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Search Results: 1 - 10 of 1428 matches for " Ilya Elfimov "
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Unbalance Level Regulating Algorithm in Power Distribution Networks  [PDF]
Eugene Alekseevich Shutov, Tatyana Evgenievna Turukina, Ilya Igorevich Elfimov
Energy and Power Engineering (EPE) , 2018, DOI: 10.4236/epe.2018.102005
Abstract: The paper dwells on the unified power quality indexes characterizing the phenomenon of voltage unbalance in three-phase systems. Voltage unbalance is one of the commonest occurrences in the town mains of 0.38 kV voltage. The phenomenon describes as inequality of vector magnitude of phase voltage and shearing angle between them. Causes and consequences of the voltage unbalance in distribution networks have been considered. The algorithm, which allows switching one-phase load, has been developed as one of the methods of reducing the unbalance level. The algorithm is written in the function block diagram programming language. For determining the duration and magnitude of the unbalance level it is proposed to introduce the forecasting algorithm. The necessary data for forecasting are accumulated in the course of the algorithm based on the Function Block Diagram. The algorithm example is given for transforming substation of the urban electrical power supply system. The results of the economic efficiency assessment of the algorithm implementation are shown in conclusion. The use of automatic switching of the one-phase load for explored substation allows reducing energy losses (active electric energy by 7.63%; reactive energy by 8.37%). It also allows improving supply quality to a consumer. For explored substation the average zero-sequence unbalance factor has dropped from 3.59% to 2.13%, and the negative-sequence unbalance factor has dropped from 0.61% to 0.36%.
Electronic polarons and bipolarons in Fe-based superconductors: a pairing mechanism
Mona Berciu,Ilya Elfimov,George A. Sawatzky
Physics , 2008, DOI: 10.1103/PhysRevB.79.214507
Abstract: Superconductivity is a fascinating example of how "more is different". It is due to electrons binding into bosonic Cooper pairs, which exhibit coherent behavior across a macroscopic sample. Finding the mechanism responsible for this binding is one of the more difficult tasks of condensed matter physics. For conventional superconductors the solution was given by the BCS theory as being due to exchange of phonons. For the cuprate high-Tc superconductors a widely-accepted explanation is still missing despite intense effort. The recently discovered Fe-based high-Tc superconductors pose now a new challenge. We present here a quantum mechanical theory for pnictides describing the influence of the large electronic polarizability of the heavy anions. We demonstrate that its inclusion results in electronic polarons as the low-energy quasi-particles and also unveils a pairing mechanism for these electronic polarons.
Hybridization effects and bond-disproportionation in the bismuth perovskites
Kateryna Foyevtsova,Arash Khazraie,Ilya Elfimov,George A. Sawatzky
Physics , 2014, DOI: 10.1103/PhysRevB.91.121114
Abstract: We propose a microscopic description of the bond-disproportionated insulating state in the bismuth perovskites $X$BiO$_3$ ($X$=Ba, Sr) that recognizes the bismuth-oxygen hybridization as a dominant energy scale. It is demonstrated using electronic structure methods that the breathing distortion is accompanied by spatial condensation of hole pairs into local, molecular-like orbitals of the $A_{1g}$ symmetry composed of O-$2p_{\sigma}$ and Bi-$6s$ atomic orbitals of collapsed BiO$_6$ octahedra. Primary importance of oxygen $p$-states is thus revealed, in contrast to a popular picture of a purely ionic Bi$^{3+}$/Bi$^{5+}$ charge-disproportionation. Octahedra tilting is shown to enhance the breathing instability by means of a non-uniform band-narrowing. We argue that formation of localized states upon breathing distortion is, to a large extent, a property of the oxygen sublattice and expect similar hybridization effects in other perovskites involving formally high oxidation state cations.
Charge disproportionation without charge transfer in the rare-earth nickelates as a possible mechanism for the metal-insulator transition
Steve Johnston,Anamitra Mukherjee,Ilya Elfimov,Mona Berciu,George A. Sawatzky
Physics , 2013, DOI: 10.1103/PhysRevLett.112.106404
Abstract: We study a model for the metal-insulator (MI) transition in the rare-earth nickelates RNiO$_3$, based upon a negative charge transfer energy and coupling to a rock-salt like lattice distortion of the NiO$_6$ octahedra. Using exact diagonalization and the Hartree-Fock approximation we demonstrate that electrons couple strongly to these distortions. For small distortions the system is metallic, with ground state of predominantly $d^8\ligand$ character, where $\ligand$ denotes a ligand hole. For sufficiently large distortions ($\delta d_{\rm Ni-O} \sim 0.05 - 0.10\AA$), however, a gap opens at the Fermi energy as the system enters a periodically distorted state alternating along the three crystallographic axes, with $(d^8\ligand^2)_{S=0}(d^8)_{S=1}$ character, where $S$ is the total spin. Thus the MI transition may be viewed as being driven by an internal volume "collapse" where the NiO$_6$ octahedra with two ligand holes shrink around their central Ni, while the remaining octahedra expand accordingly, resulting in the ($1/2,1/2,1/2$) superstructure observed in x-ray diffraction in the insulating phase. This insulating state is an example of a new type of charge ordering achieved without any actual movement of the charge.
Large electropositive cations as surfactants for the growth of polar epitaxial films
Alfred K. C. Cheung,Ilya Elfimov,Mona Berciu,George A. Sawatzky
Physics , 2014, DOI: 10.1103/PhysRevB.91.125405
Abstract: Using density functional theory (DFT) we demonstrate that the adsorption of large cations such as potassium or cesium facilitates the epitaxial growth of polar LaAlO$_3$ (LAO) on SrTiO$_3$ (STO). The low ionization potential of K favors efficient electron transfer to the STO conduction band and results in a 2D electron gas which exactly compensates for the diverging potential with increasing layer thickness. For large cations like K or Cs, DFT total energy considerations show that they remain adsorbed on the LAO surface and do not enter substitutionally into LAO. These results suggest a novel scheme for growing clean LAO/STO interface systems, and polar systems in general, by performing the growth process in the presence of large, low ionization potential alkali metal ions.
Modeling Multi-Echelon Multi-Supplier Repairable Inventory Systems with Backorders  [PDF]
Yael Perlman, Ilya Levner
Journal of Service Science and Management (JSSM) , 2010, DOI: 10.4236/jssm.2010.34050
Abstract: This paper considers an inventory system responsible for repairable equipments located at several operational sites, each in different area. When a failure occurs at the operational site, spare parts are required. We analyze a multiple-supplier inventory system that includes an internal repair shop that offers several modes of repair with different repair times and an external supplier of spare parts. The network model of the problem presented here efficiently solves the problem for deterministic demands that vary over time with backorders taken into account.
Perishable Inventory Management in Healthcare  [PDF]
Yael Perlman, Ilya Levner
Journal of Service Science and Management (JSSM) , 2014, DOI: 10.4236/jssm.2014.71002
Abstract:

