or in combination with the empirical criterions. This paper reviews the recent research advances on the attempts to predict the mechanical properties of structural materials from first principles.

Lu, GuangHong; Zhang, Lei

2012-12-01

313

First-principles study of hydrogen ordering in β-YH{sub 2+{ital x}}  

Microsoft Academic Search

The phase stability is studied for the β-phase YH{sub 2+{ital x}} system based on first-principles total energy calculations. Our study predicts that the D0ââ, ``40``, and D1{sub {ital a}} structures are stable near {ital x}=0.25, 0.5, and 0.8, respectively. Using the effective cluster interactions obtained from the first-principles total-energy data, the phase diagram for the D0ââ and ``40`` ordered phases

S. N. Sun; Y. Wang; M. Y. Chou

1994-01-01

314

Growth mechanisms of ZnO(0001) investigated using the first-principles calculation  

SciTech Connect

We investigated the dynamics of zinc (Zn) and oxygen (O) adsorbed atoms (adatoms) on a Zn-polar ZnO(0001) surface using the first-principles calculation. The results of the first-principles calculation revealed that a high-quality ZnO crystalline growth condition is induced by wurtzite structure packing under a Zn-rich growth condition using a Zn-polar ZnO(0001) surface. However, it was shown that an O adatom is not sufficient to promote surface atomic diffusion. For high-quality ZnO crystal, promoting surface diffusion of adatoms using high temperature is important.

Fujiwara, Katsutoshi; Ishii, Akira [Department of Applied Mathematics and Physics, Tottori University, Koyama, Tottori 680-8552 (Japan); Abe, Tomoki; Ando, Koshi [Department of Electrical and Electronic Engineering, Tottori University, Koyama, Tottori 680-8552 (Japan)

2012-09-15

315

First-principles calculations of the elastic constants of the cubic, orthorhombic and hexagonal phases of BaF2  

NASA Astrophysics Data System (ADS)

All the elastic constants of cubic, orthorhombic and hexagonal phases of BaF2 have been calculated using first principles methods. We have employed density-functional theory within generalized gradient approximation (GGA) using a plane-wave pseudopotentials method and a plane-wave basis set. The calculated elastic constant values for a cubic phase compare well with recent theoretical and experimental calculations. The bulk modulus derived from the elastic constant calculations of orthorhombic phase of BaF2 is 94.5 GPa and those of hexagonal phase is 161 GPa. These values are in good agreement with experimental data available. Stability of these phases of BaF2 is also estimated in different crystallographic directions.

Nyawere, P. W. O.; Makau, N. W.; Amolo, G. O.

2014-02-01

316

Unified first-principles study of gilbert damping, spin-flip diffusion, and resistivity in transition metal alloys.  

PubMed

Using a formulation of first-principles scattering theory that includes disorder and spin-orbit coupling on an equal footing, we calculate the resistivity ?, spin-flip diffusion length l(sf), and Gilbert damping parameter ? for Ni(1-x)Fe(x) substitutional alloys as a function of x. For the technologically important Ni(80)Fe(20) alloy, Permalloy, we calculate values of ? = 3.5 ± 0.15 ?? cm, l(sf) = 5.5 ± 0.3 nm, and ? = 0.0046 ± 0.0001 compared to experimental low-temperature values in the range 4.2-4.8 ?? cm for ?, 5.0-6.0 nm for l(sf), and 0.004-0.013 for ?, indicating that the theoretical formalism captures the most important contributions to these parameters. PMID:21231490

Starikov, Anton A; Kelly, Paul J; Brataas, Arne; Tserkovnyak, Yaroslav; Bauer, Gerrit E W

2010-12-01

317

Corrosion related properties of iron (100) surface in liquid lead and bismuth environments: A first-principles study  

NASA Astrophysics Data System (ADS)

The corrosion of steels in liquid metal lead (Pb) and bismuth (Bi) is a critical challenge in the development of accelerator driven systems (ADS). Using a first-principles method with a slab model, we theoretically investigate the interaction between the Pb (Bi) atom and the iron (Fe) (100) surface to assess the fundamental corrosion properties. Our investigation demonstrates that both Pb and Bi atoms favorably adsorb on the (100) surface. Such an adsorption decreases the energy required for the dissociation of an Fe atom from the surface, enhancing the dissolution tendency significantly. The segregation of six common alloying elements (Cr, Al, Mn, Ni, Nb, and Si) to the surface and their impacts on the corrosion properties are also considered. The present results reveal that Si seems to have a relatively good performance to stabilize the surface and alleviate the dissolving trend caused by Pb and Bi.

Song, Chi; Li, Dong-Dong; Xu, Yi-Chun; Pan, Bi-Cai; Liu, Chang-Song; Wang, Zhi-Guang

2014-05-01

318

Structural and elastic properties of TiN and AlN compounds: first-principles study  

NASA Astrophysics Data System (ADS)

First-principles calculations of the lattice constants, bulk modulus, pressure derivatives of the bulk modulus and elastic constants of AlN and TiN compounds in rock-salt (B1) and wurtzite (B4) structures are presented. We have used the fullpotential linearized augmented plane wave (FP-LAPW) method within the density functional theory (DFT) in the generalized gradient approximation (GGA) for the exchange-correlation functional. Moreover, the elastic properties of cubic TiN and hexagonal AlN, including elastic constants, bulk and shear moduli are determined and compared with previous experimental and theoretical data. Our results show that the structural transition at 0 K from wurtzite to rock-salt phase occurs at 10 GPa and -26 GPa for AlN and TiN, respectively. These results are consistent with those of other studies found in the literature.

Fodil, Meriem; Mounir, Amine; Ameri, Mohammed; Baltache, Hadj; Bouhafs, Bachir; Al-Douri, Y.; Ameri, Ibrahim

2014-05-01

319

Structural, electronic, optical, elastic properties and Born effective charges of monoclinic HfO2 from first-principles calculations  

NASA Astrophysics Data System (ADS)

First-principles calculations of structural, electronic, optical, elastic, mechanical properties, and Born effective charges of monoclinic HfO2 are performed with the plane-wave pseudopotential technique based on the density-functional theory. The calculated structural properties are consistent with the previous theoretical and experimental results. The electronic structure reveals that monoclinic HfO2 has an indirect band gap. The analyses of density of states and Mulliken charges show mainly covalent nature in Hf-O bonds. Optical properties, including the dielectric function, refractive index, extinction coefficient, reflectivity, absorption coefficient, loss function, and optical conductivity each as a function of photon energy are calculated and show an optical anisotropy. Moreover, the independent elastic constants, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, compressibility, Lamé constant, sound velocity, Debye temperature, and Born effective charges of monoclinic HfO2 are obtained, which may help to understand monoclinic HfO2 for future work.

Liu, Qi-Jun; Zhang, Ning-Chao; Liu, Fu-Sheng; Liu, Zheng-Tang

2014-04-01

320

First-principles investigation of the structural, magnetic, and electronic properties of olivine LiFeP O4  

NASA Astrophysics Data System (ADS)

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.

Shi, Siqi; Ouyang, Chuying; Xiong, Zhihua; Liu, Lijun; Wang, Zhaoxiang; Li, Hong; Wang, Ding-Sheng; Chen, Liquan; Huang, Xuejie

2005-04-01

321

First-principles study of the onset of noncollinearity in Mnn clusters: Magnetic arrangements in Mn5 and Mn6  

NASA Astrophysics Data System (ADS)

First-principles theoretical investigations of the noncollinearity of atomic spin moments in manganese clusters have been carried out within a gradient-corrected density-functional approach. Our studies on Mn5 and Mn6 include investigation of both collinear and noncollinear arrangements. It is shown that while the atomic structure of the ground state of Mn5 is a triangular bipyramid, the collinear and noncollinear arrangements have comparable energies and hence are degenerate. For Mn6 , while the ground state has a square bipyramid arrangement, the noncollinear configuration is most stable making it the smallest cluster to feature a noncollinear ground state. The results are discussed in view of the recent experimental Stern-Gerlach profiles.

Morisato, T.; Khanna, S. N.; Kawazoe, Y.

2005-07-01

322

First-principles study of elastic and thermodynamic properties of orthorhombic OsB4 under high pressure  

NASA Astrophysics Data System (ADS)

The first-principles study on the elastic properties, elastic anisotropy and thermodynamic properties of the orthorhombic OsB4 is reported using density functional theory method with the ultrasoft pseudopotential scheme in the frame of the generalized gradient approximation. The calculated equilibrium parameters are in good agreement with the available theoretical data. A complete elastic tensor and crystal anisotropies of the ultra-incompressible OsB4 are determined in the pressure range of 0-50 GPa. By the elastic stability criteria, it is predicted that the orthorhombic OsB4 is stable below 50 GPa. By using the quasi-harmonic Debye model, the heat capacity, the coefficient of thermal expansion, and the Grüneisen parameter of OsB4 are also successfully obtained in the present work.

Yan, Hai-Yan; Zhang, Mei-Guang; Huang, Duo-Hui; Wei, Qun

2013-04-01

323

Cation disorder in MgX2O4 (X = Al, Ga, In) spinels from first principles  

NASA Astrophysics Data System (ADS)

We have performed first-principles density functional theory calculations to investigate the possible physical origins of the discrepancies between the existing theoretical and experimental studies on cation distribution in MgX2O4 (X = Al, Ga, In) spinel oxides. We show that for MgGa2O4 and MgIn2O4, it is crucial to consider the effects of lattice vibrations to achieve agreement between theory and experiment. For MgAl2O4, we find that neglecting short-range order effects in thermodynamic modeling can lead to significant underestimation of the degree of inversion. Furthermore, we demonstrate that the common practice of representing disordered structures by randomly exchanging atoms within a small periodic supercell can incur large computational error due to either insufficient statistical sampling or finite supercell size effects.

Jiang, Chao; Sickafus, Kurt E.; Stanek, Christopher R.; Rudin, Sven P.; Uberuaga, Blas P.

