Slab Calculations Research Articles

Profiles of oxygen and titanium point defects in ferromagnetic TiO2 films

Experimentally it is shown that without any oxygen manipulation for TiO2, a strong room temperature ferromagnetism could be expected only in ultra-thin films, with the ideal thickness below 100 nm. Both bulks and nano-powders of TiO2 are diamagnetic, indicating that the surface and its nano-sublayers play very important roles in tailoring the magnetic properties in this type of compound. To shed a new light on the defect-related magnetism in the typical case of anatase TiO2 surfaces, we have performed a series of quantum-mechanical calculations for TiO2 slabs containing Ti or O vacancies in different distances from the (001) surface. The lowest formation energies were obtained for the Ti vacancies in the first sub-surface layer and the O vacancies within the surface. The computed magnetic states reflect complicated structural relaxations of atoms influenced by both the surface and vacant atomic positions. O atoms cannot contribute much to magnetic moment when Ti vacancies are isolated and far from the surface. Ti vacancies in TiO2 are only metastable. The formation energy of Ti interstitials is lower than for Ti vacancies since high-temperature annealing, especially with a lot of O2 available that would fill up O-related defects, and as a result, eliminate most of Ti vacancies. Lower temperatures, less O2, and shorter exposure times may enable not only partial elimination of Ti vacancies but also can facilitate their diffusion into different states of aggregations. In the ferromagnetic films (i.e. thin films below 100 nm), it looks like that the O atoms are located closer to the Ti vacancies.

Strength calculation of composite slabs by the partial shear connection method considering the friction

This paper aims to evaluate the behavior and longitudinal shear strength of the steel-concrete composite slab using the Steel Deck P75 through the partial shear connection method considering the contribution of the friction acting in the interface of the steel deck with the concrete at the region of supports. The method proposed by EN 1994-1-1:2004 for the design of composite slab systems is based in the analytical model similar to that of composite beams which allows to determine the degree shear connection between the steel sheeting and the concrete. This method allows to analyse the influence of friction at the support, in addition to the contribution of stud bolts on the mechanical resistance of composite slab. The composite slab was tested in the Federal University of Minas Gerais structure laboratory which used the semi-empirical "m-k" method to obtain its resistance. The behavior of the composite slab system was analysed through of the graphics deflections, end slips and strains, allowing the determination of its failure mode. Finally, the results obtained from the longitudinal shear resistance considering the friction at the support were applied in one practical example.

Absolute deformation potentials and absolute energy levels of III-N, ZnO, and II-IV-N2 semiconductors for optoelectronic applications

Absolute deformation potentials and absolute energy levels for III-N, ZnO, and II-IV-N2 semiconductors are systematically determined from hybrid-functional calculations. Separate bulk and slab calculations are combined and the vacuum level is taken as the common reference. The trends in the absolute deformation potentials are rationalized by the kinetic energy effect and the bonding (or antibonding) character of the band edge states. The calculated absolute energy levels can be used to obtain the natural band alignment between these semiconductors and are in accordance with the available results. The determined parameters are of practical importance to the optoelectronic devices designs.

