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Beswick, Matthew. "WEYL filtration dimension and submodule structures for B2." Diss., Kansas State University, 2009. http://hdl.handle.net/2097/1303.
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Doctor of PhilosophyDepartment of Mathematics
Zongzhu Lin
Let G be a connected and simply connected semisimple algebraic group over an algebraically closed field of positive prime characteristic. Let L([lambda]) and [upside-down triangle]([lambda]) be the simple and induced finite dimensional rational G-modules with p-singular dominant highest weight [lambda]. In this thesis, the concept of Weyl filtration dimension of a finite dimensional rational G-module is studied for some highest weight modules with p-singular highest weights inside the p2-alcove when G is of type B[subscript]2. In chapter 4, intertwining morphisms, a diagonal G-module morphism and tilting modules are used to compute the Weyl filtration dimension of L([lambda]) with [lambda] p-singular and inside the p[superscript]2-alcove. It is shown that the Weyl filtration dimension of L([lambda]) coincides with the Weyl filtration dimension of [upside-down triangle]([lambda]) for almost all (all but one of the 6 facet types) p-singular weights inside the p[superscript]2-alcove. In chapter 5 we study some submodule structures of Weyl (and there translations), Vogan, and tilting modules with both p-regular and p-singular highest weights. Most results are for the p[superscript]2 -alcove only although some concepts used are alcove independent.
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Beswick, Matthew. "WEYL filtration dimension and submodule structures for B₂." Manhattan, Kan. : Kansas State University, 2009. http://hdl.handle.net/2097/1303.
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Goudjil, Amar. "Data structures, binary search trees : a study of random Weyl trees." Thesis, McGill University, 1998. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=21559.
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This thesis covers the study of a particular class of binary search trees, the Weyl trees formed by consecutive insertion of numbers {theta}, {2theta}, {3theta}, ..., where theta is an irrational number from (0,1), and {x} denotes the fractional part of x. Various properties of the structure of these trees are explored and a relationship with the continued fraction expansion of theta is shown. Among these properties, an approximation of the height Hn of a Weyl tree with n nodes is given when theta is chosen at random and uniformly on (0, 1). The main result of this work is that in probability, Hn ∼ (12/pi2) log n log log n.
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Goudjil, Amar. "Data structures, binary search trees, a study of random Weyl trees." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape7/PQDD_0026/MQ50778.pdf.
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Gay, Joël. "Representation of Monoids and Lattice Structures in the Combinatorics of Weyl Groups." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLS209/document.
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La combinatoire algébrique est le champ de recherche qui utilise des méthodes combinatoires et des algorithmes pour étudier les problèmes algébriques, et applique ensuite des outils algébriques à ces problèmes combinatoires. L’un des thèmes centraux de la combinatoire algébrique est l’étude des permutations car elles peuvent être interprétées de bien des manières (en tant que bijections, matrices de permutations, mais aussi mots sur des entiers, ordre totaux sur des entiers, sommets du permutaèdre…). Cette riche diversité de perspectives conduit alors aux généralisations suivantes du groupe symétrique. Sur le plan géométrique, le groupe symétrique engendré par les transpositions élémentaires est l’exemple canonique des groupes de réflexions finis, également appelés groupes de Coxeter. Sur le plan monoïdal, ces même transpositions élémentaires deviennent les opérateurs du tri par bulles et engendrent le monoïde de 0-Hecke, dont l’algèbre est la spécialisation à q=0 de la q-déformation du groupe symétrique introduite par Iwahori. Cette thèse se consacre à deux autres généralisations des permutations. Dans la première partie de cette thèse, nous nous concentrons sur les matrices de permutations partielles, en d’autres termes les placements de tours ne s’attaquant pas deux à deux sur un échiquier carré. Ces placements de tours engendrent le monoïde de placements de tours, une généralisation du groupe symétrique. Dans cette thèse nous introduisons et étudions le 0-monoïde de placements de tours comme une généralisation du monoïde de 0-Hecke. Son algèbre est la dégénérescence à q=0 de la q-déformation du monoïde de placements de tours introduite par Solomon. On étudie par la suite les propriétés monoïdales fondamentales du 0-monoïde de placements de tours (ordres de Green, propriété de treillis du R-ordre, J-trivialité) ce qui nous permet de décrire sa théorie des représentations (modules simples et projectifs, projectivité sur le monoïde de 0-Hecke, restriction et induction le long d’une fonction d’inclusion).Les monoïdes de placements de tours sont en fait l’instance en type A de la famille des monoïdes de Renner, définis comme les complétés des groupes de Weyl (c’est-à-dire les groupes de Coxeter cristallographiques) pour la topologie de Zariski. Dès lors, dans la seconde partie de la thèse nous étendons nos résultats du type A afin de définir les monoïdes de 0-Renner en type B et D et d’en donner une présentation. Ceci nous conduit également à une présentation des monoïdes de Renner en type B et D, corrigeant ainsi une présentation erronée se trouvant dans la littérature depuis une dizaine d’années. Par la suite, nous étudions comme en type A les propriétés monoïdales de ces nouveaux monoïdes de 0-Renner de type B et D : ils restent J-triviaux, mais leur R-ordre n’est plus un treillis. Cela ne nous empêche pas d’étudier leur théorie des représentations, ainsi que la restriction des modules projectifs sur le monoïde de 0-Hecke qui leur est associé. Enfin, la dernière partie de la thèse traite de différentes généralisations des permutations. Dans une récente séries d’articles, Châtel, Pilaud et Pons revisitent la combinatoire algébrique des permutations (ordre faible, algèbre de Hopf de Malvenuto-Reutenauer) en terme de combinatoire sur les ordres partiels sur les entiers. Cette perspective englobe également la combinatoire des quotients de l’ordre faible tels les arbres binaires, les séquences binaires, et de façon plus générale les récents permutarbres de Pilaud et Pons. Nous généralisons alors l’ordre faibles aux éléments des groupes de Weyl. Ceci nous conduit à décrire un ordre sur les sommets des permutaèdres, associaèdres généralisés et cubes dans le même cadre unifié. Ces résultats se basent sur de subtiles propriétés des sommes de racines dans les groupes de Weyl qui s’avèrent ne pas fonctionner pour les groupes de Coxeter qui ne sont pas cristallographiquesAlgebraic combinatorics is the research field that uses combinatorial methods and algorithms to study algebraic computation, and applies algebraic tools to combinatorial problems. One of the central topics of algebraic combinatorics is the study of permutations, interpreted in many different ways (as bijections, permutation matrices, words over integers, total orders on integers, vertices of the permutahedron…). This rich diversity of perspectives leads to the following generalizations of the symmetric group. On the geometric side, the symmetric group generated by simple transpositions is the canonical example of finite reflection groups, also called Coxeter groups. On the monoidal side, the simple transpositions become bubble sort operators that generate the 0-Hecke monoid, whose algebra is the specialization at q=0 of Iwahori’s q-deformation of the symmetric group. This thesis deals with two further generalizations of permutations. In the first part of this thesis, we first focus on partial permutations matrices, that is placements of pairwise non attacking rooks on a n by n chessboard, simply called rooks. Rooks generate the rook monoid, a generalization of the symmetric group. In this thesis we introduce and study the 0-Rook monoid, a generalization of the 0-Hecke monoid. Its algebra is a proper degeneracy at q = 0 of the q-deformed rook monoid of Solomon. We study fundamental monoidal properties of the 0-rook monoid (Green orders, lattice property of the R-order, J-triviality) which allow us to describe its representation theory (simple and projective modules, projectivity on the 0-Hecke monoid, restriction and induction along an inclusion map).Rook monoids are actually type A instances of the family of Renner monoids, which are completions of the Weyl groups (crystallographic Coxeter groups) for Zariski’s topology. In the second part of this thesis we extend our type A results to define and give a presentation of 0-Renner monoids in type B and D. This also leads to a presentation of the Renner monoids of type B and D, correcting a misleading presentation that appeared earlier in the litterature. As in type A we study the monoidal properties of the 0-Renner monoids of type B and D : they are still J-trivial but their R-order are not lattices anymore. We study nonetheless their representation theory and the restriction of projective modules over the corresponding 0-Hecke monoids. The third part of this thesis deals with different generalizations of permutations. In a recent series of papers, Châtel, Pilaud and Pons revisit the algebraic combinatorics of permutations (weak order, Malvenuto-Reutenauer Hopf algebra) in terms of the combinatorics of integer posets. This perspective encompasses as well the combinatorics of quotients of the weak order such as binary trees, binary sequences, and more generally the recent permutrees of Pilaud and Pons. We generalize the weak order on the elements of the Weyl groups. This enables us to describe the order on vertices of the permutahedra, generalized associahedra and cubes in the same unified context. These results are based on subtle properties of sums of roots in Weyl groups, and actually fail for non-crystallographic Coxeter groups
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Borowka, Aleksandra. "Twistor constructions of quaternionic manifolds and asymptotically hyperbolic Einstein-Weyl spaces." Thesis, University of Bath, 2014. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.616872.