2012-07-01

324

First-principles Study on the Electronic Structure of Novel Titanium Yttrium Mixed-metal Nitride Clusterfullerene  

NASA Astrophysics Data System (ADS)

We present a first-principles study on the geometric, vibrational and electronic properties of a novel Y-based non-scandium mixed-metal nitride clusterfullerene (TiY2N@C80). Theoretical results indicate that the fundamental electronic properties of TiY2N@C80 are similar to that of TiSc2N@C80, but dramatically different from that of Sc3N@C80 and Y3N@C80 molecules. We find that the magnetism of TiY2N@C80 is quenched by carrier doping. The rotation energy barrier of the TiY2N cluster in C80 cage was obviously increased by exohedral chemical modification with pyrrolidine monoadduct.

Li, Shu-juan; Lei, Shu-lai; Huang, Jing; Li, Qun-xiang

2011-08-01

325

Effect of N distribution on elastic and electronic properties of hexagonal ?-Fe6Nx by first-principles calculations  

NASA Astrophysics Data System (ADS)

In this paper, the effect of N distribution on the elastic and electronic properties of hexagonal ?-Fe6Nx (x=1, 2, and 3) phases is studied by first-principles calculations. The calculated formation energies of ?-Fe6Nx phases decrease with the increasing distance between the N atoms, which demonstrates that the interaction between the interstitial N atoms is of repulsive nature. The theoretical elastic constants, bulk modulus (B), shear modulus (G), ratios of B to G, Young's modulus (E), Poisson's ratios (v) of ?-Fe6Nx phases are obtained, which are important parameters for the crystal materials and surface coatings. The more stable interstitial conifiguration enables the ?-Fe6Nx phases to possess higher B, G and E. Densities of states are given for all the configurations of ?-Fe6Nx phases to discuss the origins of the variations of energetic stability and elastic properties.

Chen, H. T.; Yan, M. F.; You, Y.

2014-03-01

326

First principle study on generalized stacking fault energy and surface energy of B2-AgRE intermetallics  

NASA Astrophysics Data System (ADS)

The generalized stacking fault energies and surface energies for AgRE (RE=Sc, Tm, Dy, Tb, Ce) intermetallics with B2 structure have been investigate d using the first principle calculations. The Perdew-Burke-Ernzerhof exchange-correlation functional for the generalized-gradient-approximation is used and the electron-ion interaction is described by the full potential frozen-core projector augmented wave. The generalized stacking fault energy along <1 0 0>, <1 1 0> and <1 1 1> directions in {1 1 0} plane have been calculated. The ductility and brittleness of AgRE have been discussed based on the Rice criterion by using the ratio between surface energies and the unstable stacking fault energies. The ideal shear strength and the theoretical cleavage strength are also presented.

Liu, Lili; Wu, Xiaozhi; Wang, Rui; Wu, Shaohua; Feng, Huifang

2012-11-01

327

First Principles Absorption Spectra of Sin (n=20-28) Clusters: TDLDA versus Predictions from Mie Theory  

NASA Astrophysics Data System (ADS)

First-principles absorption spectra calculated within the time- dependent local-density approximation for Sin (n=20-28) clusters reveal that prolate and compact clusters have distinct shape signatures, but no clear size dependence for a given shape. The shape dependence and size independence of the spectra and most of the peak positions and intensities can be explained remarkably well within the classical Mie theory, developed for light absorption by metallic particles using the bulk dielectric function of Si. Moreover, the experimental spectrum of Si21 is in very good agreement with the theoretical spectrum of the prolate cluster, which is lower in energy than the compact one at this size. Idrobo, Jackson, Yang, and Ogut, Phys. Rev. B 74, 153410 (2006)

Jackson, Koblar; Idrobo, Juan C.; Ogut, Serdar; Yang, Mingli

2007-03-01

328

Simulation of temperature-compensated voltage limit curves for aerospace Ni?Cd batteries using a first principles' model  

NASA Astrophysics Data System (ADS)

Temperature-compensated voltage limits ( V/ T limits) are routinely used in Low Earth Orbit (LEO) satellites to permit fast charging of the Ni?Cd battery with minimum overcharge and without the problems of thermal runaway during overcharge. The voltage limits are experimentally determined from extensive testing of cells for a proper design of the charge control system to achieve the desired charge/discharge ratio in orbit. Here, we demonstrate the use of first-principles', mathematical models to construct the V/ T curves theoretically. The predicted charge/discharge ratios under various orbit conditions such as different states of charge, in-rush currents and temperatures are compared with the experimental data.

Ratnakumar, B. V.; Timmerman, P.; Di Stefano, S.

329

First-Principles Calculations of Structural, Elastic and Electronic Properties of Tetragonal HfO2 under Pressure  

NASA Astrophysics Data System (ADS)

Structural, elastic and electronic properties of tetragonal HfO2 at applied hydrostatic pressure up to 50 GPa have been investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The calculated ground-state properties are in good agreement with previous theoretical and experimental data. Six independent elastic constants of tetragonal HfO2 have been calculated at zero pressure and high pressure. From the obtained elastic constants, the bulk, shear and Young's modulus, Poisson's coefficients, acoustic velocity and Debye temperature have been calculated at the applied pressure. Band structure shows that tetragonal HfO2 is an indirect band gap. The variation of the gap versus pressure is well fitted to a quadratic function.

Liu, Qi-Jun; Liu, Zheng-Tang; Feng, Li-Ping

2011-10-01

330

Study of structural, elastic, electronic and optical properties of seven SrZrO3 phases: First-principles calculations  

NASA Astrophysics Data System (ADS)

On the plane-wave ultrasoft pseudopotential technique based on the first-principles density functional theory (DFT), we calculated the structural, elastic, electronic and optical properties of the seven different phases of SrZrO3. The obtained ground-state properties are in good agreement with previous experiments and calculations, which indicate that the most stable phase is orthorhombic Pnma structure. Seven phases of SrZrO3 are mechanically stable with cubic, tetragonal and orthorhombic structures. The mechanical and thermodynamic properties have been obtained by using the Voigt-Reuss-Hill approach and Debye-Grüneisen model. The electronic structures and optical properties are obtained and compared with the available experimental and theoretical data.

Liu, Qi-Jun; Liu, Zheng-Tang; Feng, Li-Ping; Tian, Hao

2012-12-01

331

First-principles study on lithium borohydride LiB H4  

Microsoft Academic Search

First-principles calculations have been performed on lithium borohydride LiB H4 using the ultrasoft pseudopotential method, which is a potential candidate for hydrogen storage materials due to its extremely large gravimetric capacity of 18 mass % hydrogen. We focus on an orthorhombic phase observed at ambient conditions and predict its fundamental properties; the structural properties, electronic properties, dielectric properties, vibrational properties,

Kazutoshi Miwa; Nobuko Ohba; Shin-Ichi Towata; Yuko Nakamori; Shin-Ichi Orimo

2004-01-01

332

Adsorption properties of chalcogen atoms on a golden buckyball Au16? from first principles  

Microsoft Academic Search

Using first-principles density functional theory, we investigate the adsorption properties of chalcogen elements (oxygen and sulfur) on an anionic golden nanocage Au16? and its effects on the structural and electronic properties of the golden cage. In particular, we find that when a sulfur atom is encapsulated inside Au16?, its bonding character with Au atoms appears ionic due to electron transfer

Seoung-Hun Kang; Gunn Kim; Young-Kyun Kwon

2011-01-01

333

First-principles study of atomic oxygen adsorption on boron-substituted graphite  

Microsoft Academic Search

Adsorption of atomic oxygen on boron-substituted graphite has been investigated using first-principles pseudopotential density functional method with the local density approximation. Different adsorption sites on periodic basal and prismatic surfaces have been investigated and compared. Generally, adsorption of atomic oxygen is most favorable on the zigzag surface and then armchair and basal surfaces. Mulliken population and density of states have

Qianku Hu; Qinghua Wu; Guang Sun; Xiaoguang Luo; Zhongyuan Liu; Bo Xu; Julong He; Yongjun Tian

2008-01-01

334

First-principles based LIDAR simulation environment for scenes with participating mediums  

Microsoft Academic Search

Employing a modified photon mapping technique that originated within the computer graphics community, a first-principle based elastic LIDAR model was developed within the Digital and Remote Sensing Image Generation (DIRSIG) framework that calculates time-gated photon counts at a sensor from topographic reflections and multiply scattered returns. The LIDAR module handles a wide variety of complicated scene geometries, diverse surface and

Daniel D. Blevins; Scott D. Brown; John R. Schott

2006-01-01

335

Towards first principles-based identification of ternary alloys for hydrogen purification membranes  

Microsoft Academic Search

Using dense films of metal alloys offers a useful path towards fabricating membranes for hydrogen purification that simultaneously exhibit high H2 fluxes and are chemically robust. Experimental identification of ternary alloys with these properties has been limited by the large resources needed to test multiple materials. We have considered whether first principles calculations could be used to screen ternary alloys

Preeti Kamakoti; David S. Sholl

2006-01-01

336

First principles studies of band structure and electronic properties of ZnSe  

Microsoft Academic Search

The structural and electronic properties of semiconductor ZnSe are investigated by performing first principles calculations using density functional theory (DFT). The exchange correlation potentials were treated within the local density approximation (LDA) and the generalized gradient approximation (GGA) with the quantum espresso package. We calculate the density of state (DOS), projected density of state (PDOS), phonon dispersion frequencies and the

B. I. Adetunji; P. O. Adebambo; G. A. Adebayo

337

First-principles molecular dynamics study on simple cubic calcium: comparison with simple cubic phosphorus  

Microsoft Academic Search

In this paper, the dynamical behavior of simple cubic (sc) calcium is investigated by first-principles molecular dynamics (MD) simulation at 40 GPa and 300 K and is compared with that of sc phosphorus. As a result, we found that calcium shows larger structural fluctuation through the MD simulation than phosphorus but the dynamical structures of calcium and phosphorus both become the sc

Takahiro Ishikawa; Hitose Nagara; Naoshi Suzuki; Katsuya Shimizu

2012-01-01

338

Quantitative predictions of tokamak energy confinement from first-principles simulations with kinetic effects  

Microsoft Academic Search

A first-principles model of anomalous thermal transport based on numericalsimulations is presented, with stringent comparisons to experimental datafrom the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324(1992)]. This model is based on nonlinear gyrofluid simulations, which predictthe fluctuation and thermal transport characteristics of toroidal iontemperature-gradient-driven (ITG) turbulence, and on comprehensive lineargyrokinetic ballooning calculations, which provide...