RITZ METHOD IN THE DISCRETE APPROXIMATION OF DISPLACEMENTS FOR SLAB CALCULATION

Introduction: The FEM reduces the problem of structural analysis for various building structures to the formation and solution of a system of linear algebraic equations. For this purpose, there are techniques available for obtaining FE stiffness and flexibility matrices where the main structural deformation characteristics are taken into account. However, the FEM can also be considered as a special case of the Ritz method in the discrete approximation of the required functions. In the functional of full potential deformation energy with regard to the considered structure, all adopted stress-strain state characteristics are taken into account. Since it is difficult or impossible to find continuous approximation functions both in the classic version of the Ritz method and in the Bubnov–Galerkin method for some types of edge restraint in such building structures as beams, slabs, or shells, it is possible to use the Ritz method in the discrete approximation of the required functions (by analogy with the FEM). This paper presents a method of such calculations using slab calculations as an example. It is shown that, due to introducing some notations (operators), the process of finding the coefficients of the system of linear algebraic equations creates no difficulties and is easily programmable. The proposed method is not an alternative to the FEM, which is the most effective numerical method for the calculation of complex three-dimensional building structures. Purpose of the study: We aimed to create a method for calculating slabs by the Ritz method in the discrete approximation of the deflection function for edge restraint cases when it is difficult or impossible to find continuous approximation functions in the classic version of the Ritz method and the Bubnov–Galerkin method. Methods: Based on the application of the Ritz variational method in the discrete approximation of displacements for slab calculation, all the basic relations for rectangular finite elements with 12 degrees of freedom are obtained, and an algorithm for forming the coefficients of the system of linear algebraic equations is developed. Results: For the first time, the solution by the Ritz method in the discrete approximation of slab displacements is obtained for the case when two edges of the slab are rigidly restrained and other two edges are free. In this case, the correct solution of the above problem is possible only with the use of the proposed method and FEM. For the test problem, we performed a comparison of the results of the calculation using the proposed method with the results using the classic Ritz method, which showed their very close agreement. The accuracy of the obtained results was assessed.

The role of electron correlations in the electronic structure of putative Chern magnet TbMn6Sn6

A member of the RMn6Sn6 rare-earth family materials, TbMn6Sn6, recently showed experimental signatures of the realization of a quantum-limit Chern magnet. In this work, we use quantum Monte Carlo (QMC) and density functional theory with Hubbard U (DFT + U) calculations to examine the electronic structure of TbMn6Sn6. To do so, we optimize accurate, correlation-consistent pseudopotentials for Tb and Sn using coupled-cluster and configuration–interaction (CI) methods. We find that DFT + U and single-reference QMC calculations suffer from the same overestimation of the magnetic moments as meta-GGA and hybrid density functional approximations. Our findings point to the need for improved orbitals/wavefunctions for this class of materials, such as natural orbitals from CI, or for the inclusion of multi-reference effects that capture the static correlations for an accurate prediction of magnetic properties. DFT + U with Mn magnetic moments adjusted to the experiment predict the Dirac crossing in bulk to be close to the Fermi level, within ~120 meV, in agreement with the experiments. Our non-stoichiometric slab calculations show that the Dirac crossing approaches even closer to the Fermi level, suggesting the possible realization of Chern magnetism in this limit.

Electrolyte Coatings for High Adhesion Interfaces in Solid-State Batteries from First Principles.

We introduce an adhesion parameter that enables rapid screening for materials interfaces with high adhesion. This parameter is obtained by density functional theory calculations of individual single-material slabs rather than slabs consisting of combinations of two materials, eliminating the need to calculate all configurations of a prohibitively vast space of possible interface configurations. Cleavage energy calculations are used as an upper bound for electrolyte and coating energies and implemented in an adapted contact angle equation to derive the adhesion parameter. In addition to good adhesion, we impose further constraints in electrochemical stability window, abundance, bulk reactivity, and stability to screen for coating materials for next-generation solid-state batteries. Good adhesion is critical in combating delamination and resistance to lithium diffusivity in solid-state batteries. Here, we identify several promising coating candidates for the Li7La3Zr2O12 and sulfide electrolyte systems including the previously investigated electrode coating materials LiAlSiO4 and Li5AlO8, making them especially attractive for experimental optimization and commercialization.

