Network structure and dynamics of hydrogenated amorphous silicon

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.Comment: 10 pages, 5 figures, accepted in J. Non. Cryst. So

Thermally stimulated H emission and diffusion in hydrogenated amorphous silicon

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.Comment: 4 pages, 5 figures, In press EPL (Jun. 2007

Hydrogen dynamics and light-induced structural changes in hydrogenated amorphous silicon

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.Comment: 9 pages, 8 figures, submitted to PR

Microstructural and mechanical properties analysis of extruded Sn–0.7Cu solder alloy

AbstractThe properties and performance of lead-free solder alloys such as fluidity and wettability are defined by the alloy composition and solidification microstructure. Rapid solidification of metallic alloys is known to result in refined microstructures with reduced microsegregation and improved mechanical properties of the final products as compared to normal castings. The rapidly solidified Sn-based solders by melt spinning were shown to be suitable for soldering with low temperature and short soldering duration. In the present study, rapidly solidified Sn–0.7wt.%Cu droplets generated by impulse atomization (IA) were achieved as well as directional solidification under transient conditions at lower cooling rate. This paper reports on a comparative study of the rapidly solidified and the directionally solidified samples. Different but complementary characterization techniques were used to fully analyze the solidification microstructures of the samples obtained under the two cooling regimes. These include X-ray diffractometry (XRD) and scanning electron microscopy (SEM). In order to compare the tensile strength and elongation to fracture of the directionally solidified ingot and strip castings with the atomized droplet, compaction and extrusion of the latter were carried out. It was shown that more balanced and superior tensile mechanical properties are available for the hot extruded samples from compacted as-atomized Sn–0.7wt.%Cu droplets. Further, elongation-to-fracture was 2–3× higher than that obtained for the directionally solidified samples

Topological and topological-electronic correlations in amorphous silicon

In this paper, we study several structural models of amorphous silicon, and discuss structural and electronic features common to all. We note spatial correlations between short bonds, and similar correlations between long bonds. Such effects persist under a first principles relaxation of the system and at finite temperature. Next we explore the nature of the band tail states and find the states to possess a filamentary structure. We detail correlations between local geometry and the band tails.Comment: 7 pages, 11 figures, submitted to Journal of Crystalline Solid

Ab initio estimate of temperature dependence of electrical conductivity in a model amorphous material: hydrogenated amorphous silicon

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.Comment: 9 pages, 8 figures, submitted to PRB Comments: corrected typos, referee's comments include

Assessing EFL Teachers’ Perception and Their Implementation of Writing Tasks: The Case of Mekane-Eyesus, Mikrie and Koma Secondary Schools

This qualitative study endeavored to assess teachers’ perception and their implementation of writing tasks. The participants of the study were grade 10 EFL teachers at Mekane Eyesus, Koma and Mikrie secondary schools, Estie woreda. A questionnaire and interview were made to gather data on teachers’ perception and their implementation of writing tasks.  Besides, observation was held to see teachers’ implementation of writing tasks. The data was analyzed qualitatively. The questionnaire and interview results showed that the teachers have good perception on writing tasks. However, the data in observation and questionnaire revealed that teachers didn’t implement writing tasks properly. Finally, it was concluded that teachers didn’t implement writing tasks as they perceive it and they didn’t teach students following the stages of writing.  On the basis of the findings and the conclusions made, it is recommended that teachers should teach the writing tasks in stages of writing and they should support the students in each stages of writing. Key terms: task, perception, stages of writing, writing tasks DOI: 10.7176/JLLL/79-04 Publication date:May 31st 202

Characterization of genetic variation among Ethiopian barley (Hoerdeum vulgare L.) genotypes

