Browsing by Subject "Calculations"
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Item Open Access Ab initio study of hydrogenic effective mass impurities in Si nanowires(Institute of Physics Publishing, 2017-01) Peelaers, H.; Durgun, Engin; Partoens, B.; Bilc, D. I.; Ghosez, P.; Van De Walle C. G.; Peeters, F. M.The effect of B and P dopants on the band structure of Si nanowires is studied using electronic structure calculations based on density functional theory. At low concentrations a dispersionless band is formed, clearly distinguishable from the valence and conduction bands. Although this band is evidently induced by the dopant impurity, it turns out to have purely Si character. These results can be rigorously analyzed in the framework of effective mass theory. In the process we resolve some common misconceptions about the physics of hydrogenic shallow impurities, which can be more clearly elucidated in the case of nanowires than would be possible for bulk Si. We also show the importance of correctly describing the effect of dielectric confinement, which is not included in traditional electronic structure calculations, by comparing the obtained results with those of G0W0 calculations.Item Open Access Adhesive energy, force and barrier height between simple metal surfaces(1992) Çıracı, Salim; Tekman, E.; Gökçedag, M.; Batra, I. P.; Baratoff, A.Using the self-consistent field pseudopotential method we calculated the adhesive energy, perpendicular and lateral forces and barrier height between two rigid A1(001) slabs modeling the sample and a blunt tip. We found that the adhesive energy and forces are site specific, and can lead to a significant corrugation in the constant force mode with negative force gradient. Lateral forces, which determine friction on the atomic scale are not simply proportional to the perpendicular force, and are typically one order of magnitude smaller. Our results confirm that perpendicular tip force and barrier height are interrelated for separations where the force gradient is positive. © 1992.Item Open Access Adsorption of group IV elements on graphene, silicene, germanene and stanene: dumbbell formation(ACS Publications, 2014-12-09) Ozcelik, V. O.; Kecik, D.; Durgun, Engin; Çıracı, SalimSilicene and germanene derivatives constructed from periodic dumbbell units play a crucial role in multilayers of these honeycomb structures. Using first-principles calculations based on density functional theory, here we investigate the dumbbell formation mechanisms and energetics of Group IV atoms adsorbed on graphene, silicene, germanene and stanene monolayer honeycomb structures. The stabilities of the binding structures are further confirmed by performing ab-initio molecular dynamics calculations at elevated temperatures, except for stanene which is subject to structural instability upon the adsorption of adatoms. Depending on the row number of the adatoms and substrates we find three types of binding structures, which lead to significant changes in the electronic, magnetic, and optical properties of substrates. In particular, Si, Ge and Sn adatoms adsorbed on silicene and germanene form dumbbell structures. Furthermore, dumbbell structures occur not only on single layer, monatomic honeycomb structures, but also on their compounds like SiC and SiGe. We show that the energy barrier to the migration of a dumbbell structure is low due to the concerted action of atoms. This renders dumbbells rather mobile on substrates to construct new single and multilayer Si and Ge phases.Item Open Access Algorithms for efficient vectorization of repeated sparse power system network computations(IEEE, 1995) Aykanat, Cevdet; Özgü, Ö.; Güven, N.Standard sparsity-based algorithms used in power system appllcations need to be restructured for efficient vectorization due to the extremely short vectors processed. Further, intrinsic architectural features of vector computers such as chaining and sectioning should also be exploited for utmost performance. This paper presents novel data storage schemes and vectorization alsorim that resolve the recurrence problem, exploit chaining and minimize the number of indirect element selections in the repeated solution of sparse linear system of equations widely encountered in various power system problems. The proposed schemes are also applied and experimented for the vectorization of power mismatch calculations arising in the solution phase of FDLF which involves typical repeated sparse power network computations. The relative performances of the proposed and existing vectorization schemes are evaluated, both theoretically and experimentally on IBM 3090ArF.Item Open Access Anisotropic electronic, mechanical, and optical properties of monolayer WTe2(American Institute of Physics Inc., 2016) Torun, E.; Sahin, H.; Cahangirov, S.; Rubio, A.; Peeters, F. M.Using first-principles calculations, we investigate the electronic, mechanical, and optical properties of monolayer WTe2. Atomic structure and ground state properties of monolayer WTe2 (Td phase) are anisotropic which are in contrast to similar monolayer crystals of transition metal dichalcogenides, such as MoS2, WS2, MoSe2, WSe2, and MoTe2, which crystallize in the H-phase. We find that the Poisson ratio and the in-plane stiffness is direction dependent due to the symmetry breaking induced by the dimerization of the W atoms along one of the lattice directions of the