bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magnetism and Spin Tunneling in Nanostructures
Authors: Bratkovsky Alexander 
Published: April 2011

ABSTRACTIn the present paper different tunneling mechanisms in conventional and half-metallic ferromagnetic tunnel junctions are analyzed within the same general method. Theoretically calculated direct tunneling in iron group systems leads to about a 30% change in resistance, which is close but lower than experimentally observed values. It is shown that the larger observed values of the TMR might be a result of tunneling involving surface polarized states. We find that tunneling via resonant defect states in the barrier radically decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel diode structure would give a TMR of about 8%. With regards to inelastic tunneling, magnons and phonons exhibit opposite effects: one-magnon emission generally results in spin mixing and, consequently, reduces the TMR, whereas phonons are shown to enhance the TMR. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR.The model presented here is applied qualitatively to half-metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin-mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2, and an account of their peculiar band structures is presented. The implications and relation of these systems to CMR materials, which are nearly half-metallic, are discussed.
Tight-Binding Linear Muffin-Tin Orbital Implementation of the Difference Equation Green's Function Approach for 2D-Periodic Systems
Authors: Schilfgaarde Mark Van , Lambrecht Walter R. L. 
Published: April 2011

ABSTRACTThe central ideas and the advantages of the difference equation Green's function approach for layered systems with 2D-periodicity are presented. The tight-binding linear muffin-tin orbital and other practical aspects of the implementation are discussed. Preliminary test results are presented.
Three-Body Correlation in the Diluted Generalized Hubbard Model
Authors: Navarro O. , Avignon M. 
Published: April 2011

ABSTRACTA real-space method has been used to solve the generalized Hubbard Hamiltonian for a system with few electrons. The method is based on mapping the correlated many-body problem onto an equivalent tight-binding one in a higher dimensional space. For a linear chain, we have obtained an exact solution of the problem of three non-parallel electrons. The three-body correlation are studied by examining the binding energy in the ground state, for different values of the hopping parameters and of the on-site (U) and nearest-neighbor (V) interactions.
Self-Consistent Tight-Binding Approximation Including Polarisable Ions
Authors: Finnis M. W. , Paxton A. T. , Methfesselt M. , van Schilfgaarde M 
Published: April 2011

ABSTRACTUntil recently, tight-binding has been applied to either covalent or metallic solid state systems, or charge transfer treated in a simple point charge framework. We present a self-consistent tight-binding model which, for the first time, includes electrostatic ion polar-isabihty and crystal field splitting. The tight-binding eigenvectors are used to construct multipole moments of the ionic charges which are used to obtain angular momentum components of the electrostatic potential in structure constant expansions. Our first test of the model is to study the phase stability in zirconia; in particular the instability of the fluorite phase to a spontaneous symmetry breaking, and its interpretation in terms of band effects and ion polarisability. This new formalism opens up the way to apply the tight-binding approximation to problems in whichpolarisationof atomic charges is important, for example oxides and other ceramic materials and surfaces of metals.
Tight-Binding Calculations of Complex Defects in Semiconductors: Comparison with AB Initio Results
Authors: Kohyama M. , Arai N. , Takeda S. 
Published: April 2011

ABSTRACTComplex defects in Si and SiC such as coincidence tilt boundaries, planar defects and self-interstitial clusters were dealt with by using the transferable tight-binding method for Si and the self-consistent tight-binding method for SiC. These results have been compared with ab initio calculations of similar configurations. Essential features of the tight-binding results have been supported by the ab initio results. Especially, the agreement on stable atomic configurations is good, although there exits a tendency that energy increases are somewhat overestimated by the tight-binding methods. Serious faults have been found for the electronic structure by the tight-binding method for SiC.
Development of Simple SPD Tight-Binding Models for Transition Metals
Authors: Le Bacq O. , Willaime F. , Pasturel A. 
Published: April 2011

