Description Of Bulk Research Articles

A systematic DFT study of structure and electronic properties of titanium dioxide.

DFT functionals are of paramount importance for an accurate electronic and structural description of transition metal systems. In this work, a systematic analysis using some well-known and commonly used DFT functionals is performed. A comparison of the structural and energetic parameters calculated with the available experimental data is made in order to find the adequate functional for an accurate description of the TiO2 bulk and surface of both anatase and rutile structures. In the absence of experimental data on the surface energy, the theoretical predictions obtained using the high-accuracy HSE06 functional were used as a reference to compare against the surface energy values calculated with the other DFT functionals. A clear improvement in the electronic description of both anatase and rutile was observed by introducing the Hubbard U correction term to PBE, PW91, and OptPBE functionals. The OptPBE-U4 functional was found to offer a good compromise between accurately describing the structural and electronic properties of titania.

A general-purpose machine learning Pt interatomic potential for an accurate description of bulk, surfaces, and nanoparticles.

A Gaussian approximation machine learning interatomic potential for platinum is presented. It has been trained on density-functional theory (DFT) data computed for bulk, surfaces, and nanostructured platinum, in particular nanoparticles. Across the range of tested properties, which include bulk elasticity, surface energetics, and nanoparticle stability, this potential shows excellent transferability and agreement with DFT, providing state-of-the-art accuracy at a low computational cost. We showcase the possibilities for modeling of Pt systems enabled by this potential with two examples: the pressure-temperature phase diagram of Pt calculated using nested sampling and a study of the spontaneous crystallization of a large Pt nanoparticle based on classical dynamics simulations over several nanoseconds.

Lattice three-gluon vertex in extended kinematics: Planar degeneracy

We present novel results for the three-gluon vertex, obtained from an extensive quenched lattice simulation in the Landau gauge. The simulation evaluates the transversely projected vertex, spanned on a special tensorial basis, whose form factors are naturally parametrized in terms of individually Bose-symmetric variables. Quite interestingly, when evaluated in these kinematics, the corresponding form factors depend almost exclusively on a single kinematic variable, formed by the sum of the squares of the three incoming four-momenta, q, r, and p. Thus, all configurations lying on a given plane in the coordinate system (q2,r2,p2) share, to a high degree of accuracy, the same form factors, a property that we denominate planar degeneracy. We have confirmed the validity of this property through an exhaustive study of the set of configurations satisfying the condition q2=r2, within the range [0,5GeV]. This drastic simplification allows for a remarkably compact description of the main bulk of the data, which is particularly suitable for future numerical applications. A semi-perturbative analysis reproduces the lattice findings rather accurately, once the inclusion of a gluon mass has cured all spurious divergences.

Numerical Simulation of Electrochemical Systems by Coupling Analytical Calculation of the Diffuse Double Layer and Finite Element Description of Solution Bulk

AbstractNumerical modelling of electrochemical systems covers length scales from the nanometers up to the macroscopic scale. With finite element methods, the mesh must be extremely fine to describe the diffuse double layer, thus increasing the needed computational resources. We propose a method to describe the diffuse double layer by analytical equations, expressed as boundary conditions for the partial differential equations describing the solution bulk. We apply the method to a one‐dimensional system, i. e. to a cell with plane parallel electrodes, in the presence of a redox couple and a supporting electrolyte. We provide evidence of the precision of the method.

Does β-PbO2 harbor topological states?

The electronic properties of β-PbO2, have been controversial for several decades. Experiments find behavior ranging from metallic, attributed to oxygen vacancies, to indirect semiconducting for stoichiometric samples with a gap of 0.61 eV. Theory leads to similar ambiguities, and predicts this phase to be metallic (PBE, HSE06) or to possess a small bandgap (HSE06). An area where this inconsistency is amplified, is when a material behavior depends on the electronic structure in the vicinity of the Fermi energy, such as topological states. In our work, we use a self-consistent DFT + U approach and find that stoichiometric β-PbO2 to be an indirect semiconductor with a band gap of ∼0.8 eV, similar to experiment. The larger bandgap requires at least ∼4% strain, to drive β-PbO2 into a nodal line semimetallic state, significantly larger strains than reported previously. Moreover, we find that the nodal line semimetallic state is not protected against spin-orbit-coupling. Also, the surface computations do not show any evidence for topologically protected states near the Fermi energy. Therefore, our results strongly suggest that β-PbO2 is a topologically trivial material, consistent with experiment, but in stark contrast to previous computations. Previously reported topologically protected states in β-PbO2 are attributed to an inaccurate description of the (bulk) optical properties.

An effective partial charge model for bulk and surface properties of cubic ZrO2, Y2O3 and yttrium-stabilised zirconia.

