• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 201
  • 79
  • 41
  • 30
  • 29
  • 10
  • 4
  • 3
  • 3
  • 2
  • 2
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 457
  • 66
  • 48
  • 44
  • 40
  • 35
  • 34
  • 34
  • 30
  • 28
  • 28
  • 27
  • 27
  • 25
  • 25
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
171

Rebelující ženské hrdinky v dystopických románech pro mladé / Rebellious Female Protagonists in Young Adult Dystopian Novels

Drkošová, Sylvie January 2016 (has links)
The thesis is mainly concerned with popular dystopian book series Hunger Games and Divergent. The aim of the present diploma thesis is to summarize representative characteristics of a young adult dystopian novels featuring rebellious female heroes and to closely examine the social context of the aforementioned novels. The first part of the thesis is based on the analysis of young adult dystopian novels and the attitude of young female readers to the representation of strong female protagonists in literature. The second part od this thesis presents a qualitative research realized by interviews with young female readers and it attempts to answer the research questions about the attitude of readers to examined dystopian novels and the contemporary social role of women. Keywords: dystopian novel, Hunger Games, Divergent, female protagonists, gender
172

MITOCHONDRIAL AND NUCLEAR PATTERNS OF CONFLICT AND CONCORDANCE AT THE GENE, GENOME, AND BEHAVIORAL SCALES IN <em>DESMOGNATHUS</em> SALAMANDERS

Kratovil, Justin D. 01 January 2017 (has links)
Advancements in molecular sequencing have revealed unexpected cryptic genetic diversity and contrasting evolutionary histories within genes and between genomes of many organisms; often in disagreement with recognized taxonomy. Incongruent patterns between the mitochondrial and nuclear evolutionary history can have several plausible explanations, but widespread systematic conflict inevitably challenges our conceptions of species boundaries when there is discordance between coevolving and coinherited genomes. It is unknown to what degree mitonuclear conflict drives the process of divergence, or how ubiquitous these patterns are across the tree of life. To understand the evolutionary relevance of intergenomic discordance we must identify the conflicting patterns that exist in natural systems by generating robust estimates of the underlying species history, quantify support for alternative hypotheses of lineage formation, and describe patterns of genetic variation present in robust nuclear genomic datasets. Empirically testing correlations between mitonuclear genomic conflict and reduced gene flow at the organism level will contribute toward a better understanding of lineage boundaries and how intergenomic interactions shape the process of divergence. Mitochondrial introgression has been inferred in many salamander systems with limited perspective from nuclear sequence data. Within dusky salamanders (Desmognathus), these patterns have been observed between morphologically and geographically disparate populations. I sequenced regions throughout the nuclear genome to reconstruct species trees, performed population-level analyses testing concordance between the mitochondrial, nuclear datasets, and nuclear genes with mitochondrial functions with the expectation that coevolutionary interactions among genomes are more likely to manifest in these regions. I also estimated migration rates between populations that may have experienced historical mitochondrial introgression to evaluate phylogeographic patterns. Using these data we definitively reject species models in which genetic boundaries are based solely on mitochondrial data, favoring geographic models instead. Furthermore, analyses soundly reject current taxonomic models based on morphological characteristics, suggesting there is greater lineage diversity than is currently recognized. I also used empirical assays of pre-zygotic reproductive mating behavior within and among populations containing diverse mitochondrial lineages to test metrics of reproductive isolation, and to determine if introgression shapes the evolution of complex traits directly influencing rates of divergence. These results may explain incongruent patterns observed between the mitochondrial and nuclear data as a function of inheritance and population dynamics rather than directly functioning to suppress nuclear gene flow. This research builds upon recent studies suggesting that speciation is a highly complex and often non-bifurcating process in which introgression can have a profound and lasting signature on the nuclear evolutionary history. Mechanisms responsible for divergence with gene flow challenge evolutionary biologists to reevaluate our notions and definitions of species boundaries to accommodate seemingly conflicted genomic patterns within and between genomes.
173

Systematics and conservation of Colophon Gray (Coleoptera: Lucanidae)