compound. Since the semimetallic behavior of the Td phase originates from this W-W interaction (along the a crystallographic direction), tensile strain along the dimer direction leads to a semimetal to semiconductor transition after 1% strain. By solving the Bethe-Salpeter equation on top of single shot G0W0 calculations, we predict that the absorption spectrum of Td-WTe2 monolayer is strongly direction dependent and tunable by tensile strain.Item Open Access Atomic-scale tip-sample interactions and contact phenomena(1992) Çıracı, SalimTip-sample interactions become crucial owing to increased overlap at small tip-sample separation. The potential barrier collapses before the point of maximum attraction on the apex of the tip, but the effective barrier may remain significant owing to the strong confinement of current-carrying states to the constriction between tip and sample. At such separations the perpendicular tip force is still attractive and determined by ion-ion repulsion and redistribution of electronic charge. Electronic states are modified by the tip-induced perturbation of the potential in the vicinity of the tip. Self-consistent calculations reveal that local properties, such as elastic deformation, effective height and width of the tunneling barrier, electronic states and attractive tip force are site-dependent and reversible on the atomic scale. Numerical results suggest a relation between the perpendicular tip force and barrier height as a function of separation. A mechanical contact is formed with relatively strong bonds at separation near the point of zero force gradient. Whether the effective potential can collapse and hence the first channel can open to allow a transition from tunneling to ballistic conduction, and whether the conductance can show quantized steplike changes with increasing plastic deformation depends on material properties. © 1992.Item Open Access Automated web services composition with the event calculus(Springer, 2007-10) Aydın, Onur; Kesim-Çiçekli, Nihan; Çiçekli, İlyasAs the web services proliferate and complicate it is becoming an overwhelming job to manually prepare the web service compositions which describe the communication and integration between web services. This paper analyzes the usage of the Event Calculus, which is one of the logical action-effect definition languages, for the automated preparation and execution of web service compositions. In this context, abductive planning capabilities of the Event Calculus are utilized. It is shown that composite process definitions in OWL-S can be translated into Event Calculus axioms so that planning with generic process definitions is possible within this framework. © 2008 Springer-Verlag Berlin Heidelberg.Item Open Access Band Structure and Optical Properties of Kesterite Type Compounds: First principle calculations(Institute of Physics Publishing, 2017) Palaz S.; Unver H.; Ugur G.; Mamedov, Amirullah; Özbay, EkmelIn present work, our research is mainly focused on the electronic structures, optical and magnetic properties of Cu2FeSnZ4 (Z = S, Se) compounds by using ab initio calculations within the generalized gradient approximation (GGA). The calculations are performed by using the Vienna ab-initio simulation package (VASP) based on the density functional theory. The band structure of the Cu2FeSnZ4 ( Z = S, Se) compounds for majority spin (spin-up) and minority spin (spin-down) were calculated. It is seen that for these compounds, the majority spin states cross the Fermi level and thus have the metallic character, while the minority spin states open the band gaps around the Fermi level and thus have the narrow-band semiconducting nature. For better understanding of the electronic states, the total and partial density of states were calculated, too. The real and imaginary parts of dielectric functions and hence the optical functions such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant for Cu2FeSnZ4 (Z = S, Se) compounds were also calculated. © Published under licence by IOP Publishing Ltd.Item Open Access Comparative study of acceleration techniques for integrals and series in electromagnetic problems(IEEE, 1995-06) Kinayman, Noyan; Aksun, M. I.Most of the electromagnetic problems can be reduced down to either integrating oscillatory integrals or summing up complex series. However, limits of the integrals and the series usually extend to infinity. In addition, they may be slowly convergent. Therefore, numerically efficient techniques for evaluating the integrals or for calculating the sum of infinite series have to be used to make the numerical solution feasible and attractive. In the literature, there are a wide range of applications of such methods to various EM problems. In this paper, our main aim is to critically examine the popular series transformation (acceleration) methods which are used in electromagnetic problems and compare them by numerical examples.Item Open Access Coulomb drag effect in parallel cylindrical quantum wires(Pergamon Press, 1996) Tanatar, BilalWe study the Coulomb drag rate for electrons in two parallel quantum wires. The double-quantum wire structure is modeled for a GaAs material with cylindrical wires having infinite potential barriers. The momentum transfer rate between the wires (Coulomb drag effect) is calculated as a function of temperature for several wire separation distances. We employ the full wave vector and frequency