M. Kotschenreuther; W. Dorland; M. A. Beer; G. W. Hammett

1995-01-01

339

Applying Merrill's First Principles of Instruction: Practical Methods Based on a Review of the Literature  

ERIC Educational Resources Information Center

Research has shown that when Merrill's First Principles of Instruction are used as part of an instructional strategy, student learning increases. Several articles describe these principles of instruction, including specific methods for implementing this theory. However, because teachers and designers often have little time to design instruction,…

Gardner, Joel

2010-01-01

340

Calculation of structural pressure-induced phase transitions for magnesium using a local, first principles pseudopotential  

Microsoft Academic Search

A first principles, local pseudopotential is constructed from electronic densities which are calculated using the Hohenberg, Kohn and Sham formalism. With this pseudopotential the interionic potential is calculated for each value of compressed volume (which is taken using variations on the electronic density parameter, rs). From this potential, the corresponding phonon spectrum is obtained. The volume variation of the total

Gregorio Ruiz Chavarría

2005-01-01

341

Lattice Dynamics of Magnesium from a First-Principles Nonlocal Pseudopotential Approach.  

National Technical Information Service (NTIS)

Harrison's a priori theory is used to construct a first-principles nonlocal pseudopotential for magnesium, a metal with the hexagonal close-packed (hcp) structure and two atoms per unit cell. A response function for the exchange interaction among conducti...

P. H. Cutler W. F. King

1970-01-01

342

Harmonic phonons and phonon-limited resistivities for Rb and Cs from first-principle pseudopotentials  

Microsoft Academic Search

Harmonic phonons and phonon-limited resistivities have been calculated for Rb and Cs using pseudopotentials obtained from first principles. These pseudopotentials are highly non-local and the role of non-locality in influencing physical properties has been emphasised. Both phonons and phonon-limited resistivities are in excellent agreement with available experimental results.

R. Taylor; A. H. MacDonald

1980-01-01

343

On the role of the orthogonalization hole potential in Harrison's first principles pseudopotential theory  

Microsoft Academic Search

An analysis is made of the exact treatment for the orthogonalization hole potential, denoted by nu OPW*, in Harrison's first principle pseudopotential theory (1966, 1969). A corrected form of Harrison's expression for the approximation of nu OPW* is given and its effect on the calculation of phonon spectra for lithium, sodium and zinc are discussed. To demonstrate the effect of

P. H. Cutler; R. Day; W. F. King

1975-01-01

344

The lattice dynamics of zinc from a generalized first principles pseudopotential approach  

Microsoft Academic Search

The elastic shear constants, and phonon dispersion curves in three symmetry directions, are calculated for zinc and compared to experimental values. Harrison's generalized first principle pseudopotential theory in the second order, nonlocal form is used to calculate the electron gas modified ion-ion interaction energy. This 'generalized' pseudopotential method is applicable to d-band metals in that the usual small core approximation

J. K. G. Panitz

1975-01-01

345

First principles pseudopotential calculations of electronic and atomic properties of solid and liquid alkali metals  

Microsoft Academic Search

First principles fully nonlocal pseudopotentials are constructed for the alkali metals Li, Na, K. The orthogonalization hole contribution to the pseudopotential is treated exactly and comparison with approximate treatments shows significant differences. The pseudopotentials are used to calculate various solid and liquid properties of these alkalis. The phonon spectra and elastic shear constants are calculated for the solid metal. The

R. S. Day

1978-01-01

346

Electron transport in SiGe alloy nanowires in the ballistic regime from first-principles.  

PubMed

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

Amato, Michele; Ossicini, Stefano; Rurali, Riccardo

2012-06-13

347

First Principles Studies of the Structural and OptoElectronic Properties of Silicon Nanowires  

Microsoft Academic Search

We report the results of first principles studies of the structural, electronic, and optical properties of hydrogen-passivated silicon nanowires with [001], [011], and [111] growth directions and diameters ranging from 1 to 3 nm. We show that the growth direction, diameter, and surface structure all have a significant effect on the structural stability, electronic band gap, band structure, and band

Trinh Vo; Andrew Williamson; Giulia Galli

2006-01-01

348

First-principles study of contact between Ti surface and semiconducting carbon nanotube  

Microsoft Academic Search

The electronic structure of contact between Ti surface and a semiconducting carbon nanotube is investigated by using first-principles method. The nanotube placed on the Ti surface exhibits obvious cross section distortion and strong chemical bonds form between C and Ti. Our results suggest that the nanotube may become metallic and the electrons can be transferred from the nanotube to the

Tiezhu Meng; Chong-Yu Wang; Shan-Ying Wang

2007-01-01

349

First-principles studies of surface-enhanced Raman scattering: Benzene thiol on Au  

Microsoft Academic Search

First-principles calculations based on density functional theory are used to investigate how chemisorption of organic molecules on metal surfaces affects their Raman spectra. Experiments have long reported Raman intensity enhancements of many orders of magnitude for molecules on rough metal surfaces or near nanofabricated metallic tips. The goal of this work is to explore ``chemical'' effects that may contribute to

Alexey Zayak; Jeffrey Neaton

2009-01-01

350

First-principles study of superconductivity and Fermi-surface nesting in ultrahard transition metal carbides  

Microsoft Academic Search

Using a recently developed first-principles approach, we show that the variation in the superconducting behavior of group IVb and Vb transition metal carbides is associated with a significant nesting of the Fermi surfaces in group Vb compounds, while their phonon anomalies and density of states at the Fermi level play a minor role. The superconducting pairing arises from the coupling

Jesse Noffsinger; Feliciano Giustino; Steven G. Louie; Marvin L. Cohen

2008-01-01

351

The metallicity of B-doped diamond surface by first-principles study  

Microsoft Academic Search

The first-principles study is performed to boron-doped diamond (100) and (111) surface structures, respectively. The total energy values of the relaxed structures indicate that the more stable structure can be obtained for diamond (100) structure when the carbon atoms in the surface few layers are substituted by boron atoms; while for diamond (111) structure, the boron atoms have a more

C. Lu; Z. L. Wang; L. F. Xu; H. X. Yang; J. J. Li; C. Z. Gu

2010-01-01

352

First-principles calculations for the electronic band structures of small diametersingle-wall carbon nanotubes  

Microsoft Academic Search

We present first-principles calculations on the band structures of 40 different small diameter (d) single-wall carbon nanotubes (SWCNTs), including 14 chiral ones, employing density functional theory (DFT) within the local density approximation (LDA), using the Vienna ab initio simulation package (VASP). The band gaps are calculated and discussed for all of the tubes. From small to large diameters, the gap

V. Zólyomi; J. Kürti

2004-01-01

353

Integration of first-principles methods and crystallographic database searches for new ferroelectrics: Strategies and explorations  

SciTech Connect

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.

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

354

First principles calculations of the vibrational properties of icosahedral solid boron oxygen B 12O 2  

Microsoft Academic Search

In this paper, the geometrical structure and vibrational properties of icosahedral solid boron oxygen B12O2 have been calculated using first principles calculations. The symmetry of crystal vibration modes at the center of Brillouin zone is analyzed based on our numeric results and structure symmetry. The calculated results are compared with available experimental investigations.

Bin Wang; Zhaochuan Fan; Qi Zhou; Xiaoxuan Xu; Min Feng; Xuewei Cao; Yufang Wang

2011-01-01

355

First-principles study of lattice dynamics and thermodynamics of osmium under pressure  

Microsoft Academic Search

We have performed the first-principles linear response calculations of the lattice dynamics, thermal equation of state and thermodynamical properties of hcp Os metal by using the plane-wave pseudopotential method. The thermodynamical properties are deduced from the calculated Helmholtz free energy by taking into account the electronic contribution and lattice vibrational contribution. The phonon frequencies at Gamma point are consistent with

Bo Liu; Mu Gu; Xiao-Lin Liu; Shi-Ming Huang; Chen Ni; Ze-Ren Li; Rong-Bo Wang

2010-01-01

356

A first-principles study on the electromechanical effect of graphene nanoribbon  

Microsoft Academic Search

The electromechanical effect of graphene nanoribbons (GNRs) is investigated via a first-principles method. The deformations of GNRs with zigzag shaped edges (ZGNR) and armchair shaped edges (AGNR) are considered. The nonlinear stress-strain relations are found for both of the AGNRs and the ZGNRs. The AGNRs seem to be stiffer than the ZGNRs as concluded from inspecting the Young's modulus and

W. S. Su; B. R. Wu; T. C. Leung

2011-01-01

357

A first-principles study on the electromechanical effect of graphene nanoribbon  

Microsoft Academic Search

The electromechanical effect of graphene nanoribbons (GNRs) is investigated via a first-principles method. The deformations of GNRs with zigzag shaped edges (ZGNR) and armchair shaped edges (AGNR) are considered. The nonlinear stress–strain relations are found for both of the AGNRs and the ZGNRs. The AGNRs seem to be stiffer than the ZGNRs as concluded from inspecting the Young's modulus and

W. S. Su; B. R. Wu; T. C. Leung

2011-01-01

358

Global sensitivity analysis in the development of first principle-based eutrophication models  

Microsoft Academic Search

In this work, we formulate a dynamic first principle-based eutrophication model for a reservoir and perform global sensitivity analysis to determine most influential parameters. Both first-order and total sensitivity indices profiles have been calculated throughout a time horizon of one year on main differential state variables, which include cyanobacteria, diatoms and chlorophyta concentration, as well as main nutrient concentration. Numerical

Vanina Estrada; M. Soledad Diaz

2010-01-01

359

First-principles based semi-classical model for transport in magnetic layered structures  

Microsoft Academic Search

We present a first principles based semiclassical model for transport in an inhomogeneous magnetic layered structure. The approach solves the Boltzmann transport equation using the band structure, Bloch wave velocities and scattering matrices, describing the reflection and transmission of Bloch waves from interfaces, derived from ab-initio local spin density electronic structure calculations. The model has been tested for thick Co

W. H. Butler; X.-G. Zhang; J. M. MacLaren

1998-01-01

360

Band gap engineering in graphene and hexagonal BN antidot lattices: A first principles study  

