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Search Results: 1 - 10 of 461974 matches for " Tesfaye A. Abtew "
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False Prediction of Fundamental Properties of Metals by Hybrid Functionals
Weiwei Gao,Tesfaye A. Abtew,Tianyi Cai,Y. Y. Sun,S. B. Zhang,Peihong Zhang
Physics , 2015,
Abstract: The repercussions of an inaccurate account of electronic states near the Fermi level EF by hybrid functionals in predicting several important metallic properties are investigated. The diffculties in- clude a vanishing or severely suppressed density of states (DOS) at EF, significantly widened valence bandwidth, greatly enhanced electron-phonon (el-ph) deformation potentials, and an overestimate of magnetic moment in transition metals. The erroneously enhanced el-ph coupling calculated by hybrid functionals may lead to a false prediction of lattice instability. The main culprit of the problem comes from the simplistic treatment of the exchange functional rooted in the original Fock exchange energy. The use of a short-ranged Coulomb interaction alleviates some of the drawbacks but the fundamental issues remain unchanged.
Atomistic simulation of light-induced changes in hydrogenated amorphous silicon
T. A. Abtew,D. A. Drabold
Physics , 2005, DOI: 10.1088/0953-8984/18/1/L01
Abstract: We employ ab initio molecular dynamics to simulate the response of hydrogenated amorphous silicon to light exposure (Staebler-Wronski effect). We obtain improved microscopic understanding of PV operation, compute the motion of H atoms, and modes of light-induced degradation of photovoltaics. We clarify existing models of light-induced change in aSi:H and show that the Hydrogen collision model of Branz3 is correct in essentials.
Hydrogen dynamics and light-induced structural changes in hydrogenated amorphous silicon
T. A. Abtew,D. A. Drabold
Physics , 2006, DOI: 10.1103/PhysRevB.74.085201
Abstract: We use accurate first principles methods to study the network dynamics of hydrogenated amorphous silicon, including the motion of hydrogen. In addition to studies of atomic dynamics in the electronic ground state, we also adopt a simple procedure to track the H dynamics in light-excited states. Consistent with recent experiments and computer simulations, we find that dihydride structures are formed for dynamics in the light-excited states, and we give explicit examples of pathways to these states. Our simulations appear to be consistent with aspects of the Staebler-Wronski effect, such as the light-induced creation of well separated dangling bonds.
First principles molecular dynamics study of amorphous Si\sub{1-x}Ge\sub{x}:H alloys
T. A. Abtew,D. A. Drabold
Physics , 2006,
Abstract: We study the structural, dynamical and electronic properties of amorphous Si\sub{1-x}Ge\sub{x}:H alloys using first principles local basis molecular dynamics simulation. The network topology and defects in the amorphous network have been analyzed. Structural changes and an increase in number of defects have been found as the Ge atomic percentage increases from x=0.1 to x=0.5. The electronic density of states exhibits a decreasing band-gap and increased mid-gap and band-tail defect states as Ge concentration increases. Investigation of the band tails of the density of states show an exponential (Urbach) behavior. The mobility gap is estimated as a function of Ge concentration.
Thermally stimulated H emission and diffusion in hydrogenated amorphous silicon
T. A. Abtew,F. Inam,D. A. Drabold
Physics , 2006, DOI: 10.1209/0295-5075/79/36001
Abstract: We report first principles ab initio density functional calculations of hydrogen dynam- ics in hydrogenated amorphous silicon. Thermal motion of the host Si atoms drives H diffusion, as we demonstrate by direct simulation and explain with simple models. Si-Si bond centers and Si ring centers are local energy minima as expected. We also describe a new mechanism for break- ing Si-H bonds to release free atomic H into the network: a fluctuation bond center detachment (FBCD) assisted diffusion. H dynamics in a-Si:H is dominated by structural fluctuations intrinsic to the amorphous phase not present in the crystal.
Ab initio estimate of temperature dependence of electrical conductivity in a model amorphous material: hydrogenated amorphous silicon
T. A. Abtew,M. Zhang,D. A. Drabold
Physics , 2007, DOI: 10.1103/PhysRevB.76.045212
Abstract: We present an ab initio calculation of the DC conductivity of amorphous silicon and hydrogenated amorphous silicon. The Kubo-Greenwood formula is used to obtain the DC conductivity, by thermal averaging over extended dynamical simulation. Its application to disordered solids is discussed. The conductivity is computed for a wide range of temperatures and doping is explored in a naive way by shifting the Fermi level. We observed the Meyer-Neldel rule for the electrical conductivity with E_MNR = 0.06 eV and a temperature coefficient of resistance, TCR ~ -2.0% K^-1 for a-Si:H. In general, experimental trends are reproduced by these calculations, and this suggests the possible utility of the approach for modeling carrier transport in other disordered systems.
Network structure and dynamics of hydrogenated amorphous silicon
D. A. Drabold,T. A. Abtew,F. Inam,Y. Pan
Physics , 2007, DOI: 10.1016/j.jnoncrysol.2007.09.081
Abstract: In this paper we discuss the application of current it ab initio computer simulation techniques to hydrogenated amorphous silicon (a-Si:H). We begin by discussing thermal fluctuation in the number of coordination defects in the material, and its temperature dependence. We connect this to the ``fluctuating bond center detachment" mechanism for liberating H bonded to Si atoms. Next, from extended thermal MD simulation, we illustrate various mechanisms of H motion. The dynamics of the lattice is then linked to the electrons, and we point out that the squared electron-lattice coupling (and the thermally-induced mean square variation in electron energy eigenvalues) is robustly proportional to the localization of the conjugate state, if localization is measured with inverse participation ratio. Finally we discuss the Staebler-Wronski effect using these methods, and argue that a sophisticated local heating picture (based upon reasonable calculations of the electron-lattice coupling and molecular dynamic simulation) explains significant aspects of the phenomenon.
Finding the Radiation Amplitude Zero in W\GAMMA Production- is it Unique to the Standard Model?
Mark A Samuel,TESFAYE Abraha
Physics , 1997,
Abstract: In the light of recent experimental observation of the radiation amplitude zero (RAZ) in W\gamma production by CDF at Fermilab, we consider its consequences. is the RAZ unique to the standard model? Although it is not for neutrino-electron scattering, in the case of d-ubar scattering which is the case of experimental interest, observation of the RAZ implies that the SM must be correct.
Child Rights Protection in Ethiopia: Critical Analysis of the Statutory Rape Provisions of the Criminal Code and Their Application  [PDF]
Amare Tesfaye
Beijing Law Review (BLR) , 2017, DOI: 10.4236/blr.2017.84028
Abstract: Children, due to their mental and physical immaturity, are among the vulnerable section of the society. States are thus required to provide special protection and care for children by making necessary legal and institutional arrangements. To this effect, Ethiopia has ratified international and regional human rights instruments meant to provide protection for children. It also included provisions protecting child rights in its constitution. These constitutional provisions have also found expression in the subsequent legislative reform that, among others, resulted in the adoption of a New Criminal Code in 2004. This Criminal Code replaces the provisions of the earlier 1957 Penal Code on statutory rape providing better protection for children against sexual abuse. Yet, gaps still persist both in the law and practice in terms of fully realizing the rights of children in Ethiopia. This article aims to examine the legal and practical protection of children by the statutory rape provisions of the Criminal Code in the Ethiopian legal system. In doing so, it attempts to evaluate the current statutory rape provisions of the Criminal Code against child rights standards as well as evaluating the practical application of the laws by courts and prosecutors.
Applicability of Galway River Flow Forecasting and Modeling System (GFFMS) for Lake Tana Basin, Ethiopia  [PDF]
Tesfaye A. Dessalegn, Mamaru A. Moges, Dessalegn C. Dagnew, Assegidew Gashaw
Journal of Water Resource and Protection (JWARP) , 2017, DOI: 10.4236/jwarp.2017.912084
Abstract: Flow forecasting is used in activities requiring stream flow data such as irrigation development, water supply, and flood control and hydropower development. Real time flow forecasting with special interest to flooding is one of the most important applications of hydrology for decision making in water resources. In order to meet flood and flow forecasts using hydrological models may be used and subsequently be updated in accordance with residuals. Therefore in this study, different flood forecasting methods are evaluated for their potential of stream flow forecasting using Galway River Flow Forecasting and Modeling System (GFFMS) in Lake Tana basin, upper Blue Nile basin, Ethiopia. The areal rainfall and temperature data was used for the model input. Three forecast updating methods, i.e., autoregressive (AR), linear transfer function (LTF) and neuron network updating (NNU) methods were compared for stream flow forecasting, at one to six days lead time. The most sensitive parameters were fine-tuned first and modeled for a calibration period of 1994-2004 for three selected watersheds of the Tana basin. The results indicate that with the exception of the simple linear model, an acceptable result could be obtained using models embedded in the software. Artificial neural network model performed well for Gilgel Abay (NSE = 0.87) and Gumara (NSE = 0.9) watersheds but for Megech watershed, SMAR model (NSE = 0.78) gave a better forecast result. In capturing the peak flows LTF and NNU in forecast updating mode performed better for Gilgel Abay and Megech watersheds, respectively. The results of this study implied that GFFMS can be used as a useful tool to forecast peak stream flows for flood early warning in the upper Blue Nile basin.
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