dependent random-phase approximation (RPA) at finite temperature to describe the effective interwire Coulomb interaction. We find that the drag rate at high temperatures (i.e., T ≥ EF/2) is dominated by the collective modes (plasmons) of the system similar to the case in double-well structures. Including the local-field effects in an approximate way we estimate the importance of intrawire correlations to be significant. Copyright © 1996 Published by Elsevier Science Ltd.Item Open Access Density functional theory investigation of substituent effects on building blocks of conducting polymers(Elsevier, 1999) Salzner, U.Substituted heterocyclic dimers were calculated employing density functional theory (DFT) and analyzed with the natural bond orbits method (NBO). Substitution in 3- and 4-positions leads to parallel shifting of HOMO and LUMO but does not reduce energy gaps. For bridge dimers, HOMO-LUMO gaps correlate with π-electron densities in the carbon backbone and energy gap reduction correlate with the strength of π-π* interactions from the backbone to the bridging group. Alternating donor-acceptor groups do not reduce energy gaps and lead to systems with average HOMO and LUMO levels compared to the parent molecules.Item Open Access Digital computation of the fractional Fourier transform(Institute of Electrical and Electronics Engineers, 1996-09) Özaktaş, Haldun M.; Arıkan, Orhan; Kutay, M. A.; Bozdağı, G.An algorithm for efficient and accurate computation of the fractional Fourier transform is given. For signals with time-bandwidth product N, the presented algorithm computes the fractional transform in O(NlogN) time. A definition for the discrete fractional Fourier transform that emerges from our analysis is also discussed.Item Open Access Efficient parallel spatial subdivision algorithm for object-based parallel ray tracing(Pergamon Press, 1994) Aykanat, Cevdet; İşler, V.; Özgüç, B.Parallel ray tracing of complex scenes on multicomputers requires the distribution of both computation and scene data to the processors. This is carried out during preprocessing and usually consumes too much time and memory. The paper presents an efficient parallel subdivision algorithm that decomposes a given scene into rectangular regions adaptively and maps the resultant regions to the node processors of a multicomputer. The proposed algorithm uses efficient data structures to identify the splitting planes quickly. Furthermore the mapping of the regions and the objects to the node processors is performed while parallel spatial subdivision proceeds. The proposed algorithm is implemented on an Intel iPSC/2 hypercube multicomputer and promising results have been obtained. © 1994.Item Open Access Efficient use of closed-form Green's functions for the analysis of planar geometries with vertical connections(Institute of Electrical and Electronics Engineers, 1997-05) Kınayman, N.; Aksun, M. I.An efficient and rigorous method for the analysis of planarly layered geometries with vertical metallizations is presented. The method is based on the use of the closed-form spatial-domain Green's functions in conjunction with the method of moments (MoM). It has already been demonstrated that the introduction of the closed-form Green's functions into the MoM formulation results in significant computational improvement for the analysis of planar geometries. However, in cases of vertical metallizations, such as shorting pin's, via holes, etc., there are some difficulties in incorporating the closed-form Green's functions into the MoM formulation. In this paper, these difficulties are discussed and their remedies are proposed. The proposed approach is compared to traditional approaches from a theoretical point of view, and the numerical implementation is demonstrated through some examples. The results are also compared to those obtained from the commercial software em by SONNET.Item Open Access Efficient use of closed-form Green's functions for three-dimensional problems involving multilayered media(IEEE, 1994-06) Aksun, M. Irsadi; Mittra, R.With the use of casting the spatial domain Green's functions into closed forms approach, it was demonstrated that the computational efficiency of the method of moments (MoM) for the solution of the mixed potential integral equations can be improved significantly for planar microstrip geometries. However, this approach is not effective in the improvement in the computational efficiency achieved for three-dimensional geometries in planar layered media. In this paper, discussed are the difficulties involved in using the spatial domain, closed-form Green's functions in the Method of Moments formulation for three-dimensional geometries and proposed a technique to improve the computational efficiency of the MoM.Item Open Access Electronic and magnetic properties of zinc blende half-metal superlattices(A I P Publishing LLC, 2004) Fong, C. Y.; Qian, M. C.; Pask, J. E.; Yang, L. H.; Dag, S.Zinc blende half-metallic compounds such as CrAs, with large magnetic moments and high Curie temperatures, are promising materials for spintronic applications. We explore layered materials, consisting of alternating layers of zinc blende half-metals, by first principles calculations, and find that superlattices of (CrAs)1(MnAs)1 and (CrAs)2(MnAs)2 are half-metallic with magnetic moments of 7.0mB and 14.0mB per unit cell, respectively. We discuss the nature of the bonding and