Microsoft Academic Search

The effects of antidot lattices on electronic structures of graphene and hexagonal BN (h-BN) are investigated using the first principles method based on density functional theory. For graphene, we find that when the antidot lattice is along the zigzag direction, the band gap opening can be related to the intervalley scattering and does not follow the simple scaling rule previously

Aihua Zhang; Hao Fatt Teoh; Zhenxiang Dai; Yuan Ping Feng; Chun Zhang

2011-01-01

361

First principles calculations of the dark current in quantum well infrared photodetectors  

Microsoft Academic Search

A first principles model of the dark current in quantum well infrared photodetectors has been derived using a quantum mechanical approach. This is based on a combined representation of the field-induced and thermionic emission components of the dark current. It is argued that the contribution of sequential tunnelling to the dark current is affected significantly by the presence of interface

Nkaepe E. I. Etteh; Paul Harrison

2002-01-01

362

Oxygen reduction reactions on pure and nitrogen-doped graphene: a first-principles modeling  

Microsoft Academic Search

Based on first principles density functional theory calculations we explored energetics of oxygen reduction reaction over pristine and nitrogen-doped graphene with different amounts of nitrogen doping. The process of oxygen reduction requires one more step then same reaction catalyzed by metals. Results of calculations evidence that for the case of light doped graphene (about 4% of nitrogen) energy barrier for

Danil W. Boukhvalov; Young-Woo Son

2011-01-01

363

First principles study of semiconductor to metallic transition in cadmium selenide under high pressure  

NASA Astrophysics Data System (ADS)

First principles calculation were performed using density functional theory calculations with the generalized gradient (GGA) approximations to understand the electronic properties of bulk and thin slab of cadmium selenide. The stability of cadmium selenide thin slab (CdSe (111) and CdSe (100)) was analyzed via surface energy calculation.

Santhosh, M.; Kanagaprabha, S.; Sudhapriyanga, G.; Asvinimeenaatci, A. T.; Palanichamy, R. Rajeswara

2013-06-01

364

Energetics of Al doping and intrinsic defects in monoclinic and cubic zirconia: First-principles calculations  

Microsoft Academic Search

First-principles theory within the supercell approach has been employed to investigate Al doping and intrinsic defects in monoclinic and cubic zirconia. The effect of oxygen chemical potential and Fermi level on the formation energy and on the defect concentration have been taken into account. The formation of oxygen vacancies is found to be energetically more favorable in the cubic than

C. Århammar; C. Moysés Araújo; Rajeev Ahuja

2009-01-01

365

First-principles investigation on the structural stability of methane and ethane clathrate hydrates  

Microsoft Academic Search

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,

Luzhi Xu; Xu Wang; Liuxia Liu; Minghui Yang

2011-01-01

366

New high-pressure phase of Fe3S predicted from first-principles calculation  

NASA Astrophysics Data System (ADS)

It has long been recognized that the Earth's outer core must contain a significant amount of light elements, candidates for which have included hydrogen, carbon, silicon, sulfur and oxygen. High-P,T experiments (Jephcoat and Olson, 1987; Mao et al., 1998; Fiquet et al., 2001;Uchida et al., 2001) extended this argument to the inner core on the basis of the equation of state analysis of the hexagonal-closed-pack (hcp) form of pure iron, which concluded that it still has 4-5% excess density compared to the inner core values, although significant extrapolations were usually applied. At present, one of the most popular light-element candidates is sulfur. Therefore, it is crucial to determine the melting behavior of the Fe-FeS binary under core conditions, before models of core formation can be developed. The Fe-FeS binary was known to form a eutectic at low pressures (Usselman, 1975). Sherman (1995), however, suggested the stabilization of an intermediate iron sulfide compound Fe3S with AuCu3 form theoretically, and then Fei et al. (1997) found in the high-P,T experiments that Fe3S2 forms over 14 GPa, and Fe3S and Fe2S further form over 21 GPa (Fei et al., 2000). Fe3S, which is the most iron-rich sulfide compound known to exist, has a tetragonal cell isostructural with the Fe3P structure (space group No.82, Z = 8) and no phase transition has so far been identified up to 80 GPa (Seagle et al., 2006) and even at over 200 GPa (Kuwayama private comm.). These are supportive of an ab initio investigation (Martin et al., 2004), which found that the Fe3P structure is the most stable among fcc, LaF3, YF3 and Fe3P postulations. In this study, we explored higher-pressure phases of Fe3S using first-principles calculations. Comparing enthalpies among candidate structures, we found a new structure which is more stable than the Fe3P structure at the inner core pressures. In our presentation, we will make a detailed report with respect to the new stable structure and discuss phase relations in the Fe-S system. Research supported by JSPS Grant-in-Aid for Scientific Research Grants 20001005 and 21740379 and the Ehime Univ G-COE program “Deep Earth Mineralogy”.

Ishikawa, T.; Tsuchiya, T.

2010-12-01

367

The workfunction of carbon nanotube studies by first-principles calculation  

NASA Astrophysics Data System (ADS)

Nanoscale materials such as carbon nanotubes (CNT's) have attracted great attention. CNT's show interesting material properties because their unique structures and stability. The field emission properties of these quasi-one-dimensional nanomaterials are of particular interest because of their large aspect ratio. Recently, a large area CNT-based filed emission display (FED) with full color has been demonstrated.The workfunction plays an important role in understanding the field emission properties of the carbon nanotubes. Up to now, theoretical works were only limited to the calculations on the workfunction of individual carbon nanotubes and nanotube bundles. The effects of tube lengths and tube-tube distance on the workfunction of the tube are not clear. To fill this gap, we present in this paper first-principles calculations of the workfunction for capped and H-terminated (3,3) and (5,0) tubes with various tube length and tube-tube distances. The effects of tube length, tube-tube distance, chirality, and structure of the tip were investigated.We found the workfunction of capped (3,3) and (5,0) tubes decreases linearly with the inverse of tube-tube distance and the workfunction of H-terminated (3,3) and (5,0) tube increases linearly with the inverse of tube-tube distance. The workfunctions for various tube length and tube-tube distance can be fitted by WF = A + B/Dx + C/L +D/(DxL), where Dx is the tube-tube distance and L is the tube length, where A is the workfunction for an isolated infinite tube.The value of A is given by 4.61eV for capped (3,3) tube, 4.63 for H-terminated (3,3) tube, 5.40eV for capped (5,0) tube, and 5.55 eV for H-terminated (5,0) tube respectively. These are consistent with the value calculated by an infinite isolated tube.

Su, W. S.; Leung, T. C.

2004-03-01

368

[First-principles study of vibrational Raman spectra of amorphous carbon].  

PubMed

The vibrational density of states and nonresonant reduced Raman spectra of amorphous carbon at densities of 2.6, 2.9 and 3.2 g x cm(-3) were calculated by the use of a first-principles plane-wave pesudopotential method. Three structural models were generated by liquid-quench method using Car-Parinello molecular dynamics, their vibrational frequencies and eigenmodes were determined using the linear response approach, and Raman coupling tensors were calculated using the finite electric field method. The calculated results show that the sp3 fraction increases from 50% to 84.4%, the sp2 configuration changes from mainly rings to short chains, the position of the G peak moves to higher frequencies, the intensity ratio of D and G peaks decreases, the position of the T peak moves to lower frequencies and the intensity ratio of T and G peaks increases as density increases from 2.6 to 3.2 g x cm(-3). The authors' calculated Raman spectra show an overall good agreement with experimental spectra. The analysis in terms of atomic vibrations confirms that the G and D peaks both come from sp2 C contribution, G peak is due to the stretching vibration of any pair of sp2 atoms and the T peak is due to the C-C sp3 vibration. The authors' analysis also confirms that the dispersion of G and T peaks is due to bond-length changes. The bond length of chains (olefins) is shorter than that of rings, so their vibrational frequency is higher and the G-peak position moves to higher frequencies with increasing the sp3 fraction. The number of sp3-sp2 type bonds decreases as the sp3 fraction increases. These bonds are shorter than pure sp3-sp3 bonds, hence the T-peak position moves to lower frequencies. The research results provide a theoretic basis for analyzing experimental Raman spectra of amorphous carbon. PMID:19950647

Niu, Li; Zhu, Jia-qi; Gao, Wei; Du, Shan-yi

2009-09-01

369

Symbolic programming package N Coperators with applications to theoretical atomic spectroscopy  

NASA Astrophysics Data System (ADS)

A symbolic programming package N Coperators with applications to atomic physics is introduced. The package runs over Mathematica and it implements NC Algebra, the noncommutative algebra package. N Coperators features the algebra of irreducible tensor operators, the second quantization representation, the angular momentum theory, and the effective operator approach exploited in many-body perturbation theory, including Wick's theorem. The comprehensiveness is yet another characteristic feature of the present package: The generation of expressions is performed in a way as if it were done by hand. Although the theoretical atomic spectroscopy is a direct target of N Coperators, the package, with minor modifications, if any, is believed to appropriate other areas of theoretical physics as well.

Jurš?nas, Rytis; Merkelis, Gintaras

2012-12-01

370

Comparative study of the absorption spectrum of Li 2CaSiO 4:Cr 4+: First-principles fully relativistic and crystal field calculations  

NASA Astrophysics Data System (ADS)

Systematic analysis of the energy level scheme and ground state absorption of the Cr 4+ ion in Li 2CaSiO 4 crystal was performed using the exchange charge model of the crystal field [B.Z. Malkin, in: A.A. Kaplyanskii, B.M. Macfarlane (Eds.), Spectroscopy of Solids Containing Rare-earth Ions, North-Holland, Amsterdam, 1987, pp. 33-50] and recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multielectron (DVME) method [K. Ogasawara, T. Iwata, Y. Koyama, T. Ishii, I. Tanaka, H. Adachi, Phys. Rev. B 64 (2001) 115413]. Using the former method, the values of parameters of crystal field acting on the Cr 4+ ion valence electrons were calculated using the Li 2CaSiO 4 crystal structure data. Energy levels of the Cr 4+ ion obtained after diagonalizing the crystal field Hamiltonian are in good agreement with those obtained from the experimental spectra. The latter method is based on the numerical solution of the Dirac equation; therefore, all relativistic effects are automatically considered. As a result, energy level scheme of Cr 4+ and its absorption spectra in both polarizations were calculated, assigned and compared with experimental data; energy of the lowest charge transfer transition was evaluated and compared with theoretical predictions for the CrO44- complex available in the literature. The main features of the experimental spectra shape are reproduced well by the calculations. By performing analysis of the molecular orbitals (MO) population, it was shown that the covalent effects play an important role in formation of the spectral properties of Cr 4+ ion in the considered crystal.