Switala, Angelika Katrin January 2013 (has links)
The flightless Cape High-mountain stag beetle genus Colophon (Coleoptera: Lucanidae) is studied. Represented by 17 species, which are restricted to the highest mountain peaks of the Cape Floristic Region in the Western Cape, South Africa, and show a strict association with the fynbos biome. The study aimed to determine the specific and phylogenetic status of the described species of Colophon and to determine the main factors driving their evolution by testing hypotheses of relationship and of a lowland origin. This was achieved by analysing DNA sequence data from three gene regions, the mitochondrial COI and 16S rRNA and the nuclear CAD, using a Maximum Likelihood (ML) and Bayesian approach. Timing of key biogeographical events in the diversification of Colophon was estimated in BEAST. The study also undertook to determine diagnostic larval characters for Colophon species and also to determine their phylogenetic implications for the sub-familial placement of the genus. Lastly, the study aimed to collate biological information on Colophon species so as to make suggestions for their revised conservation status in terms of IUCN and ToPS criteria. Most species of Colophon showed an allopatric distribution, although contact zones between geographically adjacent species are likely. Climate seems to be the main driving factor behind Colophon evolution and the hypothesis of a lowland origin appears to be supported. Larvae are soil-living and feed on humus, a habit unique to the family. There are only small inter-specific differences between larvae, with larval characters contributed little equivocal information from which phylogenetic support for family placement of Colophon could be deduced. The main threats to Colophon survival include overexploitation by commercial collectors, decline in habitat quality and habitat loss due to projected changes in climate. It is suggested that out of the 17 currently described species, eight should be listed as Critically Endangered, seven as Endangered, two as Data Deficient. Lastly, in light of this project‟s findings, it is suggested that future considerations in terms of Colophon research should focus on obtaining more information on their biology, behaviour and population size and in so doing contribute knowledge for the effective conservation management of each species. A taxonomic revision of the species, focusing on the C. stokoei varieties and C. eastmani subspecies, should be done and a complete taxonomic key of all described species compiled. Future fieldwork should focus on sampling the five species that remained elusive during the project, to eventually be included in phylogenetic analyses. / Dissertation (MSc)--University of Pretoria, 2013. / gm2013 / Zoology and Entomology / unrestricted
174

Etude expérimentale des champs magnétiques en surface d'une cible irradiée par laser et leurs implications sur le faisceau d'électrons / Experimental study of on-surface magnetic field generated by high intensity laser and its implication on the fast electron beam