Barley (Hordeum vulgare L.) is a major cereal crop in Ethiopia and accounts for 8% of the total cereal production based on cultivation area. Farmers may face unpredictable rainfall and drought stress patterns such as terminal drought where rainfall ends before crops have completed their physiological maturity, which then poses a challenge to crop production. The absence of efficient weather forecasts and a lack of efficient communication channels for resource-poor farmers ask for the development of varieties that are robust to such irregularities. A goal of plant breeding for areas with variable climate and limited resources for agricultural inputs is to produce stable varieties with higher average yield across diverse environments and growing conditions. Genotype by environment (G x E) interactions, however, frequently interfere with the selection of widely adapted genotypes. Knowledge about the yield stability of existing Ethiopian barley varieties and landraces under changing environmental variables is important for the future development of barley varieties with high and stable yields. In addition, yield components are quantitative with substantial influence of environment. Yield components also compensate each other in trait correlation dynamics. Since grain yield is a more complex trait than its components, environmental effects and genotype-by-environment (G x E) interactions for grain yield are stronger than for its components. Therefore, indirect selection of yield components may be more efficient than selection on grain yield per se to obtain higher yielding and stable cultivars. A study, therefore, was initiated to 1) characterize the response of a diverse set of barley genotypes to different locations and variable planting dates and identify genotypes with wide adaptation and stable performance and/or genotypes with specific altitude and planting date 2) determine traits that contribute to high and stable yields across a range of different environments and planting dates 3) determine the pattern of population structure and genetic parameters among genotypes conserved in Ethiopian and German gene banks in for different period of time as well as currently growing in farmers field. In order to meet the objectives 18 genotypes were tested at four different sowing dates with 15 days interval in different locations (Ambo and Jimma) and years (2012 and 2013). The tested genotypes revealed a wide variation for both static and dynamic yield stability measures. Compared to improved cultivars, farmers landraces displayed higher average static stability and similar superiority indices (dynamic stability). These landraces are therefore a source of germplasm for breeding resilient barley cultivars. Staggered planting proved to be a useful method for evaluating genotype stability across environmental factors beyond location and season. In addition, we also noticed that compensatory relationship between kernels per spike and thousand kernel weight in landraces. Kernels per spike and number of fertile tillers can be proposed as robust traits in barley breeding for a wider adaptation as they had significant and consistent positive total effects on grain yield. In order to determine the pattern of population structure and genetic parameters among genotypes of different origin and gene banks, DNA samples were subject to double-digest by ApeK1 and Hind III enzymes. After sequencing, raw read was checked for major quality parameters. Sequence reads were then filtered for sequencing artifacts and low quality reads (preprocessing). The pre-processed reads were aligned to genome of barley cultivar Morex to call SNPs. Values of observed heterozygosity (Ho) ranged from 0.250 to 0.337 and were higher than the expected heterozygosity (He) that varied from 0.180 to 0.242 in genotypes of all origins. The inbreeding coefficient (FIS) values that ranged between -0.240 and -0.639 across the regions were also higher and negative suggesting existence of excess outcrossing than expected. Based on the inferred clusters by the ADMIXTURE, high Fst values were observed between clusters suggesting high genetic differentiation among the genotypes tested though differentiation was not based on location. In addition, genetic differentiation computed based on the predetermined location, altitude and source of genotypes suggested weak differentiation among the groups. These results indicate that, in Ethiopia, barley genetic variation between regions and altitudes were less pronounced than within region and altitude variations. This calls for the germplasm collection strategies to be cautious in considering location and altitude as a main factor of variation thus strategies should focus on exploiting the within region variation also for better germplasm conservation and utilization. The static yield stability of landrace has to be utilized by breeders for their wider recommendations for those farmers who cannot afford use of farm inputs and specific cultivars. In addition, the relative robustness as well as plasticity of traits sorted by the current study can be incorporated in the breeding strategy of barley in Ethiopia.Gerste (Hordeumvulgare L.) ist eine bedeutende Getreideart in Äthiopien und macht 8% der gesamten Getreideerzeugung, in Bezug auf die Anbaufläche, aus. Unvorhersehbaren Regen- und Dürreereignisse, wie