bluuebunny/crossref_metadata_2025_split huggingface dataset:
Empirical Tight-Binding Applied to Silicon Nanoclusters
Authors: Allan G. , Delerue C. , Lannoo M. 
Published: April 2011

ABSTRACTThe calculation of the electronic structure of silicon nanostructures is used to discuss the accuracy of results obtained by the tight-binding method. We first show that the level of refinement of the tight-binding approximation must be adapted to the calculated property. For example, an accurate description of both the valence and conduction bands which can be achieved with a 3rd-nearest neighbor approximation is necessary to calculate the variation of the gap energy with the silicon crystallite size. The sp3s* model which gives a bad description of the conduction band underestimates the confinement energy but can give good results when it is used to determine the variation of the crystallite band gap with pressure. To study Si-III (BC-8) nanocrystallites, we show that a good description of the bulk band structure can be obtained with non-orthogonal tight-binding but due to the large number of nearest neighbors one must take analytical variations of the parameter with interatomic distances. The parameters involved in these expressions can be easily fitted to the bulk band structures using the k-point symmetry without requiring the use of group theory. Finally we discuss the effect of increasing the size of the minimal-basis set and we show that it would be possible to get the values of the tight-binding parameters from a first-principles localized states band structure calculation avoiding the fit to the energy dispersion curves.
Third-Generation TB-LMTO
Authors: Andersen O. K. , Arcangeli C. , Tank R. W. , Saha-Dasgupta T. , Krier G. , Jepsen O. , Dasgupta I. 
Published: April 2011

ABSTRACTWe describe the screened Korringa-Kohn-Rostoker (KKR) method and the third-generation linear muffin-tin orbital (LMTO) method for solving the single-particle Schrödinger equation for a MT potential. In the screened KKR method, the eigenvectors CRL,i are given as the non-zero solutions, and the energies εi as those for which such solutions can be found, of the linear homogeneous equations: , where Ka (ε) is the screened KKR matrix. The screening is specified by the boundary condition that, when a screened spherical wave is expanded in spherical harmonics YR′L′ (ȓR′) about its neighboring sites R′, then each component either vanishes at a radius, rR′=aR′L′, or is a regular solution at that site. When the corresponding “hard” spheres are chosen to be nearly touching, then the KKR matrix is usually short ranged and its energy dependence smooth over a range of order 1 Ry around the centre of the valence band. The KKR matrix, K (εν), at a fixed, arbitrary energy turns out to be the negative of the Hamiltonian, and its first energy derivative, K (εν), to be the overlap matrix in a basis of kinked partial waves, φRL (εν, rR), each of which is a partial wave inside the MT-sphere, tailed with a screened spherical wave in the interstitial, or taking the other point of view, a screened spherical wave in the interstitial, augmented by a partial wave inside the sphere. When of short range, K (ε) has the two-centre tight-binding (TB) form and can be generated in real space, simply by inversion of a positive definite matrix for a cluster. The LMTOs, χRL (εν), are smooth orbitals constructed from φRL(εν, rR) and φRL(εν, rR), and the Hamiltonian and overlap matrices in the basis of LMTOs are expressed solely in terms of K (εν) and its first three energy derivatives. The errors of the single-particle energies εi obtained from the Hamiltonian and overlap matrices in the φ(εν)- and χ(εν) bases are respectively of second and fourth order in εi – εi. Third-generation LMTO sets give wave functions which are correct to order εi – εν, not only inside the MT spheres, but also in the interstitial region. As a consequence, the simple and popular formalism which previously resulted from the atomic-spheres approximation (ASA) now holds in general, that is, it includes downfolding and the combined correction. Downfolding to few-orbital, possibly short-ranged, low-energy, and possibly orthonormal Hamiltonians now works exceedingly well, as is demonstrated for a high-temperature superconductor. First-principles sp3 and sp3d5 TB Hamiltonians for the valence and lowest conduction bands of silicon are derived. Finally, we prove that the new method treats overlap of the potential wells correctly to leading order and we demonstrate how this can be exploited to get rid of the empty spheres in the diamond structure.
Structure, Bonding and Stability of Transition Metal Silicides: A Real-Space Perspective by Tight Binding Potentials
Authors: Miglio Leo , Tavazza Francesca , Garbelli Antonio , Celino Massimo 
Published: April 2011