Forestier-Colleoni, Pierre 10 March 2016 (has links)
Cette thèse porte sur la caractérisation des champs magnétiques générés par l'interaction entre un laser d'intensité de 1017 W/cm2 à 1018 W/cm2 et de cibles solides, et leurs effets sur le faisceau d'électrons chauds. En effet, les différents champs magnétiques créés lors de cette interaction ont un rôle fondamental sur les caractéristiques du faisceau d'électrons chauds : sa source et son transport dans la matière. Des diagnostics de polarimétrie et d'interférométrie croisée ont été développés lors de cette thèse pour observer le champ magnétique en surface de la cible irradiée par laser et en particulier leurs évolutions spatiale et temporelle. Deux différents régimes ont été observés selon le contraste en intensité de l'impulsion laser : un possédant une montée rapide de champ magnétique suivie d'une décroissance plus lente créées par le déplacement des électrons chauds dans la matière, et un possédant une croissance plus lente de forme logarithmique créée par la pré-impulsion du laser par effet thermoélectrique. L'interprétation de nos résultats obtenues par ces diagnostics ont permis d'évaluer la résistivité du plasma. Cette résistivité nommée anormale dans la littérature se comprend en estimant l'influence du champ magnétique sur l'anisotropie du transport des électrons et donc sur la résistivité. Le dernier diagnostic permettant l'estimation du champ magnétique détaillé dans cette thèse est la déflectométrie protonique. Elle permet d'observer la déviation d'un faisceau de protons lors de sa propagation sous l'effet de champs électrique et magnétique. D'autres expériences se sont focalisées sur la divergence de ce faisceau d'électrons. Deux diagnostics principaux ont été utilisés : l'imagerie K α et l'imagerie du rayonnement de transition cohérente (C.T.R.) en face arrière de cibles. / This thesis concerns magnetic fields, generated by the interaction between strong laser pulse (intensity up to1018 W/cm2) and solid target, and their effects on the fast electron beam. Indeed, the various magnetic fields created during this interaction can inuence the divergence of the fast electron beam. The magnetic field createdduring this interaction have a fundamental role on the fast electron beam characteristics : its source and its transportin the material. Diagnotics of polarimetry and crossed interferometry were developed during this thesis to observethe on-surface magnetic field of the target, and in particular, their spatial and temporal evolutions. Two types oftemporal evolution of the magnetic field were observed according to the contrast in intensity of the laser pulse : afast rise of magnetic field followed by a slower decrease created by the travel of the fast electrons in the material,and a slower growth of logarithmic form created by the pre-pulse of the laser by thermoelectric effect. The interpretation of our results obtained by these diagnotics allowed us to estimate the resistivity of the plasma.This resistivity named "anomalously high resistivity" in the literature can be explained by taking into account theinuence of the magnetic field on the electrons transport (creation of an anisotropy) and thus on the resitivity.The last diagnotic allowing the estimation of the magnetic field detailed in this thesis is the proton deectometry. itallows to observe the deviation of a proton beam during its propagation under the inuence of electric and magneticfields. Other experiments were focused on the fast electron beam divergence. Two main diagnotics were used : the K α imaging and the coherent transition radiation (C.T.R) imaging at the rear side of solid targets. These diagnoticsallowed to estimate the fast electron beam divergence for two distinct energetic electron populations. The differenceof divergence coming from characteristics of both diagnotics (electrons in charge of the emissions in different energies). The diagnotics of on-surface magnetic fields of target irradiated by intense laser, such as the technics of polarimetry and crossed interferometry developed in this thesis, are dedicated to be combined with diagnotics determining the evolution of the radial size of the fast electron beam generated by the laser-matter interaction. Their simultaneous use, and the correlation between their respective data, should allow to establish experimentally, in the short term, the inuence of the on-surface magnetic fields on the fast electron beam initial characteristics, in particular the angular and energy distributions. Our results of polarimetry on the spatio-temporal evolution of the magnetic fields of surface establish the state of the art for this type of measures. There are possible improvements, in particular as regards their use in conditions of irradiation by lasers of intensities > 1018 W/cm2. These perspectives are also the object of discussions in this manuscript.
175

Latitudinal Position and Trends of the Intertropical Convergence Zone (ITCZ) and its Relationship with Upwelling in the Southern Caribbean Sea and Global Climate Indices