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Let $S$ be a $2n$-dimensional complex manifold equipped with a line bundle with a real-analytic complex connection such that its curvature is of type $(1,1)$, and with a real analytic h-projective structure such that its h-projective curvature is of type $(1,1)$. For $n=1$ we assume that $S$ is equipped with a real-analytic M\"obius structure. Using the structure on $S$, we construct a twistor space of a quaternionic $4n$-manifold $M$. We show that $M$ can be identified locally with a neighbourhood of the zero section of the twisted (by a unitary line bundle) tangent bundle of $S$ and that $M$ admits a quaternionic $S^1$ action given by unit scalar multiplication in the fibres. We show that $S$ is a totally complex submanifold of $M$ and that a choice of a connection $D$ in the h-projective class on $S$ gives extensions of a complex structure from $S$ to $M$. For any such extension, using $D$, we construct a hyperplane distribution on $Z$ which corresponds to the unique quaternionic connection on $M$ preserving the extended complex structure. We show that, in a special case, the construction gives the Feix--Kaledin construction of hypercomplex manifolds, which includes the construction of hyperk\"ahler metrics on cotangent bundles. We also give an example in which the construction gives the quaternion-K\"ahler manifold $\mathbb{HP}^n$ which is not hyperk\"ahler. We show that the same construction and results can be obtained for $n=1$. By convention, in this case, $M$ is a self-dual conformal $4$-manifold and from Jones--Tod correspondence we know that the quotient $B$ of $M$ by an $S^1$ action is an asymptotically hyperbolic Einstein--Weyl manifold. Using a result of LeBrun \cite{Le}, we prove that $B$ is an asymptotically hyperbolic Einstein--Weyl manifold. We also give a natural construction of a minitwistor space $T$ of an asymptotically hyperbolic Einstein--Weyl manifold directly from $S$, such that $T$ is the Jones--Tod quotient of $Z$. As a consequence, we deduce that the Einstein--Weyl manifold constructed using $T$ is equipped with a distinguished Gauduchon gauge.
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Hadfield, Charles. "Structures de Clifford paires et résonances quantiques." Thesis, Paris Sciences et Lettres (ComUE), 2017. http://www.theses.fr/2017PSLEE010/document.
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Ce manuscrit se compose de deux parties indépendantes. La première partie de cette thèse étudie les structures de Clifford paires. Pour une variété riemannienne munie d’une structure de Clifford paire, nous introduisons l’espace de twisteurs en généralisant la construction d’un tel espace dans le cas d’une variété quaternion-hermitienne. Nous construisons une structure presque-complexe sur l’espace de twisteurs et considérons son intégrabilité lorsque la structure de Clifford est parallèle. Dans certains cas, nous pouvons aussi le fournir d’une métriquekählerienne ou, correspondant à une structure presque-complexe alternative, d’une métrique “nearly Kähler”. Dans un second temps, nous introduisons une structure appelée Clifford-Weyl sur une variété conforme. Il s’agit d’une structure de Clifford paireq ui est parallèle par rapport au produit tensoriel d’une connexion métrique sur le fibré de Clifford et une connexion de Weyl. Nous démontrons que la connexion de Weyl est fermée sauf dans certains cas génériques de basse dimension où nous arrivons à décrire des exemples explicites où les structures de Clifford-Weyl sont non-fermées. La seconde partie de cette thèse étudie des résonances quantiques. Au-dessus d’une variété asymptotiquement hyperbolique paire, nous considérons le laplacien de Lichnerowicz agissant sur les sections du fibré des formes multilinéaires symétriques.Lorsqu’il s’agit de formes bilinéaires symétriques, nous obtenonsune extension méromorphe de la résolvante dudit laplacien à l’ensemble du plan complexe si la variété est Einstein. Cela définit les résonances quantiques pour ce laplacien. Pour les formes multilinéaires symétriques en général, une telle extension méromorphe est possible si la variété est convexe-cocompacte. Dans les deux cas, nous devons restreindre le laplacien aux sections qui sont de trace et de divergence nulles. Nous utilisons ce deuxième résultat afin d’établir une correspondance classique-quantique pour les variétés hyperboliques convexescocompactes.La correspondance identifie le spectre du flot géodésique (les résonances de Ruelle) avec les spectres des laplaciens agissant sur les tenseurs symétriques qui sont de trace et de divergence nulles (les résonances quantiques)We study independently even Clfford structures on Riemannian manifolds and quantum resonances on asymptotically hyperbolic manifolds. In the first part of this thesis, we study even Clifford structures.First, we introduce the twistor space of a Riemannian manifold with an even Clifford structure. This notion generalises the twistor space of quaternion-Hermitian manifolds. We construct almost complex structures on the twistor space and check their integrability when the even Clifford structure is parallel. In some cases we give Kähler and nearly-Kähler metrics to these spaces. Second, we introduce the concept of a Clifford-Weyl structure on a conformal manifold. This consists of an even Clifford structure parallel with respect to the tensor product of a metric connection on the Clifford bundle and a Weyl structure on the manifold. We show that the Weyl structure is necessarily closed except for some “generic” low-dimensional instances,where explicit examples of non-closed Clifford-Weyl structures are constructed. In the second part of this thesis, we study quantum resonances. First, we consider the Lichnerowicz Laplacian acting on symmetric 2-tensors on manifolds with an even Riemannian conformally compact Einstein metric. The resolvent of the Laplacian,upon restriction to trace-free, divergence-free tensors, is shown to have a meromorphic continuation to the complex plane. This defines quantum resonances for this Laplacian. For higher rank symmetric tensors, a similar result is proved for convex cocompact quotients of hyperbolic space. Second, we apply this result to establish a direct classical-quantum correspondence on convex cocompact hyperbolic manifolds. The correspondence identifies the spectrum of the geodesic flow with the spectrum of the Laplacian acting on trace-free, divergence-free symmetric tensors. This extends the correspondence previously obtained for cocompact quotients
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Guggenheim, Charles Moses [Verfasser], Martin [Gutachter] Zirnbauer, and Alexander [Gutachter] Altland. "Weyl semimetals: Euler structures and disorder / Charles Moses Guggenheim ; Gutachter: Martin Zirnbauer, Alexander Altland." Köln : Universitäts- und Stadtbibliothek Köln, 2020. http://d-nb.info/1219652415/34.