ABSTRACTWe point out that the predictive power of tight binding potentials is not limited to obtaining fairly accurate total energy calculations and very satisfactory structural evolutions by molecular dynamics simulations. They also allow for a nice physical picture of the links between bonding and stability in different structures, which is particularly helpful in the case of binary suicides
Coupled Dynamics of Electrons and Nuclei in a Molecule Interacting with Ultrashort, Ultra-Intense Laser Pulses
Authors: Khosravi S. , Allen R. E. 
Published: April 2011

ABSTRACTWe have developed a technique for treating the coupled dynamics of electrons and nuclei in a molecule which is subjected to ultrashort and ultra-intense laser pulses. This technique has been employed in quantitatively accurate simulations for . Many interesting phenomena have been observed, including photodissociation, bond softening, above-threshold dissociation, ion-population trapping, electron-population trapping, sudden electronic transitions, Rabi flopping, and harmonic generation. Some representative results are shown here.
Efficient AB Initio Tight Binding
Authors: Horsfield Andrew , Kenny Steven David 
Published: April 2011

ABSTRACTTight binding is often seen as a middle ground method, lying between accurate ab iniiio methods, and fast empirical potential methods. The challenge is to make tight binding both as fast and as accurate as possible. One way to achieve this is to take established ab initio methods, and apply systematic approximations and efficient numerical techniques to obtain the greatest possible speed. A recently developed method, employing this approach, is described results presented for silicon, and recent developments (including fully self-consistent extensions) are reported.
Active Site Structure in Zeolite-Supported Lean NOx Catalysts
Authors: Blint Richard J. 
Published: April 2011

AbstractCopper ions exchanged into the zeolite, ZSM-5, are known to catalyze the reduction of NO to N2 in the oxidizing environment of lean automotive exhaust, but do require trace hydrocarbons. The interface of this catalyst provides a locally reducing environment even when the surrounding exhaust has molecular oxygen concentrations as high as 10%. The mechanism of this reduction is still unclear; however, structure calculations of the active sites and dynamics simulations of species diffusion within the zeolite pores provide insight into the mechanism of reduction. One active site is proposed to be Cu++ ionically bound to a bridge oxygen in a nonsymmetric site within a zeolitic pore. Another proposed site has the copper ion centrally (symmetrically) bound within a six member zeolitic ring. Evidence suggests that the copper ions cycle between Cu++ and Cu+ during the reduction of NO to N2. The nonsymmetric copper ions are shown here to be hydrated ions attached to the Brønsted acid sites in the zeolite. The calculations here show a four member, first hydration shell for Cu++ and a first shell of 2–3 oxygens for the Cu+ ion. An examination of the pore size in ZSM-5 indicates sufficient room for a first and second hydration shell for most of the possible acid sites. The conclusion that the copper ions are typically hydrated suggests that the catalytic mechanism may have much in common with homogeneous catalysis which is sometimes termed heterogenized homogeneous catalysis.
Diffusion of Si Adatoms on H-Terminated Si(001) Surfaces
Authors: Ohno Takahisa , Nara Jun , Sasaki Taizo 
Published: April 2011

ABSTRACTWe have investigated adsorption and diffusion processes of a Si atom on hydrogen-terminated Si(001)-(2×1) surfaces using first-principles total-energy calculations. The Si adatom segregates H atoms from surface Si dimer atoms on the monohydride terminated Si(001) surface. The migration of the Si adatom is assisted by the mobility of H atoms, that is, the Si adatom migrates on the surface by repeating release and capture of H atoms. The effects of a single H-terminated Si dimer on the Si migration are also examined. Calculated results are in good agreements with the variation in morphology of Si homoepitaxial films induced by hydrogen termination.
Microstructural Size and Alignment Effects on the Dielectric Response of Inhomogeneous Media
Authors: Ferris Kim F. , Exarhos Gregory J. , Risser Steven M. 
Published: April 2011

ABSTRACTThe dielectric response of inhomogeneous media presents a complex boundary problem dependent upon dielectric susceptibility and volume fraction of its components, as well as the physical size of the heterogeneities. In this paper, we have used the LOCALF method to examine the size dependent problem for cubic and columnar defects for ZrO2 dielectric films of varying microstructure size and volume fraction. Both the alignments of low dielectric components with respect to the applied field and their size can effectively modulate the composite response. Comparison of these results with the conventional effective medium approximation methods reinforces the need to include microstructural detail in the modeling approach.
Interfacial Properties and Mechanical Behavior of Titanium Aluminides
Authors: Yoo M. H. , Fu C. L. 
Published: April 2011