Colna, Kaitlyn E 22 March 2017 (has links)
The Intertropical Convergence Zone (ITCZ) is a feature that results from the ocean-atmosphere interactions in the tropics around the world. The ITCZ is characterized by surface wind convergence, tall storm clouds, and it forms a belt of high time-averaged precipitation around the globe. The ITCZ undergoes seasonal migrations between 5°S and 15°N roughly following the subsolar point on Earth with the seasons, with a mean annual position located slightly above the Equator, between 2° and 5°N. This study tested the hypothesis that there was a northward shift in the median position of the ITCZ in the first decade of the 2000’s relative to the 1900’s. This hypothesis has been posed in the literature given a weakening in the intensity of the Trade Winds observed in the southern Caribbean Sea during the first decade of the 2000’s, with concomitant ecological impacts due to weakening in coastal wind-driven upwelling. The hypothesis was tested by analyzing variations in the monthly latitudinal position of the ITCZ over the Atlantic Ocean relative to the median position computed for the period 1987-2011. The position of the ITCZ was derived from satellite-derived ocean surface wind measurements collected from 1987 to 2011. A Mann-Kendall analysis and a Monte Carlo simulation were used to test for trends in the median cross-basin latitudinal position of the ITCZ. The study included an analysis of regional changes across the tropical central Atlantic (50°W to 15°W), the Western Atlantic (50°W to 30°W), and the Eastern Atlantic (30°W to 15°W) within the tropics. The results show a slight southward trend in the median position of the ITCZ over the central Atlantic and also in the Eastern Atlantic in the first decade of the 2000’s relative to the 1990’s. While this trend is barely significant, it is likely simply due to interannual variation in the average annual position of the ITCZ. The data were also examined for the timing and persistence of a double ITCZ in the Atlantic. The double ITCZ over the Atlantic appeared every year in February or March, with the largest separation between the northern and southern branches of the ITCZ observed in June and July. The possible effects of changes in the average latitudinal position of the ITCZ on the upwelling in the Cariaco Basin (southeastern Caribbean Sea off Venezuela) were also examined. Anomalies of the median of the latitudinal position of the ITCZ in the Atlantic were compared with anomalies of in-situ temperature collected during the 1990’s and the first decade of the 2000’s by the CARIACO Ocean Time-Series program and with anomalies of satellite SST (from the Advanced Very High Resolution Radiometer satellite; AVHRR) from 1995 to 2016. Correlation analysis were performed between anomalies of water temperatures at various depths and anomalies of satellite SST with anomalies of the monthly mean ITCZ position with lags up to 3 months for the time series, and also just for the Cariaco basin upwelling months (December-April). For the whole Cariaco time series there were no significant correlations between the anomalies of the ITCZ position and anomalies in subsurface temperatures in the Cariaco Basin. However, during the upwelling period, the central Atlantic and Western Atlantic ITCZ position anomalies were directly correlated with Cariaco Basin temperature anomalies with no-lag (r = 0.20), and the central and Eastern Atlantic ITCZ position anomalies were inversely correlated with Cariaco Basin temperatures (r ~ -0.22 to -0.28) with ITCZ leading Cariaco temperatures by 3 months. However, these correlations were low, indicating that other factors than the position of ITCZ latitudinal position play bigger role on the Cariaco basin upwelling variability. Interannual variability in oceanographic and meteorological characteristics of the Atlantic Ocean are expected as a result of large-scale changes in other regions of the world, including due to changes such as the El Niño Southern Oscillation (ENSO) and the Atlantic Multidecadal Oscillation (AMO). Six oceanic-atmospheric variables are used to monitor ENSO over the tropical Pacific, while the AMO is determined by monitoring SST over the Atlantic. Correlations with lags of up to ± 6 months were conducted with those climate indices and the anomalies of the median monthly latitudinal position of the ITCZ. Significant direct correlations with ENSO (Multivariate ENSO Index) were seen in the Atlantic and Western Atlantic (r = 0.15), with ENSO leading the position of the ITCZ anomalies by 3 months. This implies that within three months after an El Niño event (warm ENSO anomaly in the Pacific) the ITCZ over the mid-Atlantic and Western Atlantic Ocean tends to shift to a more northerly position. The AMO also had a direct influence on the anomalies of the ITCZ position (r = 0.13) in the Central and the Western Atlantic, with the AMO leading ITCZ anomalies by 1 month (i.e. a warming of the North Atlantic led to a northward shift in the ITCZ one month later). Correlations between AMO and the ITCZ anomalies in the Eastern Atlantic were also direct but with no lag. Although significant, these correlations were low. An inverse correlation (~ -0.35) was found between ENSO and anomalies of water temperature of the Cariaco Basin. ENSO lagged ocean temperature anomalies by 3 to 4 months for both the whole Cariaco time series and for the upwelling months of CARIACO data. Correlations with AMO were direct (~ 0.4); for the whole time series AMO led Cariaco temperature anomalies by 3 months, but for the upwelling months AMO lagged Cariaco temperature anomalies by one month.
176

Analýza příčin a konsekvencí pěti krizí evropského integračního procesu v souvislosti s hospodářsko-politickým vývojem po podpisu Maastrichtské smlouvy / Analysis of the causes and consequences of five crises of the european integration process in the context of economic and political development after Maastricht Treaty