This study addresses a problem encountered in a nation-wide, large-scale healthcare supply chain that comprises several hundred medical organizations (hospitals, clinics, pharmacies, etc.) and provides highly advanced medical care to several million people. The medical products in the system are perishable, meaning that they become unusable beyond a certain expiry date. It is necessary to track the ages of units in stock and to plan and control the inventory accordingly. The models developed herein represent a multi-echelon, multi-supplier inventory system and unite together aspects of perishability and outsourcing under deterministic demand for medical products, which include both perishable and deteriorating goods. The objective of the study is to determine the optimal number of products to be purchased from regular and outsource suppliers so as to meet the required demand at the minimum operating cost. The solution is a network-flow model that can be used to determine the trade-off between the quantities of items to be ordered from the two types of suppliers in each time period. In addition, the study analyzes different distribution policies to account for the perishable nature of the products. Further insights are obtained by applying the model to a case study of a real-life healthcare supply chain from which interesting results are drawn.

Atherogenesis, the oxidative LDL modification hypothesis revisited  [PDF]
Dov Lichtenberg, Ilya Pinchuk
Advances in Bioscience and Biotechnology (ABB) , 2013, DOI: 10.4236/abb.2013.411A2007
Abstract:

The commonly-accepted “oxidized LDL hypothesis of atherogenesis” is based on a large number of indirect evidence that shows that oxidatively-modified LDL plays a role in atherogenesis. Yet, the exact role is not clear. Some researchers think that oxidatively modified biomolecules initiate atherogenesis; others believe that they “only” promote this multifactorial process. Regardless of the exact mechanism responsible for the effect of peroxidation on atherogenesis, the “oxidative theory of AS” is apparently inconsistent with the results of meta-analysis, in which (the “expected”) significant correlation between CVD and oxidative stress (OS) was found only when the OS was evaluated on the basis of the plasma concentrations of malondialdehyde (MDA), often based on the concentration of thiobarbituric acid reactive substances (TBARS). Notably, even this association is questionable due to 1) poor reliability of the laboratory assay of MDA and 2) possible publication bias. Hence, it appears that the commonly accepted paradigm regarding the role of oxidative damage in the pathogenesis of CVD has been overestimated. Furthermore, the hypothesis is apparently inconsistent with the disappointing results of most of the clinical trials that were designed to reduce OS by means of supplementation of antioxidants, mostly vitamin E. These apparent inconsistencies do not contradict the oxidative modification hypothesis of AS. The source of the apparent contradictions is probably the oversimplified considerations on which the predictions have been based. Many reasonable arguments can be raised to explain the apparent contradictions, which means that our current knowledge is insufficient to test the relationship of oxidative stress to cardiovascular disease.

Cosmological Constant and Energy Density of Random Electromagnetic Field  [PDF]
Ilya A. Obukhov
Journal of High Energy Physics, Gravitation and Cosmology (JHEPGC) , 2016, DOI: 10.4236/jhepgc.2016.23028
Abstract: It is shown that the non-equilibrium electrically neutral and relativistically invariant vacuum-like state with the negative energy density and positive pressure may exist at the non-zero temperature in the system of spinor particles, antiparticles, and random electromagnetic field generated by particle-particle, particle-antiparticle, and antiparticle-antiparticle transitions. At the temperature of the order of 10 -5 K, the energy density of its state corresponds to the dark energy density in absolute magnitude. The cosmological constant for such material medium turns out to be negative.
Density of Vacuum-Like Plasma and Hubble Constant  [PDF]
Ilya A. Obukhov
Journal of High Energy Physics, Gravitation and Cosmology (JHEPGC) , 2017, DOI: 10.4236/jhepgc.2017.34044
Abstract: The model in which expansion of the Universe leads to a generation of non-equilibrium vacuum-like electron-positron plasma is proposed and researched. The formulas that relate the Hubble’s constant with the concentration of plasma particles and the cosmological constant are obtained. The collective properties of vacuum-like plasma are investigated. It is shown, that the coefficient of a two-photon annihilation in such plasma is nine times less than for the free particles. A simple formula for dark energy density as a function of electron mass and charge is obtained. It was demonstrated that acceleration of plasma’s chemical potential fluctuations flow proportional of dark energy density.
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