In this work a newly parametrised Coulomb plus Buckingham potential formulation for cubic ZrO2, Y2O3 and yttrium-stabilised zirconia (YSZ) is presented. The density and pair distributions obtained for neat ZrO2 and Y2O3 under ambient conditions are in excellent agreement with experimental data, while the vibrational power spectra are highly similar compared to those obtained via ab initio molecular dynamics simulations at the PBEsol level. In addition, it is shown that the use of effective partial charges has several advantages compared to interaction potentials employing the oxidation states in the evaluation of the coulombic interactions: (i) the diffusion coefficient and the associated activation energy of oxygen ions evaluated for YSZn (n = 4 to 12) display the best agreement with experimental data; (ii) no unphysical reorganisation of the interface and the bulk are observed in simulations of the (110) and (111) surfaces of cubic ZrO2 and Y2O3, while due to the strong coulombic contributions in the case of the tested full-charge models a pronounced restructuring of the interface and the bulk is observed in the ZrO2 case, and (iii) the use of effective partial charges ensures compatibility with existing solvent models and force-fields for the treatment of molecular compounds.

Anatomy of magnetic anisotropy induced by Rashba spin-orbit interactions

Magnetic anisotropy controls the orientational stability and switching properties of magnetic states, and therefore plays a central role in spintronics. First-principles density-functional-theory calculations are able, in most cases, to provide a satisfactory description of bulk and interface contributions to the magnetic anisotropy of particular film/substrate combinations. In this paper we focus on achieving a simplified understanding of some trends in interfacial magnetic anisotropy based on a simple tight-binding model for quasiparticle states in a heavy-metal/ferromagnetic-metal bilayer film. We explain how to calculate the magnetic anisotropy energy of this model from the quasiparticle spin-susceptibility, compare with more conventional approaches using either a perturbative treatment of spin-orbit interactions or a direct calculation of the dependence of the energy on the orientation of the magnetization, and show that the magnetic anisotropy can be interpreted as a competition between a Fermi-sea term favoring perpendicular anisotropy and a Fermi-surface term favoring in-plane anisotropy. Based on this finding, we conclude that perpendicular magnetic anisotropy should be expected in an itinerant electron thin film when the spin magnetization density is larger than the product of the band exchange splitting and the Fermi level density-of-states of the magnetic state.

Experimental study and analytical modelling of the effect of the driving frequencies on dual frequency capacitively coupled plasmas

A capacitively coupled radio frequency discharge driven by two harmonics substantially different from each other allows some degrees of independent control of the ion energy and ion flux. The low frequency (ωLF) source controls the ion energy, while the ion flux is controlled by the high frequency (ωHF) source. The choices of the driving frequencies can influence the properties of the dual frequency capacitively coupled plasmas (2f CCP). Here, we study the effect of the driving frequencies on the properties of the 2f CCPs, i.e., on the generation of a DC self-bias, the excitation of the non-linear Plasma Series Resonance (PSR) effect as well as the independent control over the mean ion energy and the flux in three different 2f CCP experiments: namely (2.26 + 13.56) MHz, (2.26 + 27.12) MHz, and (13.56 + 27.12) MHz. We also use a non-liner global model that consists of a description of the plasma bulk based on a fluid dynamic approach coupled to a separate model of the sheath. We use argon and cover a wide range of operating conditions. We find the choices of different ωLF and ωHF's result in substantial changes of the decoupling of the mean ion energy from the ion flux. We also observe that increasing ωHF decreases the DC self-bias at a fixed value of ωLF. The PSR effect and, therefore, the electron power deposition strongly depend on the high frequency source. The experimental results are consistent with the predictions of the non-linear global model which proves the usefulness of such a rather simplistic model to study of 2f CCP discharges.

A nonlinear global model of single frequency capacitively coupled plasma and its experimental validation

The behavior of a single frequency capacitively coupled plasma (CCP) driven by 13.56 MHz rf source is investigated using an approach that integrates a nonlinear global analytical model and experimental data. The non linear model consists of a description of the plasma bulk, based on a fluid dynamics approach coupled to a separate model of the sheath. The parameters used in the model are obtained by operating the single frequency CCP experiment (13.56 MHz) in argon at working pressures 73 to 400m torr. Experimentally measured plasma parameters such as the electron density, electron temperature, the discharge symmetry parameter as well as the rf voltage waveforms are the inputs of the theoretical model. Model results of the DC self bias and rf current for various operating pressures and powers are shown. A comparison of the outputs of the numerical results is done with the experimentally obtained values of the DC self bias and rf current. A good quantitative correspondence between them is obtained. The results presents may substantially improve the understanding of the behavior of the capacitively coupled plasma.