Brik, M. G.; Ogasawara, K.

2007-11-01

371

Double-resonance versus pulsed Fourier transform two-dimensional infrared spectroscopy: An experimental and theoretical comparison  

Microsoft Academic Search

In this study we focus on the differences and analogies of two experimental implementations of two-dimensional infrared (2D-IR) spectroscopy: double-resonance or dynamic hole burning 2D-IR spectroscopy and pulsed Fourier transform or heterodyne detected photon echo spectroscopy. A comparison is done theoretically as well as experimentally by contrasting data obtained from both methods. As an example we have studied the strongly

Valentina Cervetto; Jan Helbing; Jens Bredenbeck; Peter Hamm

2004-01-01

372

First-principles modelling of Earth and planetary materials at high pressures and temperatures  

NASA Astrophysics Data System (ADS)

Atomic-scale materials modelling based on first-principles quantum mechanics is playing an important role in the science of the Earth and the other planets. We outline the basic theory of this kind of modelling and explain how it can be applied in a variety of different ways to probe the thermodynamics, structure and transport properties of both solids and liquids under extreme conditions. After a summary of the density functional formulation of quantum mechanics and its practical implementation through pseudopotentials, we outline the simplest way of applying first-principles modelling, namely static zero-temperature calculations. We show how calculations of this kind can be compared with static compression experiments to demonstrate the accuracy of first-principles modelling at pressures reached in planetary interiors. Noting that virtually all problems concerning planetary interiors require an understanding of materials at high temperatures as well as high pressures, we then describe how first-principles lattice dynamics gives a powerful way of investigating solids at temperatures not too close to the melting line. We show how such calculations have contributed to important progress, including the recent discovery of the post-perovskite phase of MgSiO3 in the D'' layer at the base of the Earth's mantle. A range of applications of first-principles molecular dynamics are then reviewed, including the properties of metallic hydrogen in Jupiter and Saturn, of water, ammonia and methane in Uranus and Neptune, and of oxides and silicates and solid and liquid iron and its alloys in the Earth's deep interior. Recognizing the importance of phase equilibria throughout the planetary sciences, we review recently developed techniques for the first-principles calculation of solid and liquid free energies, melting curves and chemical potentials of alloys. We show how such calculations have contributed to an improved understanding of the temperature distribution and the chemical composition throughout the Earth's interior. The review concludes with a summary of the present state of the field and with some ideas for future developments.

Gillan, M. J.; Alfè, D.; Brodholt, J.; Vocadlo, L.; Price, G. D.

2006-08-01

373

First-principles calculations on third-order elastic constants and internal relaxation for monolayer graphene  

NASA Astrophysics Data System (ADS)

The method of homogeneous deformation is combined with first-principles total-energy calculations on determining third-order elastic constants and internal relaxation for monolayer graphene. We employ density functional theory (DFT) within generalized-gradient-approximation (GGA). The elastic constants are obtained from a polynomial fitted to the calculations of strain-energy and strain-stress relations. Our results agree well with recent calculations by DFT calculations, tight-binding atomistic simulations, and experiments with an atomic force microscope. The internal relaxation displacement has also been determined from ab initio calculations. The details of internal lattice relaxation by first principles are basically consistent with the previous molecular dynamics (MD) simulation. But for tiny deformation, there is an anomalous region in which the behavior of internal relaxation is backward action. In addition, we have also demonstrated that the symmetry of the relationship between the internal displacement and the infinitesimal stains can be satisfied.

Wang, Rui; Wang, Shaofeng; Wu, Xiaozhi; Liang, Xiao

2010-08-01

374

Understanding Polymer Adhesion: First-principles calculations of the adsorption of organic molecules onto Si surfaces  

NASA Astrophysics Data System (ADS)

The adhesion of plastics to ceramics is important for many industrial and technological appliations. It is therefore essential to understand the underlying structure and bonding of the polymer and the surface. The aim of this research is to improve plastic adhesion using a multiscale approach. The first step involves the use of density functional calculations to understand the atomic-scale structure and bonding of polymers on surfaces. The plastic of interest is mainly composed of the polymer bisphenol-A-polycarbonate (BPA-PC). The BPA-PC monomer consists of two phenol groups, one propane group and a carbonic acid group. First-principles calculations of the adsorption of these molecules onto the Si(001)-(2x1) dimer surface will be presented. Finally, the incorporation of first-principles data into a coarse-graining method will be discussed.

Johnston, Karen; Nieminen, Risto M.

2007-03-01

375

First-principles study on elastic constants of C-S-H type minerals  

NASA Astrophysics Data System (ADS)

Calcium-Silicate-Hydrate (C-S-H) is the mineral binding phase of all Portland concrete materials, and the principle source of their strength and stiffness. Despite decades of research, the elastic properties of C-S-H mineral crystals are unknown. Here we investigate two natural analogs of C-S-H, tobermorite and jennite, and characterize their mechanical properties by first-principles calculations. First, we calculate their lattice parameters and elastic constants. Second, we show that in contrast to previous suggestions, for natural tobermorite 11 Å , the mechanically weakest directions are two inclined regions that form a hinge mechanism. By studying bond length changes under deformation in tobermorite 14 Å and jennite, we show that water molecules play a major structural role in defining their elastic properties. Averaged elastic moduli obtained by first-principles calculations of tobermorite 14 Å and jennite compare well with corresponding nanoindentation experiment on C-S-H.

Shahsavari, R.; Buehler, M.; Ulm, F.

2008-12-01

376

First-principles materials applications and design of nonlinear optical crystals  

NASA Astrophysics Data System (ADS)

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.

Lin, Zheshuai; Jiang, Xingxing; Kang, Lei; Gong, Pifu; Luo, Siyang; Lee, Ming-Hsien

2014-06-01

377

Ground state structure of BaZrO3 : A comparative first-principles study  

NASA Astrophysics Data System (ADS)

First-principles calculations, based on density-functional theory, are exploited to investigate the nature of the ground-state structure of barium zirconate. The experimentally observed simple-cubic structure is found to be dynamically unstable against an antiferrodistortive transformation. This instability manifests itself through imaginary frequency modes along the whole R-M edge of the Brillouin zone. The computations predict an orthorhombic crystal structure of the material, only slightly distorted from the cubic lattice, with an eight times larger unit cell and alternate ZrO6 octahedra slightly rotated in opposite directions around the Cartesian axes. The apparent disagreement with some of the previous first-principles results regarding the nature of the ground-state structure is considered in detail. The neglect of the barium 5s2 and 5p6 electrons in the valence configuration of Ba is found to be responsible for the previously reported erroneous results.

Bili?, Ante; Gale, Julian D.

2009-05-01

378

A biosensor based on graphene nanoribbon with nanopores: a first-principles devices-design  

NASA Astrophysics Data System (ADS)

A biosensor device, built from graphene nanoribbons (GNRs) with nanopores, was designed and studied by first-principles quantum transport simulation. We have demonstrated the intrinsic transport properties of the device and the effect of different nucleobases on device properties when they are located in the nanopores of GNRs. It was found that the device's current changes remarkably with the species of nucleobases, which originates from their different chemical compositions and coupling strengths with GNRs. In addition, our first-principles results clearly reveal that the distinguished ability of a device's current depends on the position of the pore to some extent. These results may present a new way to read off the nucleobases sequence of a single-stranded DNA (ssDNA) molecule by such GNRs-based device with designed nanopores

Ouyang, Fang-Ping; Peng, Sheng-Lin; Zhang, Hua; Weng, Li-Bo; Xu, Hui

2011-05-01

379

Large-scale first-principles molecular dynamics for electrochemical systems with O(N) methods  

NASA Astrophysics Data System (ADS)

A method for large-scale first-principles molecular dynamics (MD) simulations on electrochemical systems has been developed by combining the effective screening medium (ESM) method with O(N) density functional theory (DFT). This implementation has been significantly simplified by the introduction of neutral atom potentials, which minimizes the modifications to existing DFT code. In order to demonstrate ability of this implementation, it has been applied to an electrochemical system consisting of a H-Si(111) electrode, which is a candidate anode for high-capacity Li-ion secondary batteries, and a propylene carbonate (PC) solvent to simulate how PC molecules in the vicinity of the electrode surface respond to an imposed electric field. The large-scale MD simulation clearly demonstrates that the combination of the ESM and O(N) DFT methods provides a useful tool for first-principles investigation of complicated electrochemical systems such as high-capacity batteries.

Ohwaki, Tsukuru; Otani, Minoru; Ikeshoji, Tamio; Ozaki, Taisuke

2012-04-01

380

Structure of the (111) surface of bismuth: LEED analysis and first-principles calculations  

SciTech Connect

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.

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

381

Structure of the (111) surface of bismuth: LEED analysis and first-principles calculations  

NASA Astrophysics Data System (ADS)

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 (1×1) 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.

Mönig, H.; Sun, J.; Koroteev, Yu. M.; Bihlmayer, G.; Wells, J.; Chulkov, E. V.; Pohl, K.; Hofmann, Ph.

2005-08-01

382

Whether FeTe is superconductor: Insights from first-principles calculations  

NASA Astrophysics Data System (ADS)

We present a first-principles pseudopotential study on the electronic structure, phonon structure and the electron-phonon interaction of stoichiometric FeTe in both the nonmagnetic and double stripe antiferromagnetic phases. Our electronic structure calculations show that the nesting effect of Fermi surface is not present in stoichiometric FeTe after considering the magnetic interaction. Comparing the phonon behavior in the double stripe antiferromagnetic phase with that in the nonmagnetic phase, we find that the spin-lattice interaction can lead to the phonon softening and increase electron-phonon coupling constant ? by about 33%, which is similar to other iron-based superconductors in the single stripe antiferromagnetic phase. We suggest that the phonon softening may have no clear contact with the specific magnetic order in the ground state. Finally, we make some discussion about whether FeTe can be superconductor combining our first-principles calculations.

