bluuebunny/crossref_metadata_2025_split huggingface dataset:
Tight-Binding Hamiltonians for Carbon and Silicon
Authors: Papaconstantopoulos D. A. , Mehl M. J. , Erwin S. C. , Pederson M. R. 
Published: April 2011

ABSTRACTWe demonstrate that our tight-binding method - which is based on fitting the energy bands and the total energy of first-principles calculations as a function of volume - can be easily extended to accurately describe carbon and silicon. We present equations of state that give the correct energy ordering between structures. We also show that quantities that were not fitted, such as elastic constants and the band structure of C60, can be reliably obtained from our scheme.
Negative Cauchy Pressure within the Tight-Binding Approximation
Authors: Nguyen-Maxh D. , Pettifor D. G. , Znam S. , Vitek V. 
Published: April 2011

ABSTRACTIt is well-known that the Embedded Atom Method (EAM) predicts positive Cauchy pressures for cubic metals if physically-motivated embedding functions are used. Supris-ingly, even if the angular character of the covalent bonding is included within an orthorgonal or non-orthorgonal Tight-Binding (TB) description, the Cauchy pressure for most elemental and binary metallic systems remains positive. We describe the results of a detailed breakdown of the different contributions to the Cauchy pressure within the Harris-Foulkes approximation (HFA) to density functional theory. We show that negative values of the Cauchy pressure for both elemental transition metals such as Ir and binary intermetallics such as Ti3Al, TiAl and TiAl3 are well reproduced by the HFA. We argue that the negative Cauchy pressure (NCP) arises namely from the environment dependence of the local TB orbitals which leads to both environment-dependent bonding integrals and overlap repulsion. We discuss a particular functional form for overlap repulsion which leads to NCP and compare it with different fitting schemes proposed recently in TB theory.
Large Scale Atomistic Simulations using the Tight Binding Approach
Authors: Celino M. , Cleri F. , Colombo L. , Rosati M. , Rosato V. , Tilson J. 
Published: April 2011

ABSTRACTAtomistic modelling of Materials Science problems often requires the simulation of systems with an irreducibly-large unit cell, such as amorphous materials, fullerites, or systems containing extended defects, such as dislocations, cracks or grain boundaries. Large-scale simulations with the Tight-Binding approach must face the computational obstacle represented by the O(N3)-scaling of the diagonalization of the Hamiltonian matrix. This bottleneck can be overcome by parallel computing techniques and/or the introduction of faster, O(N)-scaling algorithms. We report the activities performed in the frame of a collaboration among several research groups on the porting of TBMD codes on parallel computers. In particular, we describe the porting of a O(N3) TBMD code on different MIMD computers, with either distributed or shared memory, by using appropriate software tools. Furthermore, preliminary results obtained in the porting of an O(N) TBMD code on an experimental, hybrid MIMD-SIMD computer architecture are reported. The new perspective of using specialized platforms to deal with large-scale TBMD simulation is discussed.
Elasticity, Thermal Properties, and Molecular Dynamics Using Non-Empirical Tight-Binding
Authors: Cohen Ronald E. , Stixrude Lars , Wasserman Evgeny 
Published: April 2011

ABSTRACTWe have further developed and applied a new non-empirical tight-binding total energy model to properties of Si, Xe, and Fe at high pressures. We have studied elasticity of various phases of each of these, demonstrating that the new model is applicable to a wide range of materials, including semiconductors, rare gases, and transition metals. We have used the particle-in-a-cell method to study the thermal equation of state of hep Fe and find excellent agreement with the shock equation of state. A molecular dynamics code has been developed based on this method, and we have studied the properties of Fe liquid at high pressures.
Structural Disorder and Localized Gap States in Silicon Grain Boundaries from a Tight-Binding Model
Authors: Cleri F. , Keblinski P. , Colombo L. , Phillpot S. R. , Wolf D. 
Published: April 2011

ABSTRACTTight-binding molecular dynamics simulations of typical high-energy grain boundaries in silicon show that the atomic structure of the interface in thermodynamic equilibrium is similar to that of bulk amorphous silicon and contains coordination defects. The corresponding electronic structure is also amorphous-like, displaying extra states in the forbidden gap mainly localized around the coordination defects, where large changes in the bond-hybridization character are observed. It is proposed that such coordination defects in disordered high-energy grain boundaries are responsible for the experimentally observed gap states in polycrystalline Si.
Microstructure Characterization of Ceramics by Fractal Geometry
Authors: Xiong Z. X. , Baba-Kishi K. Z. , Shin F. G. 
Published: April 2011