Phase field modeling of hydraulic fracturing with interfacial damage in highly heterogeneous fluid-saturated porous media

In this work, we propose an extension of the phase field model for the modeling of hydraulic fracturing or cracking in heterogeneous saturated porous media. The involved extensions comprise: (a) taking into account interfacial damage between the inclusions and the matrix; (b) modeling fluid flow within both matrix cracks and interfacial cracks; (c) the possibility to handle geometries of the heterogeneous media in the form of regular grids of voxels e.g. as obtained from experimental imaging techniques. The developed numerical framework is based on the phase field method with a regularized description of both bulk and interface discontinuities, extended to a fully coupled hydro-mechanical framework. Both 2D and 3D examples are presented for hydro-mechanical microcracking initiation and propagation in voxel-based models of complex heterogeneous media with interfacial damage between the inclusions and the matrix.

Simultaneous description of bulk and interfacial properties of fluids by the Mie potential

ABSTRACTThe vapour–liquid equilibrium of the Mie potential, where the dispersive exponent is constant (m = 6) while the repulsive exponent n is varied between 9 and 48, is systematically investigated by molecular simulation. For systems with planar vapour–liquid interfaces, long-range correction expressions are derived, so that interfacial and bulk properties can be computed accurately. The present simulation results are found to be consistent with the available body of literature on the Mie fluid which is substantially extended. On the basis of correlations for the considered thermodynamic properties, a multi-criteria optimisation becomes viable. Thereby, users can adjust the three parameters of the Mie potential to the properties of real fluids, weighting different thermodynamic properties according to their importance for a particular application scenario. In the present work, this is demonstrated for carbon dioxide for which different competing objective functions are studied which describe the accuracy of the model for representing the saturated liquid density, the vapour pressure and the surface tension. It is shown that models can be found which describe simultaneously the saturated liquid density and vapour pressure with good accuracy, and it is discussed to what extent this accuracy can be upheld as the model accuracy for the surface tension is further improved.

Applied Aspects of Pedotransfer Modelling in the Soil Physics

Purpose. To consider some examples of pedotransfer models use. Methods. Statistics, mathematical modeling. Results. On the basis of mass experimental material which is available in a database «Soil properties of Ukraine» are developed linear and nonlinear pedotransfer models for the description of equilibrium bulk of density, soil-hydrological constants and structure. As base parameters in models data of humus contents and physical clay are used. Conclusions.Various applied aspects of pedotransfer modelling mainly in the soil physics are proved, which, we hope, will promote popularization of this perspective direction.

SCC-DFTB parameters for simulating hybrid gold-thiolates compounds.

We present a parametrization of a self-consistent charge density functional-based tight-binding scheme (SCC-DFTB) to describe gold-organic hybrid systems by adding new Au-X (X = Au, H, C, S, N, O) parameters to a previous set designed for organic molecules. With the aim of describing gold-thiolates systems within the DFTB framework, the resulting parameters are successively compared with density functional theory (DFT) data for the description of Au bulk, Aun gold clusters (n = 2, 4, 8, 20), and Aun SCH3 (n = 3 and 25) molecular-sized models. The geometrical, energetic, and electronic parameters obtained at the SCC-DFTB level for the small Au3 SCH3 gold-thiolate compound compare very well with DFT results, and prove that the different binding situations of the sulfur atom on gold are correctly described with the current parameters. For a larger gold-thiolate model, Au25 SCH3 , the electronic density of states and the potential energy surfaces resulting from the chemisorption of the molecule on the gold aggregate obtained with the new SCC-DFTB parameters are also in good agreement with DFT results.

Noncollinear magnetic phases and edge states in graphene quantum Hall bars

Application of a perpendicular magnetic field to charge neutral graphene is expected to result in a variety of broken symmetry phases, including antiferromagnetic, canted, and ferromagnetic. All these phases open a gap in bulk but have very different edge states and noncollinear spin order, recently confirmed experimentally. Here we provide an integrated description of both edge and bulk for the various magnetic phases of graphene Hall bars making use of a noncollinear mean field Hubbard model. Our calculations show that, at the edges, the three types of magnetic order are either enhanced (zigzag) or suppressed (armchair). Interestingly, we find that preformed local moments in zigzag edges interact with the quantum spin Hall like edge states of the ferromagnetic phase and can induce backscattering.

Bond order potential for gold

We develop an analytic bond order potential for modelling of gold. The bond order formalism includes bond angularity and offers an alternative approach to the embedded atom type potentials frequently used to describe metallic bonding. The advantage of the developed potential is that it can be extended to describe interactions with covalent materials. Experimental and ab initio data of gold properties is used to fit the potential and a good description of bulk and defect properties is achieved. We use the potential to simulate melting of nanoclusters and find that the experimentally observed size dependent melting behaviour is reproduced qualitatively.