ABSTRACTA simple spd tight-binding scheme for atomistic simulations in transition metals is developed using an orthogonal basis set in the two-center approximation. The purpose of the present approach is to cope with the limitations of the canonical d-band model for elements at the beginning or at the end of the transition metal series while keeping a reduced number of parameters, and simple decay-functions. The parameters for the hopping integrals are fitted to FP-LMTO calculations of the volume dependence of band energies and band structures for several selected structures. Constant values are taken for the on-site energies, and the Born-Mayer pair potential is used for the repulsive term. Two approaches are compared for the total energy: the band model and the bond model. First applications are presented in the case of zirconium, where the difference between these models on phase stability results is particularly drastic. The bond model reproduces the stability of the hep phase and displays a good agreement with experiments for the elastic constants.
Optical Properties of Materials using the Empirical Tight-Binding Method
Authors: Lew Yan Voon L. C. 
Published: April 2011

ABSTRACTProcedures for calculating the optical properties of periodic systems using the empirical tight-binding method are compared. Results of the linear and nonlinear susceptibilities are presented using a recently developed exact formalism [Lew Yan Voon and Ram-Mohan. Phys. Rev. B47, 15500 (1993)].
Superlattice Calculation in an Empirical spds* Tight-Binding Model
Authors: Scholz R. , Jancu J.-M. , Bassani F. 
Published: April 2011

ABSTRACTWe propose an empirical tight-binding method for tetrahedrally coordinated cubic materials and apply it to group IV and III-V semiconductors, extending existing calculations by the inclusion of all five d-orbitals per atom in the basis set. The symmetry character of the conduction states at the surface of the Brillouin zone is considerably improved compared to calculations in smaller bases, and the corresponding band positions can be obtained within the experimental uncertainties. Because the distance dependence of the tight-binding parameters is derived from deformation potentials, the model is particularly suited for an investigation of strained superlattices where the states at direct or pseudo-direct conduction band minima are composed of wavefunctions of all the minima at Γ, X, and L of the constituents. Investigations of GaAs/AlAs and short-period superlattices indicate a strong mixing between the conduction band valleys in the miniband structure, and the results are in better agreement with experiments than state-of-the-art empirical pseudopotential calculations.
Self-Consistent Tight-Binding Methods Applied to Semiconductor Nanostructures
Authors: Carlo Aldo Di 
Published: April 2011

ABSTRACTA self-consistent tight-binding approach applied to semiconductor nanostructure is presented. This allows us to describe electronic and optical properties of nanostructured devices beyond the usual envelope function approximation. Example of applications are given for High Electron Mobility Transistors (HEMTs) and non-linear optical devices.
A Novel Scheme for Accurate Md Simulations of Large Systems
Authors: De Vita Alessandro , Car Roberto 
Published: April 2011

ABSTRACTWe present a simple and informationally efficient approach to electronic-structure-based simulations of large material science systems. The algorithm is based on a flexible embedding scheme, in which the parameters of a model potential are fitted at run time to some precise information relevant to localised portions of the system. Such information is computed separately on small subsystems by electronic-structure “black box” subprograms, e.g. based on tight-binding and/or ab initio models. The scheme allows to enforce electronic structure precision only when and where needed, and to minimise the computed information within a desired accuracy, which can be systematically controlled. Moreover, it is inherently linear scaling, and highly suitable for modern parallel platforms, including those based on non-uniform processing. The method is demonstrated by performing computations of tight-binding accuracy on solid state systems in the ten thousand atoms size scale.
Prediction Of Structure Candidates For Simple Ionic Compounds Using Global Optimisation
Authors: Schön J. C. , Jansen M. 
Published: April 2011

ABSTRACTA method is presented that allows, in principle, the prediction of the existence and structure of (metastable) compounds. We show the results of this approach for two examples of binary and ternary ionic compounds that have not been synthesized yet, but should stand a fair chance of being kinetically stable.
The Effects of the Electron-Phonon Interaction on the Vibrational Anomalies and Polymorphism in Titanium
Authors: Gavartin J. L. , Bacon D. J. 
Published: April 2011