Li, Jian; Huang, GuiQin; Zhu, XingFeng

2013-09-01

383

First-principles theory of 250 000-atom coherent alloy microstructure  

NASA Astrophysics Data System (ADS)

Microstructural issues in alloys such as precipitation have largely been outside the realm of first-principles electronic structure calculations due to the length scales involved in precipitation microstructure (typically nanometres to micrometres) and the inherent thermodynamic/statistical nature of the problem. Here, we show that modern, first-principles total energy calculations can be combined with a mixed-space cluster expansion approach (a generalized real/reciprocal space Ising model) and Monte Carlo simulations to yield a method capable of describing equilibrium coherent precipitate shapes in alloys with system sizes up to 250 000 atoms. Both the (anisotropic) interfacial free energies and the coherency strain between precipitate and matrix are accounted for in this method as well as the short-range atomic-scale ordering of the solid solution. Illustrations of the technique are given for several famous examples of coherent precipitation in aluminium alloys: Al-Mg, Al-Cu and Al-Ni.

Wolverton, C.

2000-05-01

384

Three, Two and One-dimensional MoS2: A First Principle Investigation  

Microsoft Academic Search

This study presents our study on atomic, electronic, magnetic and phonon properties of three, two and one dimensional honeycomb structures of molybdenum disulfide (MoS2) using first-principles pseudopotential plane wave method. The dimensionality effects on various properties are examined. The calculations of phonon frequencies indicate the stability of two dimensional, single layer MoS2, which consists of a positively charged molybdenum atomic

Can Ataca; Ethem Akturk; Salim Ciraci

2010-01-01

385

Thermal stabilities of delithiated olivine MPO 4 (M = Fe, Mn) cathodes investigated using first principles calculations  

Microsoft Academic Search

We present an analysis of the thermal reduction of delithiated LiMnPO4 and LiFePO4 based on the quarternary phase diagrams as calculated from first principles. Our results confirm the recent experimental findings that MnPO4 decomposes at a much lower temperature than FePO4, thereby potentially posing larger safety issues for LiMnPO4 cathodes. We find that while substantial oxygen is released as MnPO4

Shyue Ping Ong; Anubhav Jain; Geoffroy Hautier; Byoungwoo Kang; Gerbrand Ceder

2010-01-01

386

Third conformer of graphane: A first-principles density functional theory study  

Microsoft Academic Search

We propose, on the basis of our first-principles density functional based calculations, an interesting isomer of graphane in which the C-H bonds of a hexagon alternate in three-up-three-down fashion on either side of the sheet. This two-dimensional puckered structure, called ``stirrup,'' has an intermediate stability between the previously reported chair and boat conformers of graphane. The physicochemical properties of this

A. Bhattacharya; S. Bhattacharya; C. Majumder; G. P. Das

2011-01-01

387

The third conformer of graphane: A first principles DFT based study  

Microsoft Academic Search

We propose, on the basis of our first principles density functional based calculations, a new isomer of graphane, in which the C-H bonds of a hexagon alternate in 3-up, 3-down fashion on either side of the sheet. This 2D puckered structure called 'stirrup' has got a comparable stability with the previously discovered chair and boat conformers of graphane. The physico-chemical

A. Bhattacharya; S. Bhattacharya; C. Majumder; G. P. Das

2010-01-01

388

Orientational order in dense molecular hydrogen: A first-principles path-integral Monte Carlo calculation  

Microsoft Academic Search

A quantum-mechanical simulation of orientational order in solid molecular hydrogen is presented, based on path-integral Monte Carlo calculations. The effective potential between nearest-neighbor hydrogen molecules is derived from first-principles total energy calculations, taking full account of the symmetries of the Hamiltonian within a hexagonal close-packed arrangement of the molecule centers. A first-order orientational order transition is found by studying short-

Efthimios Kaxiras; Zhian Guo

1994-01-01

389

Boron nitride fullerene B36N36 doped with transition metal atoms: First-principles calculations  

Microsoft Academic Search

We perform first-principles calculations for the interaction of the transition metal atoms Fe, Co, and W, as well as the FeO molecule, with the boron nitride fullerene B36N36 . The stable structure of the atom-fullerene complexes may have the dopant atom either at the center of the cage or making covalent bonds with the fullerene wall, with similar total energies.

Ronaldo J. C. Batista; Mário S. C. Mazzoni; Helio Chacham

2007-01-01

390

First-principles calculation of the magnetic properties of paramagnetic fcc iron  

SciTech Connect

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.

Johnson, D.D.; Gyorffy, B.L.; Pinski, F.J.; Staunton, J.; Stocks, G.M.

1985-01-01

391

Hydrogen embrittlement in a magnesium grain boundary: a first-principles study  

Microsoft Academic Search

First-principles fully relaxed tensile and shear test simulations were performed on tilt Mg grain boundary (GB) models, with and without H segregation, to investigate mechanisms of H embrittlement of Mg. Strengthening as a result of covalent-like characteristics of Mg–H bonds prevailed over weakening of Mg–Mg bonds resulting from charge transfer; as a result, an H atom strengthened the GB. In

Motohiro Yuasa; Daiki Nishihara; Mamoru Mabuchi; Yasumasa Chino

2012-01-01

392

First-principles modeling of resistance switching in perovskite oxide material  

Microsoft Academic Search

We report a first-principles study on SrRuO3\\/SrTiO3 interface in the presence of the oxygen vacancy. While the oxygen vacancy on the side of SrTiO3 significantly lowers the Schottky barrier height, the oxygen vacancy close to the interface or inside the metallic electrode results in a Schottky barrier comparable to that of the clean interface. Based on these results, we propose

Sang Ho Jeon; Bae Ho Park; Jaichan Lee; Bora Lee; Seungwu Han

2006-01-01

393

The effects of hydroxyl groups on Ca adsorption on rutile surfaces: a first-principles study  

Microsoft Academic Search

Hydroxyl groups on titanium surfaces have been believed to play an important role in absorbing Ca in solution, which is crucial\\u000a in the formation of bioactive calcium phosphates both in vitro and in vivo. CASTEP, a first-principles density functional\\u000a theory (DFT) code, was employed to investigate Ca adsorption on various rutile (110) surfaces in order to clarify how hydroxyl\\u000a groups

Xiong Lu; Hong-ping Zhang; Yang Leng; Liming Fang; Shuxin Qu; Bo Feng; Jie Weng; Nan Huang

2010-01-01

394

Initial growth mode of Au on Ag(110) studied with first-principles calculations  

NASA Astrophysics Data System (ADS)

We studied the initial growth mode of Au on Ag(110) using first-principles total energy calculations. We found that a recently observed bilayer growth mode in this system is not energetically favorable and thus may not be an equilibrium process. The most favorable initial growth process up to 1 monolayer Au coverage is found to proceed via subsurface substitution, which is an interesting growth mode for a metal-on-metal system.

Chan, C. T.; Bohnen, K. P.; Ho, K. M.

1992-09-01

395

First-principles modeling of strain in perovskite ferroelectric thin films  

Microsoft Academic Search

We review the role that first-principles calculations have played in understanding the effects of substrate-imposed misfit strain on epitaxially grown perovskite ferroelectric films. We do so by analyzing the case of BaTiO$_3$, complementing our previous publications on this subject with unpublished data to help explain in detail how these calculations are done. We also review similar studies in the literature

Oswaldo Diéguez; David Vanderbilt

2008-01-01

396

First-principles study of He point-defects in HCP rare-earth metals  

SciTech Connect

He defect properties in Sc, Y, Gd, Tb, Dy, Ho, Er and Lu were studied using first-principles calculations based on density functional theory. The results indicate that the formation energy of an interstitial He atom is smaller than that of a substitutional He atom in all hcp rare-earth metals considered. Furthermore, the tetrahedral interstitial position is more favorable than an octahedral position for He defects. The results are compared with those from bcc and fcc metals.

Li, Yang; Chen, Ru; Peng, SM; Long, XG; Wu, Z.; Gao, Fei; Zu, Xiaotao

2011-05-01

397

First principles pseudopotential calculations of zone boundary phonon frequencies of III-V zincblende semiconductors  

Microsoft Academic Search

Longitudinal zone boundary X phonon frequencies have been calculated by a first principles pseudopotential method for III-V zincblende semiconductors AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs and InSb. The phonon frequencies have been evaluated from total energy calculations in the frozen phonon approximation. The calculated phonon frequencies agree very well with the experimental values.

Wang Jian-Qing; Gu Zong-Quan; Li Ming-Fu

1991-01-01

398

Truncated spherical-wave basis set for first-principles pseudopotential calculations  

Microsoft Academic Search

Analytic results for two- and three-centre integrals are derived for the truncated spherical-wave basis set designed for first-principles pseudopotential calculations within density-functional theory. These allow the overlap, kinetic energy and non-local pseudopotential matrix elements to be calculated efficiently and accurately. In particular, the scaling of the computational effort with maximum angular momentum component is dramatically improved and the projection method

B. Monserrat; P. D. Haynes

2010-01-01

399

Application of Harrison’s first-principle pseudopotential method for semiconductors  

Microsoft Academic Search

The application of theHarrison first-principle pseudopotential method is investigated for semiconductors Si and Ge. The work being limited to the study\\u000a of the electronic structure, the form factors are computed in semilocal approximation. In the calculation the following versions\\u000a are investigated: (i) for Si and Ge the use of the Hartree—Fock wave functions and core eigenvalues for the free ions

L. V. Kotova; T. Vörös

1974-01-01

400

Core-cancellation functions for evaluating exchange-correlation functionals in first-principles pseudopotential calculations  

Microsoft Academic Search

Within the framework of density-functional theory, first-principles pseudopotential methods have been highly successful in modeling the valence-electron properties of solids in their ground states. In this paper, we introduce ``core-cancellation functions'' which are designed to improve the accuracy of the treatment of the exchange-correlation interaction. This formalism, expected to be especially effective for transition metals, is tested for bulk tungsten

N. A. W. Holzwarth; Y. Zeng

1994-01-01

401

Volume dependence of the superconducting transition temperature of aluminum calculated from a first-principles potential  

SciTech Connect

Using a first-principles, local, volume-dependent pseudopotential which has been used successfully to calculate several properties of aluminum, we have calculated the electron-phonon interaction for this material. From this information we have calculated the volume dependence of the superconducting transition temperature. The pseudopotential is constructed from the nonlinear induced density around an aluminum ion in an electron gas. The volume dependence is introduced by the electron-gas density parameter r/sub s/.