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Flamencourt, Brice. "On some problems in spectral analysis, spin geometry and conformal geometry." Thesis, université Paris-Saclay, 2022. http://www.theses.fr/2022UPASM014.
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Cette thèse se divise en deux grandes parties. Dans la première, on s'intéresse à deux problèmes d'analyse spectrale portant sur la convergence des valeurs propres d'opérateurs à paramètres. D'une part, on considère l'opérateur de Schrödinger dans le plan, avec un potentiel singulier supporté par une courbe fermée Γ admettant un point de rebroussement. Ce potentiel s'écrit formellement −αδ(x−Γ), et l'on décrit le comportement du spectre de l'opérateur dans la limite α→∞. D'autre part, on étudie l'opérateur de Dirac qui apparaît dans le modèle MIT Bag, en le généralisant aux variétés spin. Lorsque le paramètre de masse de cet opérateur tend vers l'infini, on observe une convergence des valeurs propres. Dans la seconde partie, on discute différents problèmes de géométrie. On démontre tout d'abord des résultats de structure et de classification en dimension 3 pour une classe particulière de spineurs, appelés spineurs de Cauchy, qui apparaissent naturellement comme restrictions de spineurs parallèles à des hypersurfaces orientées de variétés spin. Enfin, on s'intéresse aux connexions de Weyl sur les variétés conformes. On définit les structures localement conformément produits (LCP) par la donnée d'une structure de Weyl fermée, non-exacte, non-plate et à holonomie réductible sur une variété conforme compacte. On analyse les variétés LCP afin d'initier une classificationThis thesis is divided into two main parts. In the first one, we focus on two problems of spectral analysis concerning the convergence of eigenvalues of operators with parameters. On the one hand, we consider the Schrödinger operator in the plane, with a singular potential supported by a closed curve Γ admitting a cusp. This potential is formally written −αδ(x−Γ), and we describe the behaviour of the spectrum of the operator as α→∞. On the other hand, we study the Dirac operator which appears in the MIT Bag model, by generalizing it from Euclidean spaces to spin manifolds. We observe a convergence of the eigenvalues of this operator when the mass parameter tends to infinity. In the second part, we discuss two different geometric problems. First, we prove structure and classification results in dimension 3 for a particular class of spinors, called Cauchy spinors, arising as restrictions of parallel spinors to oriented hypersurfaces of spin manifolds. Finally, we focus on Weyl connections on conformal manifolds. We define a locally conformally product (LCP) structure as a closed, non-exact, non-flat Weyl structure with reducible holonomy on a compact conformal manifold. We analyse the LCP manifolds in order to initiate a classification
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Mahajumi, Abu Syed. "InAs/GaSb quantum well structures of Infrared Detector applications. : Quantum well structure." Thesis, IDE, Microelectronics and Photonics, 2010. http://urn.kb.se/resolve?urn=urn:nbn:se:hh:diva-3848.
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The detection of MWIR (mid wavelength infrared radiation) is the important for industrial, biomedical and military applications.desirable for the radiation detector to operate in the middle wavelength IR (MWIR) band corresponding to a wavelength band ranging from about 3 microns to about 5 microns.Such MWIR detectors allow forobjects having a similar thermal signature. In addition, MWIR detectors may be used in low power applications such as in night vision for surveillance of personnel.
Now a day commercially available uncooled IR sensors operating in MWIR region (2 – 5 μm) use microbolometric detectors which are inherently slow. The novel detector of InAs/GaSb quantum well structures overcomes this limitation. However, third-generation high-performance IR FPAs are already an attractive proposition to the IR system designer. They covered such as multicolour (at least two, and maybe more different spectral bands) with the possibility of simultaneous detection in both space and time, and ever larger sizes of, say, 2000 × 2000, and operating at higher temperatures, even to room temperature, for all cut-off wavelengths.These hetero structures have a type-II band alignment such that the conduction band of InAs layer is lower than the valence band of GaSb layer. The effective bandgap of thesestructures can be adjusted from 0.4 eV to values below 0.1 eV by varying the thickness of constituent layers leading to an enormous range of detector cutoff wavelengths (3-20 This work is focused on the various key characteristics the optical (responsivity and detectivity) and electrical (surface leakage & dark current) of infrared detector and proof of concept is demonstrated on infrared P-I-N photodiodes based on InAs/GaSb superlattices with ~8.5 μm cutoff wavelength and bandgap energy ~150 meV operating at 78 K where supression of surface leakage currents is observed. In certain military applications, it isthermal imaging of airplanes, artillery tanks and otherμm).
Nice research work at Halmstad University
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Menard, Etienne. "Algèbres amassées associées aux variétés de Richardson ouvertes : un algorithme de calcul de graines initiales." Thesis, Normandie, 2021. http://www.theses.fr/2021NORMC211.