ABSTRACTFollowing Mandelbrot's fractal theory, the irregular characteristics of the microstructural features of ferroelectric Pb(Sc0.5 Ta0.5)O3 ceramics, including grain boundaries and dislocation networks, were investigated. The microstructural features were imaged by electron microscopy. The fractal analyses were carried out manually and by image processing techniques, which show the value of the fractal dimension, D, varies according to the regularity of the microstructure. The value of, D, close to unity is an indication of an increasing degree of microstructural regularity, which is in good agreement with the simulated results.
A B.F.S. Method Survey of Surface and Interfacial Properties of Multicomponent Metallic Systems
Authors: Bozzolo G. , Ferrante J. , Good B. 
Published: April 2011

ABSTRACTComputer modeling of atomic processes on surfaces and bulk materials has seen noticeable growth due to the availability of new, powerful semiempirical techniques that provide both the numerical simplicity, physical foundation and computational efficiency necessary for the study of complex systems. In an attempt to partially summarize the current status of this work, we apply one of these techniques, the BFS method for alloys, to the study of several basic topics in surface structure and analysis: surface energies of alloy surfaces, multilayer relaxation of ordered alloy surfaces, temperature-dependent segregation profiles, surface alloying and thin film growth of multicompo-nent systems.
First-Principles Simulations and Z-Contrast Imaging of Impurities at <001> Tilt Grain Boundaries in Mgo
Authors: Yan Y. , Chisholm M. F. , Duscher G. , Pennycook S. J. , Maiti A. , Pantelides S. T. 
Published: April 2011

ABSTRACTFirst-principles density-functional calculations were used to study the effects of Ca impurities on the Σ=5 (310) <001> tilt grain boundaries in MgO. An equilibrium structure and two metastable structures of the grain boundaries in pure MgO have been established. The calculations further demonstrated that Ca impurities segregate at particular sites in the metastable grain boundary and induce a structural transformation. This result is consistent with atomic resolution Z-contrast imaging. The calculations also found that the impurities at the grain boundaries do not induce states in the band gap. The mechanism of the transformation is also discussed.
First-Principles Study of Interfacial Reaction Atomic Process at Silicon Oxidation
Authors: Kageshima H. , Shiraishi K. 
Published: April 2011

ABSTRACTA theoretical atomic process model of the interfacial oxidation reaction on a silicon substrate is proposed based on first-principles calculation. In the calculation, H-terminated Si(100) surfaces are used for the first approximation of the silicon-oxide/silicon interfaces. The proposed atomic process model is based on the plausible assumption that the remaining stress in the oxidized region is kept at a minimum and does not break the grown Si-O-Si network when the oxidation proceeds. As a natural consequence, Si atoms are emitted from the interface as the oxidation proceeds to release the stress due to substitution of Si-Si bonds by Si-O-Si bonds. This emission is consistent with well-known experiments of oxidation-induced stacking faults and oxidation-enhanced diffusion. Our model presents a microscopic approach for understanding the silicon oxidation process, whereas the widely accepted Deal-Grove model presents a macroscopic one.
Etching Process of Al Oxide on Si Surface with HF Treatment
Authors: Hoshino T. , Nishioka Y. 
Published: April 2011

ABSTRACTab initio theoretical calculations have been performed to reveal the etching mechanism of HF with Al metal oxide on Si surfaces. The following sequential process has been revealed to be the most probable reaction path. HF + AlO-Si surface → HO-Si surface + AlF (1), HF + HO-Si surface → F-Si surface + H2O (1'). Both of the reactions have potential energy barriers of 0.8∼1.0eV, and are exothermic by 0.3 and l. leV, respectively. Another reaction path that proceeds with one-step process, HF + AlO-Si surface → F-Si surface + AlOH (2), is also found to be probable for AlO etching.
Density Functional Theory for Studies of Multiple States of Inhomogeneous Fluids at Solid Surfaces and in Pores
Authors: Neimark A. V. , Ravikovitch P. I. 
Published: April 2011