MaganBa, L.F.; Vazquez, G.J.

1988-11-15

402

Liquid structure of the simple alkali metals from a first-principles pseudopotential calculation  

Microsoft Academic Search

First-principles fully nonlocal pseudopotentials which predict good phonon spectra and elastic shear constants for Li, Na, and K were used to obtain the effective ion-ion pair potentials at the melting-point density. The pair potential U(r) was used to make a Monte Carlo determination of the static structure factor S(q) and pair-correlation function g(r) for liquid alkali metals. The results for

R. S. Day; F. Sun; P. H. Cutler

1979-01-01

403

MPI parallelization of the first-principles pseudopotential method program with respect to each band  

Microsoft Academic Search

We have developed an efficient parallelized program for large-scale calculations in the framework of the first-principles pseudopotential method. This program uses the residual minimization method-direct inversion in the iterative subspace method which was originally proposed by Pulay and recently implemented by Kresse and Furthmuller, to efficiently obtain the electronic ground state. This method is more suitable for parallel computations with

Tomoyuki Tamura; Guang-Hong Lu; Ryoichi Yamamoto; Masanori Kohyama; Shingo Tanaka; Yuji Tateizumi

2004-01-01

404

A first-principles pseudopotential study of the compressibility of ?-FeSi  

Microsoft Academic Search

The experimental investigations have given conflicting results for the bulk modulus of ?-FeSi. We present the results of first-principles pseudopotential plane-wave calculations of the structural properties of ?-FeSi. In these calculations the local density and generalized gradient approximations (LDA and GGA) for the exchange-correlation potential have been used. The calculated bulk modulus, using both LDA and GGA, is found to

A. Qteish; N. Shawagfeh

1998-01-01

405

First-principles pseudopotential study of the structural phases of silicon  

Microsoft Academic Search

A first-principles pseudopotential study of 11 phases of silicon is reported: diamond (cd), body-centered cubic (bcc), face-centered cubic (fcc), body-centered tetragonal (bct), simple hexagonal (sh), hexagonal-close-packed (hcp), double-hexagonal-close-packed (dhcp), simple cubic (sc), beta-tin, a body-centered cubic structure with eight atoms per unit cell (bc8), and a simple tetragonal structure with 12 atoms per unit cell (st12). For each structure and

R. J. Needs; A. Mujica

1995-01-01

406

Phonon-limited resistivity of aluminium using a first-principles pseudopotential  

Microsoft Academic Search

The phonon-limited resistivity of aluminium has been calculated using a local, first-principles pseudopotential which has been useful in the calculation of other properties of aluminium. This pseudopotential is obtained from the induced electron density around an aluminium ion in an electron gas. From this pseudopotential, the interionic potential, the phonons (which are calculated by the self-consistent harmonic approximation) and finally

G. J. Vazquez; L. F. Magana

1990-01-01

407

Lattice Dynamics of Magnesium from a First-Principles Nonlocal Pseudopotential Approach  

Microsoft Academic Search

Harrison's a priori theory is used to construct a first-principles nonlocal pseudopotential for magnesium, a metal with the hexagonal close-packed (hcp) structure and two atoms per unit cell. A response function for the exchange interaction among conduction electrons, which uses the Kohn-Sham approximation for the long-wavelength limit, is employed in the calculation of the energy-wave-number characteristic F(q). The phonon spectra

Walter F. King; P. H. Cutler

1971-01-01

408

A first-principles pseudopotential investigation of ferroelectricity in barium titanate  

Microsoft Academic Search

We have investigated the adiabatic energy surface of BaTiO3 using a first-principles molecular dynamics approach. High quality pseudopotentials were generated for barium, titanium and oxygen using a recently developed ultra-soft pseudopotential scheme. The wave-functions were expanded in a plane-wave basis set, and adequate convergence was achieved with a 25 Ry cutoff. We show that the method gives a good description

R. D. King-smith; David Vanderbilt

1992-01-01

409

First-principles pseudopotential study of GaN and BN (110) surfaces  

Microsoft Academic Search

Using the first-principles pseudopotential LDA approach, we have performed calculations of the atomic and electronic structure of the (110) surface of the cubic phase of GaN and BN. It is found that these surfaces are characterised by a much smaller rotation angle ? of the top atomic layer than many other III–V(110) surfaces. We have found a linear correlation between

R. Miotto; G. P Srivastava; A. C Ferraz

1999-01-01

410

Static structure factor and electronic transport properties of liquid rubidium from a first principles pseudopotential calculation  

Microsoft Academic Search

A first principles nonlocal pseudopotential calculation is used to obtain the ion-ion potential for liquid rubidium of 45°C. This is then used in a Monte Carlo simulation to determine the liquid structure S(q) and transport properties. It is found that the accuracy of the S(q), in the low q-region, is crucial to the calculation of electronic transport properties.

F. Sun; R. S. Day; P. H. Cutler

1978-01-01

411

Lattice Dynamics of Beryllium from a First-Principles Nonlocal Pseudopotential Approach  

Microsoft Academic Search

The lattice dynamics of beryllium, a metal with hexagonal close-packed structure and two atoms per unit cell, is investigated within the framework of Harrison's first-principles pseudopotential theory, using (i) the Slater approximation for the conduction-band-core exchange, and (ii) a modified dielectric-screening function employing the Kohn-Sham approximation for exchange among the conduction electrons. The energy-wave-number characteristic F(q) is constructed from the

Walter F. King; P. H. Cutler

1970-01-01

412

Theory of nuclear quadrupole interaction in cadmium using a first-principles pseudopotential  

Microsoft Academic Search

The field gradient in cadmium metal is calculated using a first-principles pseudopotential and including antishielding effects for the conduction electrons in an ab initio manner. The calculated field gradient in combination with the quadrupole coupling constant from perturbed-angular-correlation measurements leads to a quadrupole moment of 0.71 barns in good agreement with an earlier value obtained using an adaptation to other

Michael D. Thompson; T. P. Das; Gheorghe Ciobanu

1979-01-01

413

First-principles pseudopotential study of an aluminium grain boundary containing sulphur atoms  

Microsoft Academic Search

The electronic structure of an aluminium grain boundary with segregated sulphur impurity atoms has been calculated by a first-principles pseudo-potential method. It is found that a sulphur atom bonds to only one of the neighbouring aluminium atoms. This bond is a mixed-character metallic-covalent bond which is stronger than the metallic Al-Al bonds. Electrons that participate in forming this bond are

Guang-Hong Lu; Masanori Kohyama; Ryoichi Yamamoto

2003-01-01

414

Application of Harrison’s first-principle pseudopotential method for semiconductors  

Microsoft Academic Search

TheHarrison first-principle pseudopotential method (FPPM) is combined with theLöwdin-Brust method for the determination of electronic energy levels for Si and Ge. The FP conduction bands computed by the use of the\\u000a previously calculated FP form factors differ from the known data. For this reason the FP form factors are modified with an\\u000a empirical correction made. The resulting first three Fourier

L. V. Kotova; E. V. Galaktionov; V. E. Khartsiev; T. Vörös

1974-01-01

415

Electronic structure of cubic BaTbO3 from first-principles pseudopotential calculations  

Microsoft Academic Search

First-principles calculations of the electronic structure are performed for cubic BaTbO3 using the plane-wave pseudopotential method within the framework of density functional theory and using the generalized gradient approximation for the exchange-correlation potential. Our calculations show that cubic BaTbO3 is metallic, and that this metallic character is mainly governed by the Tb 4f electrons and the hybridization between the Tb

G. Y. Gao; K. L. Yao; Z. L. Liu

2006-01-01

416

First-principles calculations of pure elements: Equations of state and elastic stiffness constants  

Microsoft Academic Search

Using the projector-augmented wave method within the generalized gradient approximation, a systematic first-principles calculation for energy vs. volume (E–V) equations of state (EOS’s) and single crystal elastic stiffness constants (cij’s) has been performed for 76 pure elemental solids with face-centered-cubic (fcc), body-centered-cubic (bcc), and hexagonal-close-packed (hcp) crystal structures, wherein the cij’s are determined by an efficient strain–stress method, and the

S. L. Shang; A. Saengdeejing; Z. G. Mei; D. E. Kim; H. Zhang; S. Ganeshan; Y. Wang; Z. K. Liu

2010-01-01

417

First principles total energy study of NbCr{sub 2} + V Laves phase ternary system  

SciTech Connect

The C15 NbCr{sub 2} + V Laves phase ternary system is studied by using a first-principles, self-consistent, full-potential total energy method. Equilibrium lattice parameters, cohesive energies, density of states and formation energies of substitutional defects are calculated. Results of all these calculations show that in the C15 NbCr{sub 2} + V compounds, V atoms substitute Cr atoms only.