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Les algèbres amassées sont des anneaux commutatifs intègres avec une structure combinatoire particulière.Cette structure consiste en la donnée d’une famille de graines, liées entre elles par une opération appelée mutation.Chaque graine est composée de deux parties : un amas et un carquois.Les variétés de Richardson ouvertes sont des strates de la variété de drapeaux associée à un groupe linéairealgébrique de type simplement lacé. Elles sont l’intersection de cellules de Schubert respectivement à deux sous-groupes de Borel opposés. Dans [Lec16], une sous-algèbre amassée de rang maximal sur l’anneau de coordonnéesd’une variété de Richardson ouverte a été construite et cette sous-algèbre est conjecturée être égale à l’anneauentier. La construction de cette algèbre amassée provient d’une catégorie de Frobenius C v,w de modules surl’algèbre préprojective, définie comme intersection de deux catégories C w et C v déjà étudiées par Geiss, Leclerc,Schröer et Buan, Iyama, Reiten et Scott. Le lien entre les algèbres amassées et les structures amassées est donnépar le caractère d’amas défini dans [GLS06].Dans cette thèse, nous construisons un algorithme qui, étant donné les paramètres définissant une variété deRichardson ouverte, construit un module rigide maximal explicite de la catégorie de Frobenius associée et soncarquois. Cet algorithme a pour donnée de départ la graine initiale pour la structure amassée sur C w définiepar un représentant w d’un élément w du groupe de Weyl. Par le biais d’une suite de mutations déterminéecombinatoirement, on obtient à partir de la graine initiale un module rigide maximal de C w qui, à suppressionde certains facteurs directs près, est un module rigide maximal de C v,w . De plus le sous-carquois du carquoismuté est exactement le carquois de l’algèbre d’endomorphisme du module rigide maximal de C v,w donnant alorsla description complète d’une graine initiale pour la structure amassée de C v,wCluster algebras are integral domains with a particular combinatorial structure. This structure consists in thedata of a family of seeds linked together by an operation called mutation. Each seed consists in two parts : acluster and a quiver.Richardson open varieties are some strata of the flag variety associated to a simple linear algebraic groupof simply-laced type. These are the intersection of Schubert cells with respect to two opposite Borel subgroups.In [Lec16] a cluster subalgebra of maximal rank on the coordinate ring of an open Richardson variety has beenconstructed and this subalgebra is conjectured to be equal to the whole ring. The construction of this clusteralgebra comes from a Frobenius category C v,w of modules over the preprojective algebra, defined as the intersectionof two categories C w and C v already studied by Geiss, Leclerc, Schröer and Buan, Iyama, Reiten and Scott. Thebond between cluster algebras and cluster structures is given by the cluster character defined in [GLS06].In this thesis we build an algorithm which, given the parameters defining a Richardson open variety, computean explicit maximal rigid module of the associated Frobenius category and its quiver. This algorithm has aninitial seed for the cluster structure on C w defined by a representative w of an element w of the Weyl group as astarting datum. By a combinatorially defined sequence of mutation on this initial seed we obtain a maximal rigidmodule of C w which is, up to deletion of some direct summands is a maximal rigid module of C v,w . In addition,the subquiver of the mutated quiver is exactly the quiver of the endomorphism algebra of the C v,w -maximal rigidmodule, giving then the complete description of an initial seed for the cluster structure on C v,w
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Master, Cyrus Phiroze 1975. "Band structure and gain calculations for gallium nitride quantum-well structures." Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/9873.
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Thesis (M.Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1998.Includes bibliographical references (p. 113-115).
by Cyrus Phiroze Master.
M.Eng.
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Labbi, Mohammed Larbi. "Courbure riemannienne: variations sur différentes notions de positivité." Habilitation à diriger des recherches, Université Montpellier II - Sciences et Techniques du Languedoc, 2006. http://tel.archives-ouvertes.fr/tel-00267987.
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On étudie différentes notions de courbure riemanniennes: la $p$-courbure, qui interpole entre courbure scalaire et courbure sectionnelle, les courbures de Gauss-Bonnet-Weyl qui constituent une autre interpolation allant de la courbure scalaire jusqu'à l'intégrand de Gauss-Bonnet.
Les $(p,q)$-courbures que nous dégageons englobent toutes ces notions. On examine ensuite le terme en courbure de la formule classique de Weitzenböck. On étudie aussi les propriétés de positivité de la $p$-courbure, la seconde courbure de Gauss-Bonnet-Weyl, la courbure d'Einstein et de la courbure isotrope.
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Bailey, S. J. "Cathodoluminescence of quantum well structures." Thesis, University of Bristol, 1987. http://hdl.handle.net/1983/b02b03a2-c0d8-401d-a94e-8bd26b52b953.
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Lechner, Vera [Verfasser], and Sergey D. [Akademischer Betreuer] Ganichev. "Bulk and structure inversion asymmetry in semiconductor quantum well structures / Vera Lechner. Betreuer: Sergey D. Ganichev." Regensburg : Universitätsbibliothek Regensburg, 2012. http://d-nb.info/1026165601/34.
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Kowalski, Olgierd Peter. "Optical spectroscopic study of the electronic band structure of bulk AlGaInP and related quantum well structures." Thesis, University of Sheffield, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.387753.
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Liu, Chun-Hung. "Graph structures and well-quasi-ordering." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/52262.
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Robertson and Seymour proved that graphs are well-quasi-ordered by the minor relation. In other words, given infinitely many graphs, one graph contains another as a minor. An application of this theorem is that every property that is closed under deleting vertices, edges, and contracting edges can be characterized by finitely many graphs, and hence can be decided in polynomial time.
In this thesis we are concerned with the topological minor relation. We say that a graph G contains another graph H as a topological minor if H can be obtained from a subgraph of G by repeatedly deleting a vertex of degree two and adding an edge incident with the neighbors of the deleted vertex. Unlike the relation of minor, the topological minor relation does not well-quasi-order graphs in general. However, Robertson conjectured in the late 1980's that for every positive integer k, the topological minor relation well-quasi-orders graphs that do not contain a topological minor isomorphic to the path of length k with each edge duplicated.
This thesis consists of two main results. The first one is a structure theorem for excluding a fixed graph as a topological minor, which is analogous to a cornerstone result of Robertson and Seymour, who gave such a structure for graphs that exclude a fixed minor. Results for topological minors were previously obtained by Grohe and Marx and by Dvorak, but we push one of the bounds in their theorems to the optimal value. This improvement is needed for the next theorem.
The second main result is a proof of Robertson's conjecture. As a corollary, properties on certain graphs closed under deleting vertices, edges, and "suppressing" vertices of degree two can be characterized by finitely many graphs, and hence can be decided in polynomial time.
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Watling, Jeremy Richards. "Carrier transport in quantum well structures." Thesis, University of East Anglia, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.267463.
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Atminas, Aistis. "Well-quasi-ordering of combinatorial structures." Thesis, University of Warwick, 2015. http://wrap.warwick.ac.uk/67023/.
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In this work we study the notion of well-quasi-ordering for various partial orders and its relation to some other notions, such as clique-width. In particular, we prove decidability of well-quasi-ordering for factorial languages, subquadratic properties of graphs and classes of graphs with finite distinguishing number. In addition, we reveal some new classes of graphs and permutations which are or are not well-quasi-ordered. We also prove that subquadratic properties or classes of graphs with finite distinguishing number that are well-quasi-ordered have bounded clique-width and we identify two new minimal classes of graphs of unbounded clique-width.
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Cox, Bradley. "Development of well-behaved nonlinear structures." Thesis, University of Bristol, 2018. http://hdl.handle.net/1983/0fcf0eaf-4b7c-4a02-bb45-5bed5122d177.