Two-loop corrections for nuclear matter in a covariant effective field theory

Although one-loop calculations provide a realistic description of bulk and single-particle nuclear properties, it is necessary to examine loop corrections to develop a systematic finite-density power-counting scheme for the nuclear many-body problem when loops are included. Moreover, it is imperative to study exchange and correlation corrections systematically to make reliable predictions for other nuclear observables. One must also verify that the natural sizes of the one-loop parameters are not destroyed by explicit inclusion of many-body corrections. The loop expansion is applied to a chiral effective hadronic Lagrangian; with the techniques of Infrared Regularization, it is possible to separate out the short-range contributions and to write them as local products of fields that are already present in our Lagrangian. (The appropriate field variables must be re-defined at each order in loops.) The corresponding parameters implicitly include short-range effects to all orders in the interaction, so these effects need not be calculated explicitly. The remaining (long-range) contributions that must be calculated are nonlocal and resemble those in conventional nuclear-structure calculations. Calculations at the two-loop level are carried out to illustrate these techniques at finite densities and to verify that the coupling parameters remain natural when fitted to the empirical properties of equilibrium nuclear matter.

Unified Description of Bulk and Interface-Enhanced Spin Pumping

We describe a mechanism for generating nonequilibrium electron-spin accumulation in semiconductors or metals by rf magnetic field pumping. With a semiclassical model we show that a rotating applied magnetic field (or the processing magnetization inside a weak ferromagnet) generates a dc spin accumulation. For bulk systems this spin accumulation is in general given by a small fraction of h omega, where omega is the rotation or precession frequency. With the addition of a neighboring, field-free region, and allowing for the diffusion of spins across the interface, the spin accumulation is dramatically enhanced towards h omega near the interface. The interface-enhanced spin accumulation obtained within our bulk-oriented model is surprisingly similar to predictions based on interface-scattering theory [A. Brataas, Phys. Rev. B 66, 060404(R) (2002)].

A hybrid finite element-analytical method for determining the intrinsic elastic moduli of particles having moderately extended shapes and a wide range of elastic properties

The presence of a low (dilute) concentration of particulate inclusions having any shape and any property mismatch with the matrix in which they are placed, influences the elasticity of the resulting composite material. This influence is via the intrinsic moduli, which characterize the first-order term in an expansion of the effective properties in terms of the volume fraction of inclusions. The intrinsic moduli have only been solved exactly for general property mismatch in the case of ellipsoidal particles, and are analytically intractable for more general geometries. The problem of computing intrinsic elastic moduli for general geometries is approached by combining exact information that is known from analytical theory with numerical finite element computations to obtain an approximate analytical description of the intrinsic bulk ( K) and shear ( G) moduli of the particles as a function of shape and matrix and inclusion elastic properties. The approximant analytical equations are developed for a wide range of isotropic elastic property ( K, G, and Poisson’s ratio) mismatch with the matrix and a modest range in shape, so that these approximants should be useful in characterizing property changes in real composite materials (e.g., concrete and polymer nanocomposites) to which a dilute concentration of particles have been added. Particular emphasis is given in this initial study to particles having a rectangular parallelepiped shape.

Application and Extrapolation of Mean-Field Models in the Heavy and Superheavy Regions

Mean-field models have been quite successful in the description of bulk nuclear properties and the shell structure, so that they provide an attractive basis for the extrapolation of nuclear structure to the boundaries of the periodic system. We have investigated the performance of both Skyrmeforce and relativistic mean-field models for a larger sample of experimentally well known nuclei by comparing the predictions of various parameter sets corresponding to different versions of the models or simply distinguished by the data used for their fitting. This allows a critical evaluation of the uncertainty of extrapolation. The calculations show that while all parametrizations produce acceptable results in known nuclei, even the major magic shells predicted for the superheavy region depend on the choice of parameter set. The exact formulation and strength of the spin-orbit force appear to be the decisive factor. In addition a new formulation and parametrization of the relativistic point-coupling model with derivative and nonlinear terms is also discussed.

Modeling of High-Pressure Adsorption Using the Bender Equation of State

A model describing high-pressure adsorption based on the simplified local density model and the Bender equation of state is proposed. The Bender equation of state was used for the description of both bulk and adsorbed fluid. The model is capable of characterizing adsorbent material in terms of structural parameters such as the skeletal density, the micropore volume, and the energy of adsorption. The model was successfully applied to high-pressure adsorption isotherms of nitrogen and methane up to 50 MPa at 40 °C on a series of synthetic carbons and at 25−70 °C on the commercial carbon Norit R1. The structural parameters of the investigated carbons obtained by the proposed method are in good agreement with those obtained by independent methods based on standard and advanced characterization from nitrogen adsorption at 77 K.