ABSTRACTTwo new versions of the density functional theory (DFT), which correspond to the canonical ensemble and the Gibbs ensemble respectively, are applied to study multiple equilibrium states and associated hysteresis on examples of capillary condensation of argon in nanopores of MCM-41 mesoporous molecular sieves and Kr adsorption on carbon. The canonical ensemble DFT (CEDFT) allows us to trace not only the stable and the metastable states along the hysteresis loop, but also the unstable states inside the hysteresis loop. The Gibbs ensemble DFT (GEDFT) allows us to determine precisely the points of equilibrium phase transitions in confined fluids.
Melting, Freezing, Diffusion, and Colescence of Gold Nanoclusters
Authors: Lewis Laurent J. , Deltour Pierre , Jensen Pablo , Barrat Jean-Louis 
Published: April 2011

ABSTRACTWe present a detailed molecular-dynamics study of the coalescence of gold nanoclusters within the framework of the embedded-atom method. We find macroscopic sintering theories to be unable to describe the coalescing behavior of two small clusters, a failure we attribute to the fact that the nanocrystals are faceted; this has important consequences for the morphology of cluster-assembled materials. We also examine the static and dynamic properties of a 249-atom gold cluster on nickel surfaces.
Effect of Impurities on Σ3 (111) Grain-Boundary Fracture in Tungsten—Atomistic Simulation
Authors: Grujicic M. , Zhao H. , Krasko Genrich L. 
Published: April 2011

ABSTRACTThe effect of various impurities and micro-alloying additions (B, N, C, O, Al, Si, S and P) on the intrinsic resistance of the Σ3 (111) GB in Tungsten has been investigated using Molecular Dynamics simulation. The atomic interactions have been accounted for through the use of the Finnis-Sinclair interatomic potentials. The fracture resistance of the GB has been characterized by computing, in each case, the ideal work of GB separation, the Mode I stress intensity factor and the Eshelby F - conservation integral at the onset of crack propagation. The results obtained suggest that pure Tungsten is relatively resistant toward GB decohesion and that this resistance is further enhanced by the presence of B, C and N. Elements such as O, Al and Si on the other hand, have a relatively minor effect on the cohesion strength of the Σ3 (111) GB. In sharp contrast, S and P greatly reduce this strength making Tungsten quite brittle. These findings have been correlated with the effect of the impurity atoms on material evolution at the crack tip.
Molecular Dynamics Studies of Thin-Films of Sn On Cu
Authors: Ravelo R. , Aguilar J. , Baskes M. I. 
Published: April 2011

ABSTRACTUsing Molecular Dynamics, the evolution dynamics of Sn on the (111) and (100) surfaces of Cu have been investigated as a function of coverage and temperature. The interaction potentials are described by modified embedded atom method (MEAM) potentials. The calculated diffusion activation energies of Cu in Sn and Sn in Cu agree reasonably well with experimental values. We find that the structure of the overlayer depends on the morphology of the substrate and remains stable up to temperatures of the order of 70% of the melting temperature of the substrate at which diffusion of Sn into the substrate and Cu atoms onto the overlayer is observed.
Computer Simulation of Interfaces in Ceramics
Authors: Harding J. H. , Harris D. J. , Parker S. C. 
Published: April 2011

ABSTRACTThe methods for computer simulation of grain boundaries and hetero-interfaces in ceramics have long been available. We review developments in the field in two areas; the study of general grain boundaries and the simulation of hetero-interfaces. In both cases we discuss extrapolation from the symmetric case. We will also consider the validity of the continued use of classical potential models in an age of large-scale quantum calculations.
Effect of Domain Structure on the Magnetoresistance of Epitaxial Thin Films of Ferromagnetic Metallic Oxide SrRuO3
Authors: Rao R. A. , Kacedon D. B. , Eom C. B. 
Published: April 2011

ABSTRACTWe have grown epitaxial ferromagnetic metallic oxide SrRuO3 thin films with different domain structures on (001) LaAlO3 and miscut (001) SrTiO3 substrates. The effect of crystallographic domain structures on the magnetization and magnetoresistive behavior of epitaxial SrRuO3 thin films has been studied. Magnetization measurements on the single domain film on 2° miscut (001) SrTiO3 substrate showed that the in-plane [110] direction, which is aligned along the miscut direction, is the easier axis for magnetization compared to the [001] direction. This film also showed a strong anisotropie magnetoresistance (AMR) effect of ∼ 8% in magnitude. In contrast, the SrRuO3 thin film on (001) LaAlO3 substrate shows identical magnetization and magnetoresistance behavior in two orthogonal directions on the film due to the presence of 90 domains in the plane. For both the films, large negative magnetoresistance effects (-10%) were observed when the current and the applied magnetic field are parallel. The magnetoresistance behavior is explained in terms of suppression of spin fluctuations near Tc and the AMR effect.
Formation of Ferromagnetic/Ferroelectric Superlattices by a Laser MBE and Their Electric & Magnetic Properties
Authors: Tabata Hitoshi , Ueda Kenji , Kawai Tomoji 
Published: April 2011