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Historically, the onset of geometric nonlinearity in structural mechanics is viewed as akin to damage or failure. Consequently, the main focus of the practising engineer has been to mitigate any nonlinear response. In preventing nonlinear responses, engineers have inherently limited their useful design space. The motivation behind this thesis pivots around the concept of embracing geometric nonlinearities, including instabilities, and understanding the benefits of considering such phenomena, for designing novel features into structures. In encouraging a paradigm shift to view structural nonlinearities as useful, it is demonstrated that it is possible to design more efficient structures. A literature search shows that the use of geometric nonlinearities is confined to the development of meta-materials and non-load-bearing structures. This work illustrates that embracing geometric nonlinearities in conventional engineering structures yields significant improvements. In particular, this research focuses on tailoring a structure’s nonlinear response by applying minor alterations to its initial shape or geometry. This enables increases in structural capacity in the form of (i) load-carrying capacity; (ii) compliance; (iii) extended stability. Three distinct structural forms are evaluated, namely arched beams, frames, and shell structures. Case studies are introduced for each of the three structures, and a full exploration of the design space is conducted by employing a nonlinear finite element method coupled with numerical continuation algorithms. The focus is placed on the control and design of the buckling and post-buckling behaviour in these structures, whereby a well-behaved structural response absent of unexpected phenomena is sought. To this aim the work begins by tackling one of the most common examples of structural nonlinearity, a planar arch exhibiting the textbook ``snap-through'' response. In designing for nonlinearity, the topology of the arch is optimised for maximising the critical buckling load. The findings show that the design of shallow arches using linear kinematic assumptions is antithetical to that of a design accounting for the nonlinear response. These findings illustrate the limitations entailed in optimising the shape of arches using linear assumptions, particularly when the onset of an instability is the target design point. A distinct novelty to the present work is the introduction of modal nudging, a paradigm which can improve the capacity of structures without any significant increase in mass, and convert imperfection-sensitive structures into imperfection-insensitive ones. Herein it is shown that it is applicable to both frame and shell structures. The true complexity of the post-buckled behaviour of axially compressed cylindrical panels, which has often been described as ``chaotic", is shown for the first time. These results not only elucidate the complex nature of post-buckling phenomena, but also highlight the potential of modal nudging into transforming unsafe, unstable post buckling behaviour into safe and stable. In conclusion, the findings of this thesis bring about new opportunities for the design of efficient structures with greater functionality. Hence, embracing geometric nonlinearity lays the foundation for the development of lighter, safer and multi-functional structures.
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Butler, Justin John. "Quantum Well Structures for Plasma Instability-based Terahertz Radiation Sources." Thesis, Boston College, 2012. http://hdl.handle.net/2345/2886.
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Thesis advisor: Pradip BakshiThis thesis is a theoretical study of the electron transport and response properties of epitaxially grown, low-dimensional semiconductor quantum well heterostructures, under steady-state, current driven (nonequilibrium) conditions. These structures operate in the Terahertz (THz) frequency and submillimeter wavelength range, and are the leading candidates for compact, coherent sources of THz radiation. This work is divided into two parts: Part I consists of an analytical study of the individual quantum well units, and the tunneling transmission characteristics, for which reasonably accurate algebraic expressions are obtained. An underlying philosophy of this work is the desire to describe each of the key components involved, independently, through these simple analytical expressions. In Part II the numerical study of the transport and radiation response of the quantum well structures specially designed to generate THz radiation based on the plasma instability concept is presented. Several models are proposed which describe the overall electron transport and which determine the underlying nonequilibrium steady state. In particular, the key features of the experimental current-voltage (IV) curves for such structures are explained, and the corresponding response properties are determined. The modeling and simulation of these potential optoelectronic devices is a crucial tool for elucidating the precise mechanisms and interplay of the many microscopic processes which give rise to the observed behavior. Key features of the radiation response arise from the intersubband plasma instability which occurs due to the resonant interaction of an emission and an absorption mode, and these features are compared with the experimental observations
Thesis (PhD) — Boston College, 2012
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics
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Whittaker, D. M. "Theory of excitons in quantum well structures." Thesis, University of Oxford, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235974.
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Mayhew, Nicholas. "Hot carrier studies in quantum well structures." Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.335833.
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MacBean, M. D. A. "Multiple quantum well structures as optical waveguides." Thesis, University of Glasgow, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.381458.
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Guptah, Vinod Kumar. "Growth on patterned substrates for optoelectronic device applications." Thesis, University College London (University of London), 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.267027.
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Haratizadeh, Hamid. "Optical characterization of GaN/AlGaN quantum well structures /." Linköping : Univ, 2004. http://www.bibl.liu.se/liupubl/disp/disp2004/tek866s.pdf.
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Osborne, Jane Lindsey. "Excitonic properties of tunable density quantum well structures." Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.624895.
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Davies, Matthew John. "Optical studies of InGaN/GaN quantum well structures." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/optical-studies-of-ingangan-quantum-well-structures(f6c6e59b-8366-44aa-b149-9338d3f03dc0).html.
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In this thesis I present and discuss the results of optical spectroscopy performed on InGaN/GaN single and multiple quantum well (QW) structures. I report on the optical properties of InGaN/GaN single and multiple QW structures, measured at high excitation power densities. I show a correlation exists between the reduction in PL efficiency at high excitation power densities, the phenomenon so-called ``efficiency-droop'', and a broadening of the PL spectra. I also show a distinct change in recombination dynamics, measured by time-resolved photoluminescence (PL), which occurs at the excitation power densities for which efficiency droop is measured. The broadening of the PL spectra at high excitation power densities is shown to occur due to a rapidly redshifting, short-lived high energy emission band. The high energy emission band is proposed to be due to the recombination of weakly localised/delocalised carriers occurring as a consequence of the progressive saturation of the local potential fluctuations responsible for carrier localisation, at high excitation power densities. I report on the effects of varying threading dislocation (TD) density on the optical properties of InGaN/GaN multiple QW structures. No systematic relationship exists between the room temperature internal quantum efficiency (IQE) and the TD density, in a series of nominally identical InGaN/GaN multiple QWs deposited on GaN templates of varying TD density. I also show the excitation power density dependence of the PL efficiency, at room temperatures, is unaffected for variation in the TD density between 2 x107 and 5 x109 cm-2. The independence of the optical properties to TD density is proposed to be a consequence of the strong carrier localisation, and hence short carrier diffusion lengths. I report on the effects of including an InGaN underlayer on the optical and microstructural properties of InGaN/GaN multiple QW structures. I show an increase in the room temperature IQE occurs for the structure containing the InGaN underlayer, compared to the reference. I show using PL excitation spectroscopy that an additional carrier transfer and recombination process occurs on the high energy side of the PL spectrum associated with the InGaN underlayer. Using PL decay time measurements I show the additional recombination process for carriers excited in the underlayer occurs on a faster timescale than the recombination at the peak of the PL spectrum. The additional contribution to the spectrum from the faster recombination process is proposed as responsible for the increase in room temperature IQE.
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Christian, George. "Photoluminescence studies of InGaN/GaN quantum well structures." Thesis, University of Manchester, 2018. https://www.research.manchester.ac.uk/portal/en/theses/photoluminescence-studies-of-ingangan-quantum-well-structures(aa935835-26f3-4b12-8f83-21190ffa7cb9).html.