Anisotropic Electronic Structure of the 2D Electron Gas at the AlOx/KTaO3(110) Interface

AbstractOxide‐based 2D electron gases (2DEGs) have generated significant interest due to their potential for discovering novel physical properties. Among these, 2DEGs formed in KTaO3 stand out due to the recently discovered crystal face‐dependent superconductivity and large Rashba splitting, both of which hold potential for future oxide electronics devices. In this work, angle‐resolved photoemission spectroscopy is used to study the electronic structure of the 2DEG formed at the (110) surface of KTaO3 after deposition of a thin Al layer. The experiments reveal a remarkable anisotropy in the orbital character of the electron‐like dispersive bands, which form a Fermi surface consisting of two elliptical contours with their major axes perpendicular to each other. The measured electronic structure is used to constrain the modeling parameters of self‐consistent tight‐binding slab calculations of the band structure. In these calculations, an anisotropic Rashba splitting is found with a value as large as 4 meV at the Fermi level along the [−110] crystallographic direction. This large unconventional and anisotropic Rashba splitting is rationalized based on the orbital angular momentum formulation. These findings provide insights into the interpretation of spin‐orbitronics experiments and help to constrain models for superconductivity in the KTO(110)‐2DEG system.

РАСЧЕТНОЕ ОБОСНОВАНИЕ ТОНКИХ ФУНДАМЕНТНЫХ ПЛИТ ПОД ОБОРУДОВАНИЕ НА ПРИМЕРЕ ДИЗЕЛЬ-ГЕНЕРАТОРНОЙ УСТАНОВКИ.

The presented article contains a summary of the methodology for the numerical calculation of thin foundation slabs for the action of dynamic loads from vibroactive equipment. The considered scheme makes it possible to take into account the vibration isolation and compliance of the supporting structure, the mutual influence of the load application areas on the stress-strain state of the slab. Two installation options are considered - with vibration isolation and without it. The calculation results show that small pieces of the slab account for large overall and heavy vibroactive equipment.

ON THE NONLINEARITY OF DEFORMATION OF REINFORCED CONCRETE FLEXURAL ELEMENTS

It is shown that in most calculations of reinforced concrete elements, the descending branch of the concrete deformation diagram should not be taken into account. This is due to the fact that in the calculation it is impossible to exceed the value of the stress in concrete, the value of the design resistance. It is shown that the "moment-curvature" dependence has a linear character both before the formation of cracks and after the formation of cracks. At the same time, calculations using software systems are greatly simplified with almost no loss of accuracy. The use of nonlinear methods for determining the stress-strain state in design leads to significant difficulties. In addition, these methods do not add accuracy to the calculation, but lead to ill-conditioned design schemes. The ascending branch of the concrete diagram can be approximated by a 63 straight line, which greatly simplifies the calculations without a noticeabledecrease in accuracy. The use of a non-linear form of the concrete diagram complicates the calculations, especially when used in software systems. This is because the roots of any equation greater than the first degree have two or more values for the same value of moment or stress. Therefore, it is difficult to choose the desired root in the software package. The use of a linear function always has a unique solution, while the accuracy decreases quite slightly. The statement about the linear relationship "moment-curvature" is also proved by the approach that is adopted in the US standards. If we pass from the formula for determining the moment of inertia of an element with cracks to the “moment-curvature” dependence, then we can see that this dependence is linear. The article shows how to use linear dependencies in the calculations of beam elements and slabs. The proposed approach to the design of reinforced concrete slabs using the finite element method involves changing the rigidity of the finite element depending on the presence or absence of cracks. In this case, the stiffness can have only two values, which are approximated by linear dependencies

Van der Waals Engineering of Ultrafast Carrier Dynamics in Magnetic Heterostructures

Heterostructures composed of the intrinsic magnetic topological insulator MnBi2Te4 and its nonmagnetic counterpart Bi2Te3 host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBi2Te4 and MnBi4Te7 following near-infrared optical excitation using time- and angle-resolved photoemission spectroscopy and disentangle surface from bulk dynamics based on density functional theory slab calculations of the surface-projected electronic structure. We gain access to the out-of-equilibrium charge carrier populations of both MnBi2Te4 and Bi2Te3 surface terminations of MnBi4Te7, revealing an instantaneous occupation of states associated with the Bi2Te3 surface layer followed by carrier extraction into the adjacent MnBi2Te4 layers with a laser fluence-tunable delay of up to 350 fs. The ensuing thermal relaxation processes are driven by phonon scattering with significantly slower relaxation times in the magnetic MnBi2Te4 septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBi2Te4-based van der Waals compounds.