ABSTRACTWe have constructed artificial superlattices with a combination of magnetic/magnetic and ferroelectric/ferromagnetic materials using a laser ablation technique. An ideal hetero-epitaxy can be obtained due to the similar crystal structure of the perovskite type di/ferroelectric BaTiO3, Pb(Zr,Ti)O3 (so-called PZT), SrTiO3 and ferro/antiferromagnetic LaFeO3, LaCrO3, (La,Sr)MnO3. First of all, we have controlled ferromagnetic order on LaFeO3/LaCrO3 superlattices formed on SrTiO3(l11) substrate. Such a spin structure(ferromagnetic order) can't be got in bulk condition. In the heterostructured ferromagnetic /ferroelectric devices, (La,Sr)MnO3/PZT there are remarkable and interesting phenomena. The electric properties of the ferromagnetic material can be controlled by the piezoelectric effect via distortion of the crystal structure.
Evaluation of Raman Scattering in La1-xMxMnO3 Single Crystals Due to Structural and Magnetic Transitions
Authors: Podobedovm V. B. , Weber A. , Romero D. B. , Rice J. P. , Drew H. D. 
Published: April 2011

ABSTRACTThe study of doped and undoped La1-xMxMnO3 single crystals (x = 0 to x = 0.3) in the temperature range from 5 K to 423 K is reported. The activity of phonon vibrational modes in optical spectra is analyzed and a comparison to experimental Raman data is presented. Different contributions from optical phonons to the Raman scattering process are discussed. It is shown that both the value of doping and temperature have a significant influence on Raman features, and these are strongly affected by structural and magnetic phase transitions. Due to the symmetry present in distorted lanthanum manganese compounds their Raman spectra were found to contain comparable contributions from first- and second-order phonon scattering. The common features related to both the reduction of the Jahn-Teller (JT) distortion and the temperature-induced phase transition were found in the Raman spectra of La1-xMxMnO3 crystals. An anomalous behavior of the low-frequency Raman mode in doped systems was found and explained in terms of the temperature-dependent ionic radius of the La/Sr ion sites. It was found that the strong dependence of the Ag Raman mode on the value of doping may serve as a useful tool for optical characterization of La1-xMxMnO3 compounds, including the inhomogeneity of the surface.
Lattice deformation and magnetic properties in epitaxial thin films of Sr1-xBaxRuO3
Authors: Fukushima Noburu , Sano Kenya , Schimizu Tatsuo , Abe Kazuhide , Komatsu Shuichi 
Published: April 2011

ABSTRACTThe crystal structure and magnetic properties in epitaxially grown Sr1-xBaxRuO3 on SrTiO3 substrates were determined. Epitaxial Sr1-xBaxRuO3 exhibits a simple perovskite structure in the whole region of Ba/Sr ratio, in contrast to a complex hexagonal layered perovskite in the case of Ba-rich bulk Sr1-xBaxRuO3 which has plane-sharing oxygen octahedra. Tetragonal deformation was enhanced from the pseudo cubic structure of SrRuO3 to a highly distorted tetragonal lattice in BaRu03. Electronic properties such as conductivity and magnetization were examined and compared to the results of band calculation in which a tetragonal distortion was taken into account. A metal-insulator transition was not observed in this system either in the experiment or in the simulation, and metallic conductivity was maintained in the whole region of Ba content. The ferromagnetic ordering which occurs at 160K in pseudo-cubic bulk SrRuO3 was suppressed in Sr1-xBaxRuO3 films with increasing tetragonal deformation and Curie temperature decreased to 50K in BaRuO3.
Resonant X-Ray Fluorescence Spectroscopy at The VL-Edges of Vanadium Oxides
Authors: Duda L.-C. , Stagarescu C. B. , Downes J. E. , Smith K. E. , Dräger G. 
Published: April 2011

ABSTRACTWe have studied resonant VLα-fluorescence spectra of vanadium oxides with V in several different oxidation states. The spectra are dominated by the O 2p-contribution centered at about 6 eV below the top of the valence band (VB-top). The V 3d-contribution, found close to the VB-top, increases with decreasing valency of the vanadium atoms. Resonant inelastic (Raman) x-ray scattering is fairly weak in these compounds and overlaps with the ordinary fluorescence spectrum. Large spectral changes of VLα-fluorescence in the metal-insulator transition of V2O3have been observed.
Phase Stability of The Mixed-Conducting Sr-Fe-Co-O System
Authors: Ma B. , Balachandran U. , Hodges J. P. , Jorgensen J. D. , Miller D. J. , Richardson J. W. 
Published: April 2011