ABSTRACTThe role of various interfaces in deformation and fracture behavior of two-phase TiAl-Ti3Al alloys is analyzed on the basis of the specific interfacial and surface energies determined from ab initio calculations. The propensity of twinning observed in these alloys is consistent with the low true-twin boundary energy. The strong plastic anisotropy reported in TiAl polysynthetically twinned (PST) crystals is attributed partly to the localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. Interfacial fracture energies are estimated to be the highest for the α2/γ lamellar boundary and the lowest for the 120° rotational γ/γ boundary. The fracture mode mixity plays an important role in the crack-tip plasticity by ordinary slip and true-twinning, leading to translamellar and interfacial fracture.
Simulating Stm Images for the Gaas (110) Surface
Authors: Chelikowsky James R. , Kim Hanchul 
Published: April 2011

ABSTRACTScanning tunneling microscopy (STM) is one of the most successful experimental tools for probing the structure of semiconductor surfaces. However, care must be taken in interpreting the images at the atomistic limit. Often a “naive” interpretation of the STM image can yield an incorrect surface structure. We illustrate this situation via ab initio pseudopotential calculations for the STM image of the (110) GaAs surface. We will compare theoretical STM images to experimental images for the relaxed surface and for a surface with an As vacancy.
Low-Field Colossal Magnetoresistance in Manganite Tunnel Junctions
Authors: Nassar J. , Viret M. , Drouet M. , Contour J. P. , Fermon C. , Fert A. 
Published: April 2011

ABSTRACTLarge magnetoresistance values are obtained on tunnel junctions epitaxially deposited by pulsed-laser deposition and consisting of ferromagnetic manganite La0.67Sr0.33MnO3 electrodes separated by various tunnel barriers: SrTiO3, PrBaCu2.8Ga0.2O7 and CeO2. The magnetoresistance can be decomposed into a low-field and a high-field contribution. The latter is attributed to the presence of canted interfacial manganite phases, as confirmed by the temperature behaviour of the resistance. A low-field magnetoresistance ratio of 450% below 100 Oe is obtained on a sample with a SrTiO3 barrier, indicating a spin polarization value in excess of 0.83 for the manganite.
In-Plane Grain Boundary Effects on the Transport Properties of La0.7Sr0.3MnO3-δ Thin Films
Authors: Gu J. Y. , Ogale S. B. , Ghosh K. , Venkatesan T. , Ramesh R. , Radmilovic V. , Dahmen U. , Thomas G. , Noh T. W. 
Published: April 2011

ABSTRACTC-axis oriented La0.7Sr0.3MnO3.δ (LSMO) films were fabricated on the top of SrTiO3/YBa2Cu3O7 grown on MgO(001) substrates. From x-ray φ-scan and planar transmission electron microscopy measurements, the LSMO layer in the LSMO/SrTiO3/YBa2Cu3O7/MgO heterostructure is found to have coherent in-plane grain boundaries with a predominance of 45° rotations (between [100] and [110] grains) in addition to the cube-on-cube epitaxial relationship. Also, epitaxial LSMO/Bi4Ti3O12/LaAl03 (001) and c-axis textured LSMO/Bi4Ti3O12/SiO2/Si(001) with random in-plane grain boundaries are introduced as the counterparts for comparison. The resistivity and magnetoresistance (MR) of LSMO layer were measured and compared in these three different heterostructures. The low field MR at low temperature shows a dramatic dependence on the nature of the grain boundary. An attempt is made to interpret these results on the basis of correlation between the magnetic properties and grain structures.
The Effect of Elastic Strain on the Electrical and Magnetic Properties of Epitaxial Ferromagnetic SrRuO3 Thin Films
Authors: Gan Q. , Rao R. A. , Garrett J. L. , Lee Mark , Eom C. B. 
Published: April 2011