AbstractWe apply the frozen phonon and molecular dynamics methods within the semiempirical orthogonal tight-binding framework to study the anomalous behaviour of the (0001) optical longitudinal (LO) and transverse (TO) phonons in the low temperature hep phase of Ti, and the ⅔[111]L and ½[110]T1 phonons in the high temperature bec phase. We demonstrate that, in agreement with previous findings in Zr, the anomalous thermal frequency shifts in hep Ti are related to the strong coupling of the electron density of states (DOS) to the particular lattice distortions. The distortions along the bec ⅔[111]L and ½[110]T1 phonons also significantly affect the DOS, resulting in the instability of these modes at low temperatures and triggering the bcc-hep and bcc-ω phase transformations.
Electronic Structure of Amorphous Silicon
Authors: Allan G. , Delerue C. , Lannoo M. 
Published: April 2011

ABSTRACTThe electronic structure of a continuous network model of tetrahedrally bonded amorphous silicon (a-Si) and of a model hydrogenated amorphous silicon (a-Si:H) that we have built from the a-Si model are calculated in the tight binding approximation. The band edges near the gap are characterized by exponential tails of localized states induced mainly by the variations in bond angles. The spatial localization of the states is compared between a-Si and a-Si:H. Valence band offset between the amorphous and the crystalline phases is calculated.
Electronic Structure and Atomic Configuration of Extended Defects in Metals by First-Principles and Semiempirical TB-LMTO Methods
Authors: Šob M. , Turek I. , Vitek V. 
Published: April 2011

ABSTRACTWe present two tight-binding linear muffin-tin orbitals (TB-LMTO) techniques for electronic structure calculations of extended defects (such as grain boundaries, interphase interfaces, surface layers etc.) in metals. The first is based on the first-principles self-consistent surface Green's function approach within the atomic-sphere approximation (ASA) utilizing two-dimensional periodicity in the layers parallel to the interface. In the second approach the Hamiltonian is constructed within the TB-LMTO-ASA as well, but semiempirical terms are employed to characterize the repulsive part of the interaction and the effect of electrons in interstitial space. While the adjustable parameters have only been fitted to the properties of ideal ground state structure, the semiempirical approach describes correctly the structural energy differences, phonon frequencies etc. Two examples are presented: the electronic structure of the Σ = 5(210)/[001] tilt grain boundary in tungsten is determined and the sensitivity of 4d magnetic moments in thin films to local environment is discussed. A comparison of the semiempirical TB-LMTO-ASA with the first-principles full-potential LMTO results is performed along the trigonal deformation path connecting the bcc, simple cubic and fee structures and the applicability of the semiempirical approach for simulating atomic structure of extended defects is assessed.
Defects in Aluminum: A Density Functional Study
Authors: Ramprasad R. , Atlas S. R. 
Published: April 2011

ABSTRACTWe describe a new method of examining defects and their interactions with each other at a microscopic level of detail. This involves efficiently calculating local properties at arbitrary spatial points, applicable in periodic supercell calculations. In particular, we have identified two quantities, the local energy density, ε(r) and the local stress density, σαβ(r), which are constrained to yield the total energy and the average macroscopic stress tensor, respectively, when integrated over the entire volume of the supercell. In systems with defects, these microscopic quantities provide important insights about the local nature and environment of defects. For the test case of bulk Al with a point defect, we demonstrate that these concepts result in meaningful local quantities characteristic to the point defect. We propose to use these methods to study the microscopic nature of vacancies at a grain boundary and their interaction with each other.
Potential Energy Calculation of two Structures of the Σ=11 <011> Tilt Grain Boundary in Silicon and Germanium
Authors: Chen J. , Hairie A. , Lebouvffir B. , Nouet G. , Paumter E. 
Published: April 2011

ABSTRACTAvailable experimental data concerning two possible atomic structures (A and B) of the Σ=11 <011> tilt grain boundary in silicon and germanium are analyzed. Previous empirical calculations concerning low and high temperature stability of these structures are summarized and criticized. New calculations using the semi-empirical tight-binding method (SETBM) are presented for low temperature stability. The tight-binding parameters of Mercer and Chou give results in agreement with experimental observations by high resolution electron microscopy.
H2S and HS-Adsorption on a Charged Cu Surface in an Electrolyte: Effect of Ionic Strength
Authors: Wilson W. D. , Schaldach C. M. 
Published: April 2011