ABSTRACTMixed-conducting ceramic oxides have potential uses in high-temperature electrochemical applications such as solid oxide fuel cells, batteries, sensors, and oxygen-permeable membranes. The Sr-Fe-Co-O system combines high electronic/ionic conductivity with appreciable oxygen permeability at elevated temperatures. Dense ceramic membranes made of this material can be used to separate high-purity oxygen from air without the need for external electrical circuitry, or to partially oxidize methane to produce syngas. Samples of Sr2Fe3-xCoxOy (with x = 0,0.6,1.0, and 1.4) were prepared by solid-state reaction method in atmospheres with various oxygen partial pressures (pO2) and were characterized by X-ray diffraction, scanning electron microscopy, and electrical conductivity testing. Phase components of the sample are dependent on cobalt concentration and pO2. Electrical conductivity increases with increasing temperature and cobalt content in the material.
Molecular Dynamic Simulation and Electrical Properties of Ba2In2O5
Authors: Kanzaki Masami , Yamaji Akihiko , Kawakami Kazuya 
Published: April 2011

ABSTRACTBrownmillerite(Ca2Al2O5-Ca2Fe2O5 solid solution) structure can be regarded as an oxygen-ion deficient perovskite structure. Because of high proportion of the oxygen vacancies in the structure, this material could be a candidate of fast oxide-ion conductor. Goodenough et al. indeed observed a first-order transition to a fast oxide-ion conductor at 930° C for Ba2In2O5 which adapts brownmillerite structure at ambient temperature. Molecular dynamics simulation was employed to study oxygen ion diffusion and phase transition of Ba2In2O5. The structure was well simulated at 300 K. When the system was heated, the original orthogonal cell transformed to a tetragonal cell at 2300 K. Inspection of the structure revealed that oxygen ions started to migrate from their original sites to nearest vacant oxygen sites at this temperature. The diffusion was restricted for the oxygen sites around In-tetrahedron, resulting highly anisotropie diffusion on the ac plane. At 4600 K it further transformed to an oxygen vacancies-disordered cubic perovskite structure. Although predicted transition temperature were apparently overestimated, the transition way to the phases with high oxygen ion diffusivity is consistent with the experimental results from electrical conductivity measurements. The high temperature cubic phase shows large ion conductivity. It is of interest to examine whether or not the cubic phase stabilizes in the low temperature region by making solid solution of another elements. We found that the cubic phase is stabilized below 500° C without any decrease of conductivity in BaIn1.9Ce0.1Oy and Ba2In1.8Nb0.2O5.
Mechanisms Causing Capacity Loss on Long Term Storage in NiMH System
Authors: Singh P. , Wu T. , Wendung M. , Bendale P. , Ware J. , Ritter D. , Zhang L. 
Published: April 2011

ABSTRACTCapacity recovery after long term storage and loaded storage is a critical issue with the NiMH system since its inception. A measurable loss in capacity is observed when cells are stored for long periods of time or discharged deeply to zero volts. The different mechanisms that are known to cause self discharge and capacity loss after storage and loaded storage will be the focus of this paper. Capacity loss after long term storage involves two main events. One is self discharge which causes the open circuit voltage(OCV) of the cell to drop. Self discharge is caused by decomposition of NiOOH, migration of metal ions and possible degradation of separator. Self discharge can be prevented by using separators which are stable at high temperatures and pH and have good ion trapping capability. Various separator types and treatments can play an important role in inhibiting metal ions from migrating thus reducing self discharge. Self discharge during storage causes a severe suppression in the voltage of the foam positive electrode. This drop in voltage causes a breakdown of the cobalt conductive network in the nickel positive electrode. Reduction of high valence cobalt(III) which forms the electrode's conductive network takes place at these low voltages. A permanent breakdown in the conductive network results in low efficiency of the cell on consecutive charge and discharge cycles. In addition, the cobalt in its lower valence states can migrate away from the electrode into the separator causing shorts. These events effect the charge and discharge efficiency of these cells thereby resulting in capacity loss. Various mechanisms causing self discharge which affect capacity recovery after long term storage and loaded storage are discussed in this paper.