ABSTRACTWe report the direct measurement of elastic strain effect on the electrical and magnetic properties of single domain epitaxial SrRuO3 thin films, using a lift-off technique. The as-grown films on vicinal (001) SrTiO3 substrates are subjected to elastic biaxial compressive strain within the plane and tensile strain normal to the plane. In contrast, the lift-off films prepared by chemical etching of SrTiO3 substrates, are completely strain free with bulk like lattice. Our measurements indicate that the elastic strain can significantly affect the electrical and magnetic properties of epitaxial ferromagnetic SrRuO3 thin films. For the strained films, the Curie temperature (Tc) was suppressed to 150K and the saturation magnetic moment (Ms) was decreased to 1.15μB/Ru atom as compared to a Tc of 160K and Ms of 1.45μB/Ru atom for the strain free films. These property changes are attributed to the structural distortion due to the elastic strain in the as-grown epitaxial thin films. Our results provide direct evidence of the crucial role of lattice strain in determining the properties of the perovskite epitaxial thin films.
Magneto-Transport Properties in Layered Manganite Crystals
Authors: Kimura T. , Tomioka Y. , Okudat T. , Kuwahara H. , Asamitsu A. , Tokura Y. 
Published: April 2011

ABSTRACTAnisotropie charge transport and magnetic properties have been investigated for single crystals of the layered manganite, La2–2xMn2O7 (0.3≤x≤0.5). Remarkable variations in the magnetic structure as well as in the charge-transport properties are observed with changing doping-level x. A crystal with x=0.3 behaves like a 2-dimensional ferromagnetic metal in the temperature region between ∼90 K and ∼270 K, and shows the interplane tunneling magnetoresistance at lower temperatures. These characteristic charge-transport properties are attributed to the interplane magnetic coupling between the adjacent MnO2 bilayers, and are strongly affected by the application of pressure as well as low magnetic fields through the change in magnetic structure. With increase of the carrier concentration toward x=0.5, the charge-ordered phase is stabilized and dominates the charge transport and magnetic properties.
Mn K-Edge X-Ray Absorption Spectroscopy (XAS) Studies of La1-xSrxMnO3
Authors: Mini S. M. , Mitchells J. F. , Hinks D. G. , Alatasi Ahmet , Rosenmann D. , Kimball C. W. , Montano P. A. 
Published: April 2011

ABSTRACTSystematic Mn K-edge x-ray absorption spectroscopy (XAS) measurements on samples of La1-xSrxMnO3, which are precursors to colossal magnetoresistive (CMR) materials, are reported. Detailed results on the edge or chemical shift as a function of Sr concentration (hole doping) and sample preparation (air vs oxygen annealed), are discussed. For comparison, a systematic XANES study of the Mn K-edge energy shift, denoting valence change in Mn, has been made in standard manganese oxide systems. Contrary to expectations, the variation in near-edge energies for Mn in La0.725Sr0.275MnO3 were small when compared to the difference between that for manganese oxide standards of nominal valence of +3 and +4 (Mn2O3 and MnO2).
Hydrothermal Synthesis and Properties of Ceria Solid Electrolytes
Authors: Greenblatt M. , Huang W. , Shuk P. 
Published: April 2011

ABSTRACTThe structure, thermal expansion coefficients and ionic/electronic conductivity of (Ce1−xSmx)1-y(Tb/Pr)yO2-x/2)+δ (x=0–0.30; y=0–0.10) and Ce1−xCaxO2-x ( x= 0–0.17) solid electrolytes prepared hydrothermally were investigated. The uniformly small particle size (7–68 nm) of the hydrothermally prepared materials allows sintering of the samples into highly dense ceramic pellets at 1400°C, a significantly lower temperature, compared to that at 1600–1650°C required for samples prepared by solid state techniques. The maximum ionic conductivity in Ce1−x(Sm/Ca)xO2-δ was found at x= 0.17 for the Sm (σ600°c = 5.7×10−3 S/cm). In (Ceo0.83Sm0.17)1-y(Tb/Pr)yO1.915+δ the maximum conductivity was found at y= 0.17 for the Pr and Tb substituted ceria (σ600°c=7.6x10−3S/cm, Ea= 0.55 eV and σ600°c = 10−2 S/cm, Ea= 0.72 eV respectively) with electronic contribution to total conductivity around 20–30 %. When the Tb or Pr substitution in Ce0.83Sm0.17OO1.915 is reduced, the conductivity becomes more ionic, and is purely ionic at 2 %. However the conductivity at this lower level doping is not significantly lower.
Development of Electrolyte plate for Molten Carbonate Fuel Cell
Authors: Shoji C. , Matsuo T. , Suzuki A. , Yamamasu Y. 
Published: April 2011