ABSTRACTWe present a method for the calculation of the binding and rotational energies of neutral (H2S) and charged (HS-) molecules impinging upon a charged (Cu <100>) surface in the presence of an electrolyte. A molecular surface is constructed surrounding the H2S and HS-molecules forming boundary elements. A coupled Schrödinger-Poisson-Boltzmann iterative procedure treats the electronic structure of the molecules at the 6–31G**/MP2 level of theory and includes solvation effects through the single and double layers of charge induced by the electronic distribution. The molecule, together with its charged layers, forms a Molecular Single and Double Layer (MSDL), an object which then interacts with a Gouy-Chapman plane within the electrolyte. The additional induced charge at the molecular surface resulting from this electric field is obtained by solving a second set of boundary element equations. Repulsive interactions between the atoms of the molecule and those of the surface are obtained using a rigid-ion Hartree-Fock method. Binding energies of the molecule to the surface are determined as a function of the real surface charge imposed and also the ionic strength of the solution. It is found that surface charges can completely (180°) reorient these molecules and that the counterions in the solution can completely screen binding effects of even large surface charges.Work supported by the United States Department of Energy under contract #DE-AC04–94AL85000.
Diffusion Processes and Pre-Exponential Factors in Homo-Epitaxial Growth on Ag(100)
Authors: Kürpick Ulrike , Rahman Talat S. 
Published: April 2011

ABSTRACTWe provide a recipe for the evaluation of pre–factors for various diffusion processes relevant to crystal growth on Ag(100), based on explicit calculations of appropriate vibrational density of states using realistic many-body interaction potentials. Our calculated diffusion coefficients show that while hopping processes are favored for terrace self-diffusion, exchange processes dominate step-edge diffusion. We also find the characteristics of adatom diffusion along the < 100 >-step-edge to be strikingly different from the < 110 >-step-edge with interesting consequences for interlayer transport on this surface.
Monte Carlo Simulation of Magnetization Reversal Via Domain-Wall Motion in Fe Sesquilayers on W(110)
Authors: Kolesik M. , Novotny M. A. , Rikvold Per Arne 
Published: April 2011

ABSTRACTIron sesquilayers are ultrathin films with coverages between one and two atomic mono-layers. They consist of an almost defect-free monolayer with compact islands of a second atomic layer on top. This variation of the film thickness results in a strong interaction between domain walls and the island structure. It makes these systems an ideal laboratory to study the dynamics of domain walls driven by weak external fields. We present computer simulations which provide insight into the role of the thermally activated nucleation processes by which a driven domain wall overcomes the obstacles created by the islands.
Molecular Dynamics Study of the Effect of Magnetic Field on the Static and Dynamical Properties of Two-Dimensional Coulomb Systems
Authors: Gumbs Godfrey , Dubey Girija S. 
Published: April 2011

ABSTRACTMolecular dynamics simulations are used to examine the effect of a uniform perpendicular magnetic field on a two-dimensional (2D) interacting electron system and we analyze how the magnetic field affects the single-particle properties of the system. In this simulation, we include the effect of the magnetic field classically through the Lorentz force. Both the Coulomb interaction and the magnetic field are included directly in the electron dynamics to study their combined effect on the transport properties of the 2D system. Results are presented for the pair correlation function, the mean square displacement and the density correlation function, in the presence and absence of an external magnetic field. Our simulation results, obtained from a first-principles calculation, clearly show that the external magnetic field has no effect on the static properties, but it affects the dynamics.
Contribution to the Dielectric Behavior of Relaxers from the Surface of Nano-Order Cluster in the Materials
Authors: Cheng Z. -Y. , Guo Aqiang , Katiyar R. S. 
Published: April 2011