Ševčíková, Michaela January 2013 (has links)
This paper analyses the essential aspects, causes and consequences of contemporary European integration process and examines the performance of individual EU countries. The analysis is done through the view of five crises: first the political and institutional; second the public finance, respectively the debt; third the different quality and competitiveness of single economies; fourth the banking, financial, respectively economic; and fifth the view, which focuses on the psychological aspect of trust. Substantial measures proposed or implemented by the EU institutions are subjects of analysis (European bonds; tax on financial transactions, banking union, fiscal pact and bail-out fund), including their critical evaluation, especially activist policy of the European Central Bank. Summary is contained in the SWOT analysis. This work also proposes recommendation for further development in the Czech Republic. The research section made by testing selected variables among 2003-2012 based on the method Beta convergence confirmed, that instead of theoretical assumption of faster real convergence of EU countries, the divergence between the core and periphery countries took place in the examined period. Beta convergence is confirmed within the designated groups in the new EU member states in the pre-crisis growth and in the overall monitored period. The second method -- the cluster analysis created groups, respectively clusters of countries that are the most similar based on selected indicators. On the one hand, the repeated grouping of core countries into one common cluster was verified with different options of the model. On the other hand, the used Ward's method showed, among others, that in majority of observed periods the countries commonly categorized into periphery groups (such as Italy and Ireland) have greater economic similarities to the core countries rather than countries such as Spain, Portugal and Greece.
177

Quantiles univariés et multivariés, approches probabilistes et statistiques : applications radar / Univariate and multivariate quantiles, probabilistic and statistical approaches : radar applications

Decurninge, Alexis 26 January 2015 (has links)
La description et l’estimation des modèles aussi bien univariés que multivariés impliquantdes distributions à queue lourde est un enjeu applicatif majeur. Les L-moments sontdevenus des outils classiques alternatifs aux moments centraux pour décrire les comportementsen dispersion, asymétrie, kurtosis d’une distribution univariée à queue lourde. Eneffet, contrairement aux moments centraux correspondants, ils sont bien définis dès quel’espérance de la distribution d’intérêt est finie. Les L-moments peuvent être vus comme laprojection de la fonction quantile sur une famille orthogonale de polynômes, récupérant lalinéarité inhérente aux quantiles. Nous estimerons dans un premier temps les paramètresde modèles semi paramétriques définis par des contraintes sur ces L-moments par des méthodesde minimisation de divergences.Nous proposons dans un second temps une généralisation des L-moments aux distributionsmultivariées qui passe par la définition d’un quantile multivarié défini comme untransport entre la distribution uniforme sur [0; 1]d et la distribution d’intérêt. Cela nouspermet de proposer des descripteurs pour des distributions multivariées adaptés à l’étudedes queues lourdes. Nous détaillons leurs expressions dans le cadre de modèles possédantdes paramètres de rotation.Enfin, nous proposons des M-estimateurs de la matrice de dispersion des distributions complexeselliptiques. Ces dernières forment un modèle multivarié semi-paramétrique contenantnotamment des distributions à queue lourde. Des M-estimateurs spécifiques adaptésaux distributions elliptiques avec une hypothèse supplémentaire de stationnarité sont égalementproposés. Les performances et la robustesse des estimateurs sont étudiées.Les signaux radar provenant de fouillis tels les fouillis de mer ou les fouillis de sol sontsouvent modélisés par des distributions elliptiques. Nous illustrerons les performances dedétecteurs construits à partir de l’estimation de la matrice de dispersion par les méthodesproposées pour différents scénarios radar pour lesquels la robustesse de la procédure d’estimationest cruciale. / The description and the estimation of univariate and multivariate models whose underlyingdistribution is heavy-tailed is a strategic challenge. L-moments have becomeclassical tools alternative to central moments for the description of dispersion, skewnessand kurtosis of a univariate heavy-tailed distribution. Indeed, contrary to correspondingcentral moments, they are well defined since the expectation of the distribution of interestis finite. L-moments can be seen as projections of the quantile function on a family oforthogonal polynomials. First, we will estimate parameters of semi-parametric modelsdefined by constraints on L-moments through divergence methods.We will then propose a generalization of L-moments for multivariate distributions using amultivariate quantile function defined as a transport of the uniform distribution on [0; 1]dand the distribution of interest. As their univariate versions, these multivariate L-momentsare adapted for the study of heavy-tailed distributions. We explicitly give their formulationsfor models with rotational parameters.Finally, we propose M-estimators of the scatter matrix of complex elliptical distributions.The family of these distributions form a multivariate semi-parametric model especiallycontaining heavy-tailed distributions. Specific M-estimators adapted to complex ellipticaldistribution with an additional assumption of stationarity are proposed. Performancesand robustness of introduced estimators are studied.Ground and sea clutters are often modelized by complex elliptical distributions in the fieldof radar processing. We illustrate performances of detectors built from estimators of thescatter matrix through proposed methods for different radar scenarios.
178