z.B. Dürren, bei denen der Regen endet, bevor die Gerste ihre physiologische Reife erreicht hat, stellen die Pflanzenproduktion in Äthiopien vor große Herausforderungen. Der mangelnde Zugang für Landwirte zu effizienten Wettervorhersagesystemen und Kommunikationskanäle erfordern die Entwicklung von Sorten, die extremen Wetterereignissengegenüber tolerant sind. Ein Ziel der Pflanzenzüchtung, für Gebiete mit Extremwetterereignissen und begrenzten Ressourcen für landwirtschaftliche Betriebsmittel, ist die Erzeugung umweltstabiler Sorten mit höherem Durchschnittsertrag in unterschiedlichen Umwelten und unter verschiedenen Wachstumsbedingungen. Genotyp - Umwelt Interaktionen (G x E) erschweren jedoch häufig die Auswahl von Genotypen die sich an unterschiedliche Umweltbedingungen anpassen können. Das Wissen über die Ertragsstabilität bekannter äthiopischer Gerstensorten und Landsorten, unter sich ändernden Umweltbedingungen, ist für die zukünftige Entwicklung von Gerstensorten mit hohen und stabilen Erträgen wichtig. Darüber hinaus sind Ertragskomponenten, quantitative Merkmale,die stark von der Umwelt beeinflusst werden. Da der Kornertrag einkomplexeres Merkmal ist als die ertragsbestimmenden Komponenten, sind die Umwelteinflüsse und die Wechselwirkungen zwischen Genotyp und Umwelt (G x E) für denKornertrag stärker als für Komponenten die den Kornertrag bestimmen. Daher kann eine indirekte Selektionanhand vonErtragskomponenten effizienter sein als die per se Selektion auf Basis des Kornertrags, um Ertragreichere und stabilere Sorten zu erhalten. Daher wurde diese Studie initiiert um 1) die Reaktion verschiedener Gerstengenotypen auf verschiedene Standorte und Aussaattermine zu beurteilen und Genotypen mit einerweiten Anpassung und stabilen Leistung zu identifizieren, 2) Merkmale zu bestimmen, die zu hohen und stabilen Erträgen in einer Reihe unterschiedlicher Umwelten beitragen, 3) die Populationsstruktur und genetische Parameter von äthiopischen und deutschen Genbankakzessionen, sowie aktuellen Gerstensorten, zu erfassen. Um diese Ziele zu erreichen, wurden 18 Genotypen an vier verschiedenen Aussaatterminen im Abstand von 15 Tagen an verschiedenen Orten (Ambo und Jimma) und Jahren (2012 und 2013) getestet. Die getesteten Genotypen zeigen eine große Variation, sowohl für die statische als auch für die dynamische Messung der Ertragsstabilität. Im Vergleich zu aktuellen Sorten zeigen die Landrassen der Landwirte eine höhere durchschnittliche statische Stabilität und eine ähnliche dynamische Stabilität. Diese Landrassen sind daher ein wertvoller Genpool für die Züchtung von widerstandsfähigen Gerstensorten. Die zeitversetzteAussaat erwies sich als nützliche Methode zur Beurteilung der Stabilität von Genotypen über Umweltfaktoren hinweg. Darüber hinaus haben wir auch eine kompensatorische Beziehung zwischen Kornanzahl pro Ähre und Tausendkorngewicht in Landrassen festgestellt. Die Kornanzahl pro Ähre und die Anzahl der fruchtbaren Bestockungstriebeerwiesen sich als robuste Merkmale für eine breitere Adaption an unterschiedliche Umweltbedingungen, da sie einen signifikanten und beständig, positiven Effekt auf den Kornertrag haben. Um die Populationsstruktur und genetische Parameter zwischen Genotypen verschiedener Herkunft und Genbanken zu bestimmen, wurden DNA-Proben mit ApeK1- und HindIII-Enzymenbearbeitet. Nach der Sequenzierung wurde die Rohdaten auf wichtige Qualitätsparameter überprüft. Sequenzen wurden gefiltert, um Artefakte zu eliminieren und Sequenzen mit geringer Qualität zu entfernen (Vorverarbeitung). Die vorverarbeiteten Sequenzen wurden an dem Genom der Gerstensorte Morexausgerichtet, um SNPs zu identifizieren. Die Werte der beobachteten Heterozygosität (Ho) lagen im Bereich von 0,250 bis 0,337 und waren höher als die erwartete Heterozygosität (He), die in der gesamten Population von 0,180 bis 0,242 variierte. Die Werte für den Inzuchtkoeffizienten (FIS), liegen zwischen -0,240 und -0,639, sind ebenfalls höher und negativ, was auf eine übermäßige Auskreuzung als erwartet hindeutet. Basierend auf den von ADMIXTURE abgeleiteten Clustern wurden hohe Fst-Werte zwischen den Clustern beobachtet. Dies lässt auf eine hohe genetische Differenzierung zwischen den getesteten Genotypen schließen, jedoch ließ sich keine ortsabhängige Differenzierung feststellen. Darüber hinaus deutet die genetische Differenzierung, die basierend auf Ort, Höhe und Sortentyp berechnet wurde, auf eine schwache Differenzierung zwischen den Gruppen hin. Diese Ergebnisse zeigen, dass in Äthiopien die genetische Variation der Gerste zwischen Regionen und Höhenlagen weniger ausgeprägt war als innerhalb von Region und Höhenlagen. Dies erfordert, dass Strategien zur Sammlung von genetischen Ressourcen sich nicht nur an Standort und Höhenlage als Hauptvariationsfaktor ausrichten. Vielmehr sollten sich Strategien zum Erhalt des Gersten-Genpools darauf konzentrieren, die Variationen innerhalb der Regionenund Höhenlagen zu nutzen um die genetische Diversität aufrecht zu erhalten. Die statische Ertragsstabilität der Landsortensollte von Züchtern für ihre Empfehlungen für Landwirte berücksichtigt werden. Darüber hinaus kann empfohlen werden die relative Robustheit sowie die Plastizität von Merkmalen, die durch die aktuelle Studie identifiziert wurden, in die Züchtungsstrategie von Gerste in Äthiopien einzubeziehen