ABSTRACTBased on the analysis of the dielectric behavior of relaxor ferroelectrics, it is assumed that the material has two polarization processes. Thus, a formula, which can fit the experimental results very well, is proposed to describe the temperature and frequency dependence of the dielectric constant. The fitted results show that there is a resonance process in the material and the peak of the dielectric constant is determined with both the relaxation and resonance processes. The relaxation time analysis shows that the peak of the dielectric constant indeed consists of two parts.
Monte Carlo Simulations of Fluid Phase Transitions in Micropores: Hysteresis and Van Der Waals-Type Loops
Authors: Gusev V. Yu. 
Published: April 2011

ABSTRACTWe report the results of the Monte Carlo simulation study of the fluid phase transitions in micropores. The Gibbs, Canonical, and Grand Canonical ensembles were employed to obtain the adsorption isotherms of the Lennard-Jones nitrogen in carbon slit micropores at 77.4 K. It is shown for the first time that depending on the choice of the parameters of the Monte Carlo simulations, such as the number of particles in the simulation cells, the sampling from the Gibbs ensemble may supply either van der Waals-type or hysteresis loops in the phase transition zones. The existence of these loops is confirmed by independent sampling from the Canonical ensemble using Widom method of test particles. The Grand Canonical ensemble simulations always produce hysteresis loops within the phase transition zone. Outside the phase transition zones, sampling from all three ensembles yield essentially identical isotherms.
A Method to Predict the Adhesion Energy of Nonreactive Ceramic-Metal Wetting Based on Chemical Thermodynamics Considerations
Authors: Wu Jianxin , Wang Xiaodong 
Published: April 2011

ABSTRACTA previous model is refined to predict the adhesion energy of nonreactive ceramic-liquid metal wetting using the radii of metal atoms and ceramic ions and the enthalpies of mixing at infinite dilution. This method is convenient in that no fit to experiment data is required.
Interfacial Strain-Field in Thin-Film Microstructures
Authors: Shen Qun , Kycia Stefan 
Published: April 2011

ABSTRACTWe present x-ray diffraction analysis of interfacial strain fields in crystalline semiconductor microstructures. In particular we report the observations of diffuse interference fringes, asymmetric grating diffraction patterns, and asymmetric crystal-truncation-rod profiles due to the straw-varying regions in a microstructure. From these diffraction features, both the longitudinal and the transverse strain gradients are determined, in addition to the average stram. These experimental results can be used to interpret other physical properties in microstructures such as band-gap energies and surface morphologies.
Atomistic Simulations of the Work of Adhesion at Metal Oxide Interfaces
Authors: Ohira T. , Inoue Y. 
Published: April 2011

ABSTRACTIn this work we have studied magnetite scale adhesion on a tube made from some different type materials, and treated metal oxide interfaces between magnetite scales (Fe3O4) and stainless steel tube surface oxides (NiFe2O4) or chromium coating tube surface oxides (Cr2O3, FeCr2O4). In this paper we have defined new MEAM (Modified Embedded Atom Method) parameters for Fe-O, Cr-O, Ni-O, Fe-Ni, and Fe-Cr pair interactions based on experimental information such as lattice constants, cohesive energies, bulk moduli of those metal oxides, and have calculated the work of adhesion at Fe3O4 (110) / Fe3O4 (110), NiFe2O4(110), Cr2O3(110), Cr2O3(100), FeCr2O4(110) interfaces where a cluster-surface interfacial model is adopted. As the results of the calculation, it was found that there is an energy barrier, which prevents scales from approaching a tube surface, in a potential energy curve vs. a separation between scales and chromium coating surfaces. We further discuss the effect of surface directions at interfaces on the work of adhesion.
First Principles Calculations to Describe Zirconia Pseudopolymorphs
Authors: Jomard G. , Petit T. , Magaud L. , Pasturel A. 
Published: April 2011

ABSTRACTThe structural and electronic properties of four different structures of zirconia (ZrO2) are studied using ab initio total energy calculations. The calculations are made in the framework of density functional (DFT) and pseudopotential theory. We compare results given within the LDA (Local Density Approximation) and including Generalized Gradient Corrections (GGCs) in the Perdew Wang and Perdew Becke formalisms. We present results for pure and defective zirconia (oxygen vacancies and Zr substitution by Fe) showing the effects of such point defects on tne relative structural stabilities of the different pseudopolymorphs.