Relationen mellan chefer och medarbetare - Ett principal-agent-perspektiv

Andersson, Jens, Karlsson, Olle, Pettersson, Mattias January 2018 (has links)
Titel: Relationen mellan chefer och medarbetare - Ett principal-agent-perspektiv Universitet: Malmö Universitet Kurs: TR128C – Företagsekonomi: Examensarbete i Transport Management Författare: Jens Andersson, Olle Karlsson & Mattias Pettersson Handledare: Benedikte Borgström Nyckelord: Agentteori, agentproblem, måldivergens, informationsasymmetri, moral hazard, adverse selection, programmerbarhet Syfte: Att utifrån ett principal-agent-perspektiv studera hur relationen mellan chefer och medarbetare i den studerade miljön påverkas av förekomsten av informationsasymmetri och skilda målbilder, samt hur motivationen att prestera på arbetsplatsen kan påverkas av incitamentsstrukturer kopplade till kontraktsupplägget och olika nivåer av övervakning. Metod: Studien har genomförts via en kvalitativ ansats där data insamlats genom intervjuer, observationer och textanalys. Teori: Studiens teoretiska ramverk bygger på principal-agent-teorin och dess komponenter med fokus på informationsasymmetri och skilda målbilder. Slutsatser: I den studerade miljön där relationen mellan principal och agent kännetecknas av hög informationsasymmetri, hög programmerbarhet och parterna motiveras av olika målbilder står principalen inför valet att antingen komplettera ett beteendebaserat kontrakt med tillräckliga övervakningsmekanismer för att utröna agentens beteende, eller försöka upprätta ett resultatbaserat kontrakt. Hög informationsasymmetri kopplat till ett beteendebaserat kontrakt riskerar att lägga grunden för problem kopplade till moral hazard då agenterna blir helt fria att agera efter sina egna målbilder. / Title: The relationship between managers and workers - An agency perspective University: Malmö Universitet Course: TR128C – Business Administration: Bachelor thesis in Transport Management Writers: Jens Andersson, Olle Karlsson & Mattias Pettersson Mentor: Benedikte Borgström Keywords: Agency theory, agency problem, goal divergence, information asymmetry, moral hazard, adverse selection, programmability Purpose: From an agency perspective study how the relationship between managers and workers in the studied environment is affected by the existence of information asymmetry and diverging goals, and also how the motivation to perform at the workplace can be affected by different incentive structures related to the contract form and different levels of surveillance. Methods: The study has been conducted via a qualitative approach and the data has been collected through interviews, observations and text analysis. Theories: The studies theoretical framework is based on the agency theory and its components with a focus on information asymmetry and diverging goals. Conclusion: In the studied environment the relationship between principal and agent is characterized by asymmetrical information flows, high programmability, and diverging goals. This leaves the principal to either reinforce a behavioral based contract with sufficient surveillance to uncover the agent’s behavior, or to try and establish a contract based on results. A high degree of information asymmetry in combination with a behavioral based contract runs the risk of laying the foundation for problems related to moral hazard as the agents are free to act according to their own personal goals.
179