Ormeci, A. [Koc Univ., Istanbul (Turkey); Chen, S.P.; Wills, J.M.; Albers, R.C. [Los Alamos National Lab., NM (United States)

1999-04-01

418

Effect of oxygen on single-wall silicon carbide nanotubes studied by first-principles calculations  

Microsoft Academic Search

According to the predictions of first-principles calculations, single-wall silicon carbide nanotubes exhibit several unusual properties: they are semiconducting independently of their chirality, superior material for hydrogen storage, and have strong nonlinear optical coefficients. Nevertheless, only a single experiment indicates, in our knowledge, that a tubular form of silicon carbide (SiC) exists. It is known that the surface of bulk silicon

Á. Szabó; A. Gali

2009-01-01

419

First-principle component models for control system simulation of MHD-steam plants  

Microsoft Academic Search

The first-principle, lumped-parameter subsystem models for the CNCD unit, typical steam-water path units, the boiler feed pump, the steam drum, steam turbines, the air compressor, and the spray attemperator are discussed. The models can predict time-varying changes in combustion gas temperature and enthalpy, and temperature within each major component of the combined-cycle system. Methods of connecting the component models for

D. A. Pierre; D. A. Rudberg

1979-01-01

420

First-principles study on anatase TiO2 codoped with nitrogen and praseodymium  

Microsoft Academic Search

The crystal structures, electronic structures and optical properties of nitrogen or\\/and praseodymium doped anatase TiO2 were calculated by first principles with the plane-wave ultrasoft pseudopotential method based on density functional theory. Highly efficient visible-light-induced nitrogen or\\/and praseodymium doped anatase TiO2 nanocrystal photocatalyst were synthesized by a microwave chemical method. The calculated results show that the photocatalytic activity of TiO2 can

Pan Gao; Jing Wu; Qing-Ju Liu; Wen-Fang Zhou

2010-01-01

421

First-Principles Study of GaSb(001) surface reconstructions  

Microsoft Academic Search

Different models for the GaSb-(001) surface reconstruction have been proposed on the basis of experimental observations in typical GaSb growth conditions (V\\/III flux ratio > 1). We have analyzed their relative stability by means of first-principles pseudopotential plane-wave calculations. We constructed the surface phase diagram as a function of the chemical potential of the elemental constituents to mimic different growth

M. C. Righi; Rita Magri; C. M. Bertoni

2005-01-01

422

Effect of Subsurface Boron on H2O Adsorption on Si(100):A First Principles Study  

Microsoft Academic Search

Precise control of surface chemical reactions becomes essential for fabrication of ever smaller semiconductor devices. The interaction of H2O with silicon surfaces is of particular interest because of oxidation in silicon device manufacture. In this talk, we will present our recent first principles study of H2O adsorption on a B-modified Si (001) surface. A major p-type dopant B can be

Yun Wang; Gyeong Hwang

2003-01-01

423

First-Principles Study of the Surface Electronic Structures of Transition Metal Carbides  

Microsoft Academic Search

Surfaces of transition metal carbides (TiC, ZrC, NbC, HfC and TaC(001)-1×1) are investigated using the first-principles molecular dynamics (FPMD) method. By the full structural optimization of the surface, the carbon and transition metal atoms on the top layer move outward and inward, respectively. All the calculated electronic states of surfaces are metallic. A non-linear core correction is considered for pseudopotentials

Kazuaki Kobayashi

2000-01-01

424

First-principles study of Ti-doped sodium alanate surfaces  

Microsoft Academic Search

We have performed first-principles calculations of thick slabs of Ti-doped sodium alanate (NaAlH4), which allows one to study the system energetics as the dopant progresses from the surface to the bulk. Our calculations predict that Ti stays on the surface, substitutes for Na, and attracts a large number of H atoms to its vicinity. Molecular dynamics simulations suggest that the

Jorge Íñiguez; Taner Yildirim

2005-01-01

425

First-principles study of the (001) surface of cubic CaTi O3  

Microsoft Academic Search

We present first-principles calculations on the (001) surfaces of cubic CaTiO3 with CaO and TiO2 terminations. For the TiO2 -terminated surface, the largest relaxation is on the second layer atoms, not on the first layer ones. This behavior is different to SrTiO3 and BaTiO3 . The large relaxation of the Ca atoms in the second layer deeply affects the band

Yuan Xu Wang; Masao Arai; Taizo Sasaki; Chun Lei Wang

2006-01-01

426

First-principles study of bulk ordering and surface segregation in Pt-Rh binary alloys  

Microsoft Academic Search

The cluster expansion technique in conjunction with first-principles calculations has been applied in Monte Carlo simulations to derive the configurational thermodynamics of the bulk and (111) surface of Pt-Rh alloys. Lattice-dynamics calculations reveal that the vibrational contribution to Pt-Rh bulk phase stability is fairly negligible. Calculated short-range-order parameter, ground state, and ordering transition temperature Tc of bulk Pt50Rh50 are in

Koretaka Yuge; Atsuto Seko; Akihide Kuwabara; Fumiyasu Oba; Isao Tanaka

2006-01-01

427

A first-principles study on the Rashba effect in surface systems  

Microsoft Academic Search

The Rashba effect in several surface systems, Au(111), Au(110), Ag(111), Sb(111) and Si(111)-Bi, is studied by means of first-principles relativistic density-functional calculations. The importance of the asymmetric behavior around the surface atom is emphasized as a crucial factor to determine the magnitude of Rashba spin splitting in addition to the size of the spin-orbit coupling. The Rashba effect at the

Miki Nagano; Ayaka Kodama; T. Shishidou; T. Oguchi

2009-01-01

428

First-principles study of surface phonons on the AlN(1 1 0) surface  

Microsoft Academic Search

Using a linear-response approach based on the pseudo-potential method and the local density functional scheme, we have performed calculations of the atomic and dynamical properties of the (110) surface of the cubic phase of AlN. Surface localised modes are found with energies below and above the bulk spectrum, as well as in the acoustic–optical and optical–optical gap regions. The first-principles

?. U?ur; H. M. Tütüncü; G. P. Srivastava; ?. Akgün

2004-01-01

429

First-principles study of adsorption and migration on the (001) surfaces of cubic BN  

Microsoft Academic Search

This first-principles study examines the behavior of boron and nitrogen adatoms on the dimer-reconstructed (001) surfaces of cubic boron nitride (cBN), to find low-barrier migration pathways that control the vapor-growth of cBN. Adatom dynamics on the surfaces of wide band gap semiconductors such as cBN (with ˜6 eV gap) and diamond is an uncharted field of surface science: Because these

Hiroaki Koga; Tsuyoshi Miyazaki; Satoshi Watanabe; Takahisa Ohno

2006-01-01

430

The first-principle study of the iodine-modified silver surfaces  

Microsoft Academic Search

The properties of the I-modified Ag(111), (100) and (110) surfaces are studied via the first-principle local density functional calculation with the ultrasoft pseudopotential and the generalized gradient approximation. The preferred adsorption site, relaxation of the surface structure, adsorption energy, work function change and changes of the properties with the coverage as well as total density of state are investigated. The

Yun Wang; Wenning Wang; Kangnian Fan; Jingfa Deng

2001-01-01

431

First-Principles Studies of Antiferromagnetic MnO and NiO Surfaces  

Microsoft Academic Search

Surface properties of antiferromagnetic MnO(001) and NiO(001) are investigated by means of first-principles electronic structure calculations. It is shown that the surface O sites have finite spin magnetic moments of 0.04 muB and 0.07 muB for MnO and NiO surfaces, respectively. Because of non-zero energy gap even at the surface, the spin magnetic moment of Mn and Ni sites are

Hiroyoshi Momida; Tamio Oguchi

2003-01-01

432

First-principles study of the polar (111) surface of Fe3O4  

Microsoft Academic Search

We performed a systematic full-potential density functional theory study with the generalized gradient and local density approximation+U approaches on five possible (1×1) terminations of the low-index polar (111) surface of Fe3O4 . Applying the concepts of first-principles thermodynamics, we analyze the composition, the structure, and the stability of the Fe3O4 (111) orientation at equilibrium with an arbitrary oxygen environment. The

L. Zhu; K. L. Yao; Z. L. Liu

2006-01-01

433

First-principles studies on surface electronic structure and stability of LiFePO 4  

Microsoft Academic Search

The stabilities and electronic structure of LiFePO4 (001), (010) and (100) surfaces have been studied using the first-principles method. Calculated cleavage energies show that LiFePO4 (010) is the most stable termination. Large differences in electronic density of states are observed between the LiFePO4 (001) and (100) surfaces comparing to the counterpart of the bulk, due to the broken symmetry at

Xiaofang Ouyang; Molin Lei; Siqi Shi; Cuilan Luo; Desheng Liu; Diyou Jiang; Zhiqing Ye; Minsheng Lei

2009-01-01

434

First Principles Study of the Cross-Sectional Surface Structure of III-V Superlattices  

Microsoft Academic Search

Using first-principles density-functional theory and scanning tunneling microscopy, we study the detailed interface structure and cross-sectional STM topography of 6.1-Å III-V superlattices. We first determine the interface geometry of InAs\\/GaSb superlattices within the Local Density Approximation (LDA) and pseudopotential formalism. By integrating the resulting local density of states, we then generate simulated STM images over the cross-sectional surface and compare

Seong-Gon Kim; S. C. Erwin; B. Z. Nosho; L. J. Whitman

2000-01-01

435

First-principles study of TiB2(0001) surfaces  

Microsoft Academic Search

The TiB2(0001) surfaces are calculated using the first-principles total-energy plane-wave pseudopotential method based on density functional theory. It is found that there are large relaxations within the top three layers for both termination surfaces, and the outermost and second interlayer relaxations for B-terminated surfaces are much larger than those for Ti-terminated surfaces. The charge depletion in the vacuum and the

Yanfeng Han; Yongbing Dai; Da Shu; Jun Wang; Baode Sun

2006-01-01

436

First principles study of Ag, Au, and Cu surface segregation in FePt-L10  

Microsoft Academic Search

Doping FePt nanoparticles could be a possible approach to achieve high L10 order and magnetic anisotropy. To address stability, first-principles studies of surface segregation of dilute Ag\\/Au\\/Cu solutes at and near the (001)\\/(100)\\/(111) surfaces of FePt-L10 are performed. It is found that a strong surface segregation tendency at first outer layer is present in all the cases. For Cu, segregation

Roman V. Chepulskii; Stefano Curtarolo

2010-01-01

437

First-principle study of GaN polar and nonpolar surfaces  

Microsoft Academic Search

The polarity of GaN surface may have a substantial influence on its electronic structure, So the (0001) and (112?0) surfaces and their electronic structures are studied based on density-functional theory using the local-density approximation (LDA) as well as the hybrid functional approach were studied by First-principles study. It presents the change of surface atoms position and analyses the surface electronic

Hong-Tao Zhao; Zhi-You Guo; Hua-Xiong Zhao; Yu-Fei Zhang

2010-01-01

438

First-principles study of bulk and (001) surface of TiC  

Microsoft Academic Search

The structural and electronic properties of bulk and (001) plane of TiC were investigated by the first-principles total-energy pseudopotential method based on density functional theory. The calculated bulk properties indicate that bonding nature in TiC is a combination of ionicity, covalency and metallicity, in which the Ti-C covalent bonding is the predominate one. The calculated results of structural relaxation and

Li-hong FANG; Li WANG; Jian-hong GONG; Hong-shang DAI; De-zhuang MIAO

2010-01-01

439