half-metallicity in these materials and, based on the understanding acquired, develop a simple expression for the magnetic moment in such materials. We explore the range of lattice constants over which half-metallicity is manifested, and suggest corresponding substrates for growth in thin film form.Item Open Access First-principles calculations of Pd-terminated symmetrical armchair graphene nanoribbons(Elsevier, 2013) Kuloglu, A. F.; Sarikavak-Lisesivdin, B.; Lisesivdin, S. B.; Özbay, EkmelThe effects of Palladium (Pd) termination on the electronic properties of armchair graphene nanoribbons (AGNRs) were calculated by using ab initio calculations. After a geometric optimization process, the electronic band structures, density of states, and binding energies of AGNRs with Na = 5-15 were calculated. Pd-termination was found to significantly influence the electronic properties of AGNRs. In DOS, many Q0D and Q1D type states were observed. Binding energy (BE) for single-side or both-side Pd-terminated structures represents characteristic drops with the increasing GNR width. With the increasing GNR width, the BEs of these structures become similar to hydrogenated structures. Because of the GNR width, dependent BE also gave information on the possible stiffness information, in which all of this information can be used in studies where controlled binding to graphene is required.Item Open Access Fracture of femoral neck: Analysis of new implant models with a slit and without a slit by the finite element method(WHO Office in Azerbaijan, 2017) Jafarov, A. A.; Ozer, Z.; Alizadeh, Ch. A.; Mammadov, A. M.During fractures of the neck of the femur (PBHB) for the completion of postoperative fusion, there is a need for stable fixation - interfragmental immobility. The stability of used implants in a living person is difficult to calculate. For this purpose, the analysis is carried out using the finite element method (the final analysis of the limited elements). The aim of this study is to study the features of the proposed new hip implant with finite element analysis. Based on the digital geometry of the anatomy of the femur, a 3D model of the femur was developed. Stress and strain, obtained with the help of the computer program ANSYS as a result of loads on the head of the thigh, were investigated by the finite element analysis method. Based on the Pawel classification, 3 groups of femoral neck fracture models were created, corresponding to the fracture angles closer to 30, 50 and 70 degrees (type 1, type 2 and type 3). In each group, the corresponding implants are analyzed in 2 types: without a slit and with a slit. For the spongiform bone, the UTS (Ultimate Tensile Stres) is defined as 20 MPA, and for the cortical bone, 150 MPA. In all analyzes, the force loaded in the vertical direction onto the head of the computer model of the femur was calculated to be 4000 N. Given that the slits on the surface of the implant can cross waves, homogeneously distribute the force and pressure throughout the entire implant, on the basis of this, a decrease in pressure on the surface of the bone tissue was observed. It is believed that this process can increase the stability of the implant and minimize the level of damage to the bone tissue.Item Open Access From a calculus to an execution environment for stream processing(ACM, 2012) Soulé, R.; Hirzel, M.; Gedik, Buğra; Grimm, R.At one level, this paper is about River, a virtual execution environment for stream processing. Stream processing is a paradigm well-suited for many modern data processing systems that ingest high-volume data streams from the real world, such as audio/video streaming, high-frequency trading, and security monitoring. One attractive property of stream processing is that it lends itself to parallelization on multicores, and even to distribution on clusters when extreme scale is required. Stream processing has been co-evolved by several communities, leading to diverse languages with similar core concepts. Providing a common execution environment reduces language development effort and increases portability. We designed River as a practical realization of Brooklet, a calculus for stream processing. So at another level, this paper is about a journey from theory (the calculus) to practice (the execution environment). The challenge is that, by definition, a calculus abstracts away all but the most central concepts. Hence, there are several research questions in concretizing the missing parts, not to mention a significant engineering effort in implementing them. But the effort is well worth it, because using a calculus as a foundation yields clear semantics and proven correctness results. Copyright © 2012 ACM.Item Open Access A hierarchical model for the cell loading problem of cellular manufacturing systems(Taylor & Francis, 1998) Aktürk, M. S.; Wilson, G. R.A hierarchical cell loading approach is proposed to solve the production planning problem in cellular manufacturing systems. Our aim is to minimize the variable cost of production subject to production and inventory balance constraints for families and items, and capacity feasibility constraints for group technology cells and resources over the planning horizon. The computational results indicated that the proposed algorithm was very efficient in finding an optimum solution for a set of randomly generated problems.
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