ASSOCIATION OF TOO SHORT ARCS USING ADMISSIBLE REGION

Surabhi Bhadauria (8695017) 24 April 2020 (has links)
<p>The near-Earth space is filled with over 300,000 artificial debris objects with a diameter larger than one cm. For objects in GEO and MEO region, the observations are made mainly through optical sensors. These sensors take observations over a short time which cover only a negligible part of the object's orbit. Two or more such observations are taken as one single Too Short Arc (TSA). Each set of TSA from an optical sensor consists of several angles, the angles of right ascension, declination, along with the rate of change of the right ascension angle and the declination angle. However, such observational data obtained from one TSA because it is covering only a very small fraction of the orbit, is not sufficient for the complete initial determination of an object's orbit. For a newly detected unknown object, only TSAs are available with no information about the orbit of the object. Therefore, two or more such TSAs that belong to the same object are required for its orbit determination. To solve this correlation problem, the framework of the probabilistic Admissible Region is used, which restricts possible orbits based on a single TSA. To propagate the Admissible Region to the time of a second TSA, it is represented in closed-form Gaussian Mixture representation. This way, a propagation with an Extended Kalman filter is possible. To decide if two TSAs are correlated, that is if they belong to the same object, respectively, an overlap between the regions is found in a suitable orbital mechanic's based coordinate frame. To compute the overlap, the information measure of Kullback-Leibler divergence is used. <br></p>
180

Extrinsic Effects on Heat and Electron Transport In Two-Dimensional Van-Der Waals Materials- A Boltzmann Transport Study

Majee, Arnab K 07 November 2016 (has links)
Two-dimensional van der Waals materials have been a subject of intense research interest in recent years. High thermal conductivity of graphene can be utilized for many thermal management applications. In spite of possessing very high electron mobility, graphene can’t be used as transistors because of the absence of band gap; however transition metal dichalcogenides are another class of two-dimensional van der Waals materials with inherent band gap and show a great promise for future nanoelectronic applications. But in order to tailor these properties for commercial applications, we should develop a better understanding of the effect of extrinsic factors like size, rough edges, grain boundaries, mass-impurities, interaction with substrate etc. on thermal and electrical transport. Most materials exhibit a smooth ballistic-to-diffusive type of thermal transport in which when the sample size is small as compared to mean-free-path of phonons the transport is ballistic, whereas, when the sample size is large as compared to phonon mean-free-path, phonons undergo multiple scattering events and the thermal transport becomes diffusive in nature. However, graphene exhibits an atypical thermal transport behavior where thermal conductivity shows an increasing logarithmic trend even for samples far greater than the mean-free-path of phonons. We show that this anomalous behavior can be attributed to the significant contribution coming from momentum-conserving normal phonon-phonon scattering. Secondly, graphene grain boundaries have been found to significantly reduce thermal conductivity even in the presence of substrates. In spite of numerous studies on the effect of grain boundaries (GBs) on thermal conductivity in graphene, there lacks a complete correlation between GB resistance and misorientation angle across graphene GBs. We show a direct correlation between thermal GB resistance and mismatch angles with low angle mismatch can be captured only by GB roughness, whereas, large mismatch angles will lead to the formation of a disordered patch at the interface and it could significantly deteriorate the overall thermal conductivity even in the presence of substrates. GBs are found to affect electrical transport in two-dimensional systems as well. Owing to the excellent electronic properties and compactness of these two-dimensional materials, high quality 2D heterojunctions are the subject of intense research interest in recent years. Graphene-MoS2 heterojuctions are found to form ohmic contacts and show great potential for future nanoelectronic applications. We show that the interface resistance in Gr-MoS2 heterojuctions can affect the overall resistance of the device if the channel (MoS2) length is small at low carrier densities, whereas, at high carrier densities interface resistance do not play much role in determining the resistance of the entire device. However, if graphene and MoS2 grains are misorientated then interface resistance can play a crucial role in determining the overall resistance of the device. We also show a weak dependence of misorientation angles on GB resistance across MoS2 grain boundaries.

Page generated in 0.0565 seconds