ANALYTICAL METHOD FOR CALCULATING ANNULAR PLATES ON A VARIABLE ELASTIC BASE

The paper considers the application of the method of direct integration to calculations of annular plates and slabs on a continuous variable elastic base. Ring-shaped plates with variable geometric and mechanical parameters are increasingly used. Not only the elastic base, but also the plate thickness and cylindrical stiffness can be variable parameters here. The need for an analytical method for calculating such structures raises no doubts, since it makes it possible to evaluate the accuracy of finite-element analysis. To date, there are no proposals in the literature regarding a general analytical method for the calculation of annular plates on a variable elastic base. A detailed description of the algorithm of the direct integration method is not given in the paper, and all the calculation formulas for the annular plate are taken from the authors' already published article. The results of numerical implementation of this algorithm for specific examples are considered: a concrete plate, which is rigidly pinch on the inner contour, and its outer contour is free, and a steel plate, which is rigidly pinch on the outer contour, and its inner contour is free. To estimate the results of calculation by the author's method, computer modeling of the considered structures in PC LIRA-SAPR and their calculations by the finite-element method have been executed. The foundation reaction is described by Winkler model with a variable bedding factor. In the first case a bed factor is assumed constant, and in the second case it changes under the linear law. Calculations have shown that discrepancy between deflections calculated by the finite-element method and the author's method does not exceed 1 %, and the results of radial and circumferential moments calculation differ more considerably, amounting to 10 %. The authors explain this difference by the inaccuracy of the numerical analysis associated with the semi-automatic method of constructing a finite-element mesh, which should be made finer. The densification of the mesh in the manual mode of its partitioning significantly reduces the discrepancy between the results of calculating the deflections, radial and circumferential bending moments by the finite-element method and the author's method.

Development of a ReaxFF potential for Au–Pd

The bimetallic alloys often outperform their single-component counterparts due to synergistic effects. Being widely known, the Au–Pd alloy is a promising candidate for the novel heterogeneous nanocatalysts. Rational design of such systems requires theoretical simulations under ambient conditions. Ab initio quantum-mechanical calculations employ the density functional theory (DFT) and are limited to the systems with few tens of atoms and short timescales. The alternative solution implies development of reliable atomistic potentials. Among different approaches ReaxFF combines chemical accuracy and low computational costs. However, the development of a new potential is a problem without unique solution and thus requires accurate validation criteria. In this work we construct ReaxFF potential for the Au–Pd system based on ab initio DFT calculations for bulk structures, slabs and nanoparticles with different stoichiometry. The validation was performed with molecular dynamics and Monte-Carlo calculations. We present several optimal parametrizations that describe experimental bulk mechanical and thermal properties, atomic order-disorder phase transition temperatures and the resulting ordered crystal structures.

Directional growth of quasi-2D Cu2O monocrystals on rGO membranes in aqueous environments.

The preparation technology of unconventional low-dimensional Cu2O monocrystals, which exhibit specific crystal planes and present significantly unique interfacial and physicochemical properties, is attracting increasing attention and interest. Herein, by integrating a high-temperature oxidation process under vacuum and a pure-water incubation process under ambient conditions, we propose the self-assembled growth and synthesis of quasi-two-dimensional Cu2O monocrystals on reduced graphene oxide (rGO) membranes. The prepared Cu2O crystals have a single (110) crystal plane, regular rectangular morphology, and potentially well conductivity. Experimental and theoretical results suggest that this assembly is attributed to the pre-nucleation clusters aggregation and directional attachment of Cu and O on the rGO membranes in aqueous environment and cation-π interactions between the (110) crystal plane of Cu2O and rGO surface. Our findings offer a potential avenue for the discovery and design of advanced low-dimensional single-crystal materials with specific interfacial properties in a pure aqueous environment.

Electronic Band Structure and Surface States in Dirac Semimetal LaAgSb2.

LaAgSb is a Dirac semimetal showing charge density wave (CDW) order. Previous angle-resolved photoemission spectroscopy (ARPES) results suggest the existence of the Dirac-cone-like structure in the vicinity of the Fermi level along the –M direction. This paper is devoted to a complex analysis of the electronic band structure of LaAgSb by means of ARPES and theoretical studies within the ab initio method as well as tight binding model formulation. To investigate the possible surface states, we performed the direct DFT slab calculation and the surface Green function calculation for the (001) surface. The appearance of the surface states, which depends strongly on the surface, points to the conclusion that LaSb termination is realized in the cleaved crystals. Moreover, the surface states predicted by our calculations at the and X points are found by ARPES. Nodal lines, which exist along the X–R and M–A paths due to crystal symmetry, are also observed experimentally. The calculations reveal other nodal lines, which originate from the vanishing of spin–orbit splitting and are located at the X–M–A–R plane at the Brillouin zone boundary. In addition, we analyze the band structure along the –M path to verify whether Dirac surface states can be expected. Their appearance in this region is not confirmed.

Use of analytical method for calculating stiffnesses of straight waffle slabs

Introduction. The analysis of analytical and computer-assisted calculations of waffle slabs shows that, depending on the established finite-element model and structure geometry, the forces in beams can vary significantly. As a rule, the analytical model is used as a reference when comparing bending moments. Examples available in the literature show that regardless of the slab geometry, the stiffness of individual central orthogonal beams or conditionally selected beams is used in the calculations when determining structural loads.Aim. In this work, the accuracy of resulting forces in the beams of straight waffle slabs was assessed using the stiffness of individual beams in the analytical calculation.Materials and methods. The work was carried out by comparing the bending moments obtained analytically and in the SCAD software by the finite-element method for beams having various stiffnesses in the central zones of straight waffle slabs. 12.0 × 12.0 m slabs square in plan having different aspect ratios of waffle were considered, as well as slabs having square 1.5 × 1.5 m waffles and different aspect ratios of spans. A T-beam-and-girder construction was used in the computer model.Results. The values of bending moments of slabs square in plan having square caissons calculated analytically and using the computer-assisted finite-element method coincide, being a particular case. The bending moments calculated analytically and using the finite-element method for rectangular slabs or slabs having rectangular caissons differ. As the ratio of the span or caisson sides increases, these deviations increase.Conclusions. In the general calculation, using the stiffnesses of individual central orthogonal beams or conditionally selected beams for the analytical determination of forces in the beams of straight waffle slabs leads to erroneous results.

Влияние переходных режимов работы оборудования на напряженно- деформированное состояние конструкций

The article presents the results of the expected calculations of floor slabs for loads excited in operating and transient modes. Particular attention is paid to the issue of the influence of the time of starting and stopping the machine on the amplitude of vibrations of structures in transient conditions. The main purpose of the study is to determine the magnitude of the resonant increase in the oscillation amplitudes compared to oscillations in the operating mode, as well as to analyze the effect of the equipment start and stop time on the resonant amplitude.

Electronic Band Structures of the Possible Topological Insulator Pb2BiBrO6 and Pb2SeTeO6 Double Perovskite: An Ab Initio Study

Using the frameworks of density functional theory, we found a new class of three-dimensional (3D) topological insulators (TIs) in Pb2BiBrO6 and Pb2SeTeO6 double perovskites. Our ab initio theoretical calculations show that Pb2BiBrO6 and Pb2SeTeO6 are Z2 nontrivial, and their bandgaps are 0.390 eV and 0.181 eV, respectively. The topology comes from two mechanisms. Firstly, the band inversion occurs at Γ point in the absence of spin-orbit interactions and secondly, the bandgap is induced by the SOC. This results in a larger bandgap for this new class of topological insulators than conventional TI. In Pb2BiBrO6 double perovskites, our slab calculations confirm that the topology-protected surface metallic bands come from the BiBrO4 surface which means that one can build a transport device using Pb2BiBrO6 double perovskites with a PbO layer as an outmost protection layer. The mechanical stabilities such as bulk, shear, Young’s moduli, Poisson’s and Pugh’s ratio, longitudinal, transverse, and average sound velocity, together with Debye temperature are also studied. Our results show that these Pb2AA’O6 (A = Sb and Bi; A’ = Br and I) and Pb2SeTeO6 are mechanically stable.

Density functional Bogoliubov-de Gennes analysis of superconducting Nb and Nb(110) surfaces

We report on the implementation of the Bogoliubov-de Gennes method into the JuKKR code, an implementation of the relativistic all-electron, full-potential Korringa-Kohn-Rostoker Green function method, which allows a material-specific description of inhomogeneous superconductors and heterostructures on the basis of density functional theory. We describe the formalism and report on calculations for the $s$-wave superconductor Nb, a potential component of the materials platform enabling the realization of the Majorana zero modes in the field of topological quantum computing. We compare the properties of the superconducting state both in the bulk and for (110) surfaces. We compare slab calculations for different thicknesses, the effect of surface relaxations, and the influence of a softening of phonon modes on the surface for the resulting superconducting gap.

Limit Carrying Capacity Calculation of Two-Way Slabs with Three Simply Supported Edges and One Clamped Edge under Fire

Fire-resistance experiments were conducted on two full-scale two-way reinforced concrete slabs with three simply supported edges and one clamped edge. This paper presents the design of the furnace, specimens and clamped-end device, the test plan, and the measuring contents and methods. The cracking and failure characteristics of the tested two-way reinforced concrete slabs were introduced. The temperature field distribution of the concrete and steel in the direction of the section thickness, out-of-plane deflections, in-plane deflections, and angle of the slab edges of the two-way concrete slab under fire conditions were analyzed. Cracks form on the slab surface, which is shaped similar to the bottom of a shallow bowl, with two transverse cracks in the mid-area of the medially clamped side and intensively annular diagonal cracks on the corner side. There were several main transverse cracks in the central area of the slab surface. The results indicate that there was a significant decrease in frequency under fire conditions. Relations between the frequency and the central vertical deflection of the two slabs were analyzed by the regression method. This research generates valuable test data that can be used to validate the numerical models developed by fellow researchers in the field of structural fire engineering. Based on the plate balance method and energy method, calculation formula of the limit carrying capacity calculation formula of reinforced concrete two-way slabs with four different boundaries under fire are given, which are simply supported on three sides and clamped on one side. The influence of membrane effect under large deflection is considered in the formula, and the calculated results are in good agreement with the experimental results.

Ab-Initio Investigation of Finite Size Effects in Rutile Titania Nanoparticles with Semilocal and Nonlocal Density Functionals

In this work, we employ hybrid and generalized gradient approximation (GGA) level density functional theory (DFT) calculations to investigate the convergence of surface properties and the electronic gap of rutile titania (TiO2) nanoparticles with particle size. The surface energies and electronic gaps are calculated for cuboidal particles with minimum dimension ranging from 3.7 Å (24 atoms) to 10.3 Å (384 atoms) using a highly parallel real-space DFT code to enable hybrid-level DFT calculations of larger nanoparticles than are typically practical. We deconvolute the geometric and electronic finite size effects in surface energy and evaluate the influence of defects on the electronic gap and density of states (DOS). The electronic finite size effects in surface energy vanish when the minimum length scale of the nanoparticles becomes greater than 10 Å. We show that this length scale is consistent with a computationally efficient numerical analysis of the characteristic length scale of electronic interactions. The surface energy of nanoparticles having a minimum dimension beyond this characteristic length can be approximated using slab calculations that account for the geometric defects. In contrast, the finite size effects on the electronic gap and DOS are highly dependent on the shape and size of these particles. The DOS for cuboidal particles and more realistic particles constructed using the Wulff algorithm reveal that defect states within the electronic gap play a key role in determining the eigenvalue distribution of nanoparticles and the electronic gap does not converge to the bulk limit for the particle sizes investigated.