ABSTRACTIt is important for the commercialization of molten carbonate fuel cell (MCFC) to improve the endurance and the reliability of the electrolyte plate. The electrolyte-loss in the electrolyte plate increases the cell resistance and deteriorates the cell voltage. The formation of cracks in the electrolyte plate causes a gas cross leakage between the fuel gas and the oxidizer gas. The pore structure of electrolyte plate must be stable and fine to support liquid electrolyte under MCFC operation. It is necessary to prevent the formation of cracks in electrolyte plate during thermal cycling. We have improved the stability of electrolyte plate using advanced LiAIO2powder and improved the durability of electrolyte plate for thermal cycling by the addition of the ceramic fiber.The initial cell voltage using electrolyte plate with advanced LiAIO2powder was 820mV at current density 150mA/cm2and the decay rate of cell voltage was under 0.5%/1000h for 8,800h. According to the post analyses, the pore structure of the electrolyte plate did not change. The stability of advanced LiAIO2powder was confirmed. It was proved that the electrolyte plate reinforced with ceramic fiber is effective for thermal cycling.
Pulsed Field Gradient NMR Investigation of Molecular Mobility of Trimethoxymethane in Nafion Membranes
Authors: Wu Y. , Za Wodzinski T. A. , Smart M. C. , Greenbaum S. G. , Prakash G.K.S. , Olah G. A. 
Published: April 2011

ABSTRACTMolecular mobility of water and trimethoxymethane (TMM) in NAFION membranes of two different equivalent weights (EW), 1100 and 1500, were investigated. Self-diffusion coefficients were determined by the NMR pulsed field gradient method, using the water and methyl protons NMR signals, in saturated NAFION samples containing concentrations of TMM in water varying between 0.5 and 14 M, and at temperatures varying from 30°C to 80°C. Diffusion of molecular species containing methyl protons is more than a factor of two slower in the 1500 EW membrane than in the 1100 EW membrane at 30°C and 1 M concentration. The difference rises to about a factor of four at 80°C and 14 M concentration. These differences are attributed to the lower extent of plasticization of the higher EW material as well as the greater effective distance between acid functional groups. NAFION samples containing methanol/water mixtures were also investigated. Comparison with the methanol results and the permeation behavior, as characterized by gas Chromatographie methods, show that more than half of the TMM is hydrolyzed to methanol as it passes through the acidic membrane. The implications of these findings for alternative fuels in direct oxidation fuel cells are discussed.
Preparation Of Pt/WO3 Powders and Thin Coatings on Carbon Black and Metal Supports by The Complex Sol-Gel Process (CSGP)
Authors: Deptula A. , Lada W. , Olczak T. , Sartowska B. , Ciancia A. , Giorgi L. , Di Bartolomeo A. 
Published: April 2011

ABSTRACTIn this work the CSGP was applied for preparation of Pt-WOx and its coatings on carbon black, silver and titan substrates. Saturated tungsten sol (0.15M) was prepared by dissolving (NH4)4W5O17 2.5 H2O in ascorbic acid solution. To this solution H2PtCl6 was added to obtain the molar ratio Pt:W=l. After evaporation and heating at 700°C for 2h homogeneous yellow-green powders were obtained. Another part of this solution was diluted with ethanol and after ultrasonic mixing used for preparation of coatings on Ag and Ti substrates by the immersion technique. Gel layers were dried at 200°C for 20h and then calcined at 700°C for 2h. The resulting white colored layers (thickness 10–65μm) were very adherent. Ascorbate aqueous tungsten-platinum sol diluted to 0.03 or 0.06 M ΣMe was used for preparation of Pt-W (10 and 20 %) catalyst by impregnation of carbon black Vulcan XC-72, followed by thermal treatment in air, argon and hydrogen atmospheres.
Determination of Oxygen Permeation Kinetics in A Ceramic Membrane with The Composition SrFeCo0.5O3.25-δ
Authors: Kim S. , Yang Y. L. , Christoffersen R. , Jacobson A. J. 
Published: April 2011

ABSTRACTThe oxygen permeation through an oxide membrane with bulk composition SrFeCo0.5O3.25-δ has been measured as a function of both oxygen partial pressure and temperature. The results of the pressure dependence of the permeation indicate that the oxygen transport in this membrane is dependent primarily on the bulk diffusion rate. Although the permeation experiments were carried out at temperatures within, or very close to, the range where SrFeCo0.5O3.25-δ is stable as a pure single phase, the membrane was found to consist of SrFe1.5-xO3.25-δ(x = ∼0.42) together with fractions of Sr(Co,Fe)O3-δ perovskite and Co-Fe oxide that formed as stable phases during densification of the membrane at high temperature (1090°C). These additional phases persisted in the membrane during the permeation measurements.