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In this thesis, optical studies of c-plane InGaN/GaN quantum well (QW) structures are presented. The effects of a Si-doped underlayer (UL) on the optical properties of multiple quantum well (MQW) structures are investigated. The QW photoluminescence (PL) emission peak energy and radiative recombination rate decrease and increase respectively with increasing number of QWs. These observations are attributed to the increasing net electric field across the MQW structure as the strength of the surface polarisation field, which acts in the opposite sense to the piezoelectric polarisation fields across the QWs, reduces with increasing distance of the UL from the sample surface. This leads to a reduction in the electron-hole recombination energy and wavefunction overlap. It is also shown that the internal quantum efficiency of the MQW structures may decrease with increasing number of QWs due to the reducing radiative recombination rate, which could indicate that carrier losses due to thermionic emission or interface recombination are mitigated by the inclusion of an UL. Optical studies of single QW structures containing Si-doped ULs with different net electric fields across the QW are presented. The net electric field across the QW is changed by varying the thickness of the GaN cap layer. The full width at half maximum of the emission peak increases with increasing net electric field across the QW. This is attributed to the increasing variation in electron ground state energies due to the role of the electric field in the localisation of electrons at quantum well width fluctuations. For one sample, a smaller Huang-Rhys factor compared to the rest of the samples is calculated. The non-exponential PL decays detected on the low energy side of the QW emission peak from this sample are also of a different shape to the other PL decays detected at all energies for the other samples. This may be due to the reversal of the net electric field across these QW regions. Observations of a broad emission band on the high energy side of single QW structures at high excited carrier densities are presented. This band occurs in the carrier density regime at which the efficiency droop is observed. The emission band is attributed to higher energy weakly localised or delocalised electron and hole states that are populated following the saturation of the localised ground states. PL decay curves detected across this emission band exhibit plateaus where the PL intensity remains constant until the higher energy emission has decayed. These are similar to decays observed in semiconductor quantum dots, which are characteristic of Pauli state blocking.
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Kirinda, Viraj C. "Well-Controlled Ortho-Phenylene-Based Higher-Order Structures." Miami University / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=miami1625137591219366.
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Moran, Martin. "Optical properties of low dimensional semiconductor structures." Thesis, Manchester Metropolitan University, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.336966.
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Greatorex, Barry. "The structure of well-being in dementia." Thesis, University of Leicester, 2002. http://hdl.handle.net/2381/31328.
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The nature and structure of the concept of well-being, both in general psychological research and in relation to dementia, is reviewed. Well-being is important in understanding subjective emotional experience and life satisfaction, and central to the concept of 'quality of life'. The research between the structure of well-being in dementia and in wider research is explored, bringing together work from the United States and Tom Kitwood's theory of dementia care. The principal hypothesis investigated relates to this body of work: specifically whether 'well-being' in people-with-dementia can be understood as one- or more than one factor. Subsidiary hypotheses concerning well-being in dementia and other variables are also investigated. Seven point rating scales were used to measure indicators of well-being used in 'Dementia Care Mapping'. A pilot study was used to develop a rater's manual, to test acceptability, and to obtain feed-back for improvements in method. A reliability study measured inter-rater reliability on fifteen indicators of well-being. Further data was gathered to produce data on ratings of indicators of well-being in 102 persons with dementia. Additional data on demographic characteristics and cognitive functioning were collected. Findings indicate that indicators of well-being in people-with-dementia can be reliably measured using observation. The research findings support a two-factor model of well-being in dementia, comprising a factor of 'Social engagement/Pleasure', and a factor of 'Negative affect vs. Relaxed calm'. These factors are uncorrelated. Investigation of subsidiary hypotheses reveals evidence of a negative correlation between cognitive impairment and the 'Social engagement/Pleasure' factor, but no correlation between cognitive impairment and 'Negative Affect'. No gender differences in factor scores are found for well-being or ill-being, although gender differences for indicators of ill-being not included in the factor analysis are found. The theoretical, service, policy and clinical implications for the above findings are discussed. Suggestions for future research are made.
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Qiu, Bocang. "Photonic integration in InGaAs/InGaAsP multiple-quantum well laser structures using quantum well intermixing." Thesis, University of Glasgow, 1998. http://theses.gla.ac.uk/6887/.
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The damage introduced into an InGaAs/InGaAsP quantum well structure during CH4/H2 reactive ion etching (RIE) processes was measured, for plasma powers from 20 W to 100 W, using low temperature photoluminescence. The damage depth profile is estimated to be around 12 nm - 70 nm after annealing at 500 °C for 60 seconds using a rapid thermal annealer (RTA). A reduced damage RIE process has been developed to fabricate InGaAs/InGaAsP multi-quantum well ridge waveguide lasers. The performance of these lasers has been compared to that of lasers fabricated from the same epilayer using wet etching to form the ridge. The resultant threshold currents were essentially indistinguishable, being 44.5 mA and 43 mA respectively for dry and wet etched lasers with 500 μm long laser cavities. Quantum well intermixing in the InGaAs/InGaAsP material system was demonstrated using two techniques. The first was a laser irradiation process, which combines irradiation by continuous wave and Q-switched pulsed Nd: YAG lasers. Differential shifts up to 70 nm have been obtained. The second was a plasma process which involves sputtering a thin layer of Si02 and subsequent high temperature annealing using either a CW laser or rapid thermal annealer (RTA). Differential blue-shifts of the bandgap of up to 120 nm were obtained. The bandgap shift in the control regions is very insignificant. Measurement of the spatial selectivity of this technique shows that the spatial resolution is better than 50 μm. The design, fabrication and characterisation for 3-dB MMI couplers were carried out using both as-grown (peak emission wavelength of 1.48 μm) and bandgap widened material. The measured results show good agreement with the design. A splitting ratio of around 0.12 dB (51: 49) has been achieved for an MMI section length of 470 gm. Low loss waveguides have been fabricated using the laser process. A loss as low as 2.1 dB/cm was obtained for an operation wavelength of 1.556 um. Extended cavity ridge lasers (ECL) in InGaAs/InGaAsP multiple-quantum well structures have been successfully fabricated using the two QWI technique developed. The increases in threshold current were only 10 mA and 8 mA for cavity length of 800 4m active section and 1000 μm passive section, compared to the all active lasers with cavity length of 800 μm, and the losses in the passive sections of ECLs were calculated which were 2.4 cm-1 and 4.4 cm-1, for the two processes, respectively. Considerable theoretical work was carried out, which included the calculation of the optical confinement and gain in the InGaAs/InGaAsP MQW structure used throughout this thesis. Modelling of the intermixing of quantum wells was also performed and the results indicate that the changes of bulk bandgap energy are mainly responsible for the blue-shift of the photoluminescence.
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Wee, Siew Fong. "Interdiffusion of semiconductor alloy heterostructures." Thesis, University of Surrey, 1998. http://epubs.surrey.ac.uk/844156/.
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This thesis is concerned with a quantitative study of intermixing in GaAs/AlGaAs and ZnSe/ZnCdSe single quantum well semiconductor structures. In this study, a method of iterative isothermal anneals and photoluminescence used to characterize this phenomenon has enabled the evolution of the diffusion coefficients for the interdiffusion process with anneal time to be followed. The blue-shift emissions arising from this method are predicted by a model based on Fick's law of diffusion. This model is developed in an attempt to relate the energy shift that is observed experimentally to the diffusion length. The mixing is modelled using an error function expression to solve the diffusion equation so as to describe the variation in well shape which is attributed to compositional disordering induced during thermal processing. Using this approach, where evidence of intermixing was monitored, the emission would be expected to shift measurably. Data has been taken to cover a wide temperature range to establish values for the activation energy EA. From this data, it has been found that the diffusion coefficients at various temperatures are thermally activated with an energy of 3.6 +/- 0.2 eV in GaAs/AlGaAs. The data is compared with the available literature data taken under a wide range of experimental conditions. We show that despite the range of activation energies quoted in the literature all the data appears to be consistent with a single activation energy. Departures from the 'mean' value are ascribed to experimental uncertainties in determining the diffusion coefficients for example, to fluctuations in the composition of the material, to techniques used, or to a wide range of perturbations. Photoluminescence observations on ZnSe/ZnCdSe show that an improvement in the optical quality of these quantum well structures was found for anneals at temperatures (~500°C). A value of EA = 2.9 +/- 0.3 eV was derived from the experiments for the interdiffusion process over a 250 K temperature range and four decades of interdiffusion coefficient. The interdiffusion process of both these systems was inferred to be Fickian with no dependence on alloy composition or strain.
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Daumer, Volker. "Phase coherent transport phenomena in HgTe quantum well structures." Doctoral thesis, [S.l.] : [s.n.], 2005. http://deposit.ddb.de/cgi-bin/dokserv?idn=979460352.
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Chen, Peili. "Magneto-optical studies of CdTe/CdMnTe quantum well structures." Thesis, University of Hull, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.363190.
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Chabrol, Gregoire Robert. "Optical investigations of group III-nitride quantum well structures." Thesis, University of Manchester, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.492711.
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Hylton, Nicholas. "Optical spectroscopy of group III-nitride quantum well structures." Thesis, University of Manchester, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.515223.
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Presented in this thesis is a detailed investigation of the optical properties of sets of InGaN/GaN and InGaN/AIGaN quantum well structures, supported by the results of microstructural experiments from the University of Cambridge. The effect of the depth of a single InGaN/GaN quantum well below the sample surface on its optical properties was investigated. Oscillations in the photoluminescence excitation spectra of the samples were observed at temperatures below 50 K and at photon energies above the GaN band gap. These features were attributed to the excitation of electrons at multiples of the LO-phonon energy above the GaN conduction band minimum. The rapid relaxation of the electrons and their capture into the quantum well led to a shift in the photoluminescence peak energy, and therefore the oscillations in the excitation spectra. The shifts in the photoluminescence spectra were attributed to a modification to the carrier distribution amongst localised states in the quantum well. A comparison of the optical properties of a set of semi-polar InGaN/GaN quantum well structures with those of nominally identical polar control samples was made. Evidence for a reduction in the electric field strength across the quantum wells was observed in the form of blue-shifts of the photoluminescence peak energy, increased rates of radiative recombination and sharpening of the quantum well excitation spectra, with respect to the controls. However, the reduction in the electric field strength across the wells does not result in an improvement in the room temperature internal quantum efficiency; an observation which was attributed to an increased density of non-radiative recombination centres due to an order of magnitude increase in the threading dislocation density in the semi-polar samples. The optical properties of a set of InGaN/AIGaN multiple quantum well samples with no capping layer, a GaN capping layer and a magnesium doped capping layer were also compared. It was found that the intensity of the quantum well luminescence in the capped samples was reduced at T = 6 K compared to that of the uncapped quantum wells, due to carriers generated in the cap layer not being captured into the wells. At room temperature the difference in quantum well luminescence intensity between the capped and uncapped samples increased due to the lower carrier density in the wells in the capped samples. In the case of the uncapped sample the higher carrier density resulted in the saturation of the impurity luminescence bands meaning that more carriers could contribute to the quantum well luminescence. However, in the capped samples the impurity luminescence was not saturated and so it is possible that carriers could be thermalised out of the wells and be captured at impurity sites, hence not contributing to the quantum well luminescence. Finally, the optical properties of InGaN/GaN multiple quantum well structures grown on 2 and 6 inch silicon substrates were studied. It was discovered that the reflectivity of silicon varies at low temperatures, which inevitably affects the outcome of measurements of internal quantum efficiency. After taking into account the effects of the changes to the reflectivity of the substrate, the room temperature photoluminescence internal quantum efficiencies of the samples were estimated to be 33 %,57 % (2 inch substrates) and 38 % (6 inch substrate).
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Mazzucato, Simone. "GaInNAs/GaAs quantum well structures for uncooled 1.3mum lasers." Thesis, University of Essex, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.275858.
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Blenkhorn, William Eric. "Optical studies of polar InGaN/GaN quantum well structures." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/optical-studies-of-polar-ingangan-quantum-well-structures(7f610cae-ba98-44d8-ae54-dbc4c44725d4).html.
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In this thesis, I will present and discuss research performed on InGaN/GaN multiple quantum well (QW) structures. The results of which were taken using photoluminescence (PL) spectroscopy and PL time decay spectroscopy. In the first two experimental chapters, I report on the effects of QW growth methodology on the optical properties of c-plane InGaN/GaN QWs. I compare structures grown using the single temperature (1T), quasi-two temperature (Q2T), temperature bounced (T-bounced) and two temperature (2T) QW growth methodologies. The T-bounced and 2T structures are observed to have gross well width fluctuations (GWWF), where the QW width varies from 0 to 100 % created when the QWs are exposed to a temperature ramp. Whereas, the 1T and Q2T structures have continuous QWs with only one or two monolayer well width fluctuations. The structures with GWWFs are observed to have a larger room temperature internal quantum efficiency (RT-IQE) at low excitation conditions i.e. below efficiency droop compared to those without. The larger RT-IQE is ascribed to several factors which include an increased radiative recombination rate, increased thermal activation energy of non-radiative recombination and reduced defect density of the QWs. The effect of barrier growth temperature is also investigated. No clear trend is observed between barrier growth temperature and RT-IQE.In the last experimental chapter I report on studies of carrier localisation in InGaN/GaN QWs using resonant PL spectroscopy. The effect of carrier localisation on the independently localised electrons and holes are investigated and the resonant PL spectrum is studied in detail. The InGaN/GaN QW structure is observed to exhibit an effective mobility edge at 12 K where delocalised carriers are created above a particular excitation energy. The emission from the resonantly excited localised states which are accompanied by the emission of a longitudinal optical phonon (resonant LO feature) is investigated as a function of temperature and excitation energy. The integrated PL intensity of the resonant LO feature is observed to quench rapidly with temperature up to around 45 K, independent of excitation energy. The integrated PL intensity of the resonant LO feature is fitted to an Arrhenius model and a thermal activation energy of ∼ 1(±1) meV is extracted. This activation energy is speculated to be consistent with the localisation energy of electrons.
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Rezaee, Amirabbas, and amirabbas rezaee@rmit edu au. "Phase-Periodic Quantum Structures and Perturbed Potential Wells." RMIT University. Electrical and Computer Engineering, 2009. http://adt.lib.rmit.edu.au/adt/public/adt-VIT20091218.160522.
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The restrictions of micro-scale systems are approaching rapidly. In anticipation of this development, nano-scale electronics has become the focus of many researchers and engineers in academia and industry since early 1990s. The basic building blocks of modern integrated circuits have been diodes and transistors with their current-voltage I-V characteristics being of prime significance for the design of complex signal processing and shaping devices and systems. Classical and semi-classical physical principles are no longer powerful enough or even valid to describe the phenomena involved. The application of rich and powerful concepts in quantum theory has become indispensable. These facts have been influential in undertaking this research project. This research is built upon the determination of the Eigenpairs of one and two dimensional positive differential operators with periodic boundary conditions. The Schrödinger equation was solved for positive operators in both one and two dimensions. Fourier series were used to express the derivatives as the summation of Fourier terms. This led to a novel approach for the calculation of the eigenmodels of a perturbed potential well. The perturbation can be done via an electric field applied to the potential well. The research in this thesis includes a thorough understanding of quantum mechanics fundamentals, mastering of different approximation techniques such as the variational technique and results that have been generated and published using the novel techniques.
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Aganoglu, Ruzin. "Non-linear Optical Properties Of Two Dimensional Quantum Well Structures." Master's thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/3/12607089/index.pdf.
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In this work optical properties of two dimensional quantum well structures are studied. Variational calculation of the eigenstates in an isolated quantum well structure with and without the external electrical field is presented. At weak fields a quadratic Stark shift is found whose magnitude depends strongly on the finite well depth. It is observed that under external electrical field, the asymmetries due to lack of inversion symmetry leads to higher order nonlinear optical effects such as second order optical polarization and second order optical susceptibility.
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Collins, Stephen. "Theoretical studies of wavepacket propagation in semiconductor quantum well structures." Thesis, University of Warwick, 1986. http://wrap.warwick.ac.uk/73492/.
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In this thesis a heuristic expression for the current through a GaAs/GaAlAs heterostructure is derived. This expression is shown to give rise to agreement between experiment and theory. The expression itself is derived within the effective mass formalism, which is discussed to show that its use will not generate large errors. This conclusion is contrary to previous work which will be shown to be in error due to misunderstandings concerning effective mass theory. To justify the approach used to obtain the tunnel current expression the behaviour of a wavepacket incident upon a square potential barrier is studied. The study shows that the wavepacket traverses the potential sufficiently rapidly to allow scattering to be neglected, and that the total transmission probability can be calculated from the solution of the time independent Schrodinger equation. The current expression is reduced to a one dimensional integral by assuming parabolic conduction bands, position independent mass and a thermalised electron distribution. The resulting expression is different from the usual Tsu-Esaki formula, a difference which can be seen to arise because the Tsu-Esaki formula does not account for the different velocities on each side of the barrier. The final stage, before any comparison is made to experimental results, is to show that the numerical technique of Vigneron and Lambin is more accurate than the WKB technique. A comparison of experimental results and the results of the numerical integration of the current density expression shows that they can only be reconciled if a resistance or diode is assumed in series with the tunnel barrier. This fitting parameter is then shown to be sufficient for good fits to be obtained between experiment and theory for the first time.
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Becker, Christian Eberhard. "Transport properties of modulation doped Si/SiGe quantum well structures." Thesis, University College London (University of London), 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.404404.
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Ng, Inn-Khuan. "TEM characterisation of III-V semiconductor multiple quantum well structures." Thesis, University of Cambridge, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.245224.
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Xie, Feng. "Resonant optical nonlinearities in cascade and coupled quantum well structures." [College Station, Tex. : Texas A&M University, 2008. http://hdl.handle.net/1969.1/ETD-TAMU-3101.
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Allam, Jeremy. "Impact ionization and electrical transport in multiple quantum well structures." Thesis, University of Surrey, 1988. http://epubs.surrey.ac.uk/843801/.
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This thesis describes experimental studies of perpendicular transport in multi-layer III-V semiconductor structures. Impact ionization, charge trapping at heterointerfaces and resonant tunnelling in multiple quantum well photodiodes were studied. Long-wavelength high-speed multiple quantum well avalanche photodiodes were studied. Ga0.47In0.53As/InP structures grown by trichloride, and metal-organic, vapour phase epitaxy exhibited avalanche multiplication at room temperature. The pulse response at high fields was not limited by carrier pile-up at the heterointerfaces. High quality Al0.48In0.52As/Ga0.47In0.53As photodiodes, grown by molecular beam epitaxy, exhibited multiplication of ≈ 70, microwave gain of ≈12 and an intrinsic response time of ≈100 ps at room temperature and high electric fields, indicating no carrier pile-up despite the very abrupt heterointerfaces. In multiple quantum well photodiodes which exhibited large reverse-bias leakage currents, a new carrier multiplication effect was observed. This was attributed to impact ionization across the heterojunction discontinuity of carriers "stored" in the quantum wells. Experimental evidence for this effect is presented for several material systems. Single carrier-type multiplication was demonstrated in a multilayer photodiode with compositionally-graded interfaces. Sequential resonant tunnelling and resonant Zener tunnelling were studied in high quality Al0.48In0.52As/Ga0.47In0.53As structures with low background doping. Carrier transport in undepleted quantum wells was studied in Ga0.47In0.53As/InF structures grown by gas source molecular beam epitaxy.
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Eccleston, Richard. "Time-resolved photoluminescence studies of GaAs-AlGaAs quantum well structures." Thesis, Imperial College London, 1989. http://hdl.handle.net/10044/1/47419.
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Xu, Bin 1968. "Development of intersubband terahertz lasers using multiple quantum well structures." Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/49673.
Full textAbstract:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1998.Includes bibliographical references (p. 221-241).
This thesis describes an experimental and theoretical effort in developing intersubband THz lasers using multiple-quantum-well structures. Scarcity of compact solid state sources in this frequency range, and to demonstrate a novel unipolar laser technology, motivated this research. Transport studies for realizing THz intersubband population inversion, new methods for long-wavelength mode confinement, and farinfrared spectral measurement techniques are critical steps in achieving this goal. Conduction-band three-level subband systems in triple-quantum-well structures using GaAs/Alo.3Gao.7As heterostructures were proposed, designed, and simulated by a numerical method. The numerical simulation is a self-consistent solution among the Schrödinger equation, Poisson equation, and rate equations. Electrons are injected by resonant tunneling to populate the upper subband; the lower subband is depopulated by fast longitudinal optical (LO) phonon scattering. THz emission devices consist of many modules of such triple-quantum-structures with the three-level systems cascade connected to each other. Dynamic charge of electron is provided by the 6-doping per module. Temperature-dependent intersubband scattering plays a key role in transport modeling and therefore the degree of population inversion. Systematic calculations were performed to address issues of hot electron effect, lattice heating, and non-equilibrium optical phonons. Guidelines for device design and optimization were provided. The measured dc I-V at cryogenic temperature confirmed the design expectations. Plasma confinement is used for making THz laser cavities. The minimum cavity loss can only be achieved by using metallic waveguides. The first metallic waveguide, which incorporates non-alloyed ohmic contact, was successfully fabricated by combining wafer bonding and selective etching techniques. Schemes for THz emission couplings were investigated by quantifying coupling loss, including surface coupling by gratings and edging coupling by facets. The first free-space THz spectral measurement system was developed using a Fourier Transform Infrared (FTIR) spectrometer. This experimental set-up was successfully demonstrated in resolving THz emission by using step-scan and lock-in techniques, and a fast Ge:Ga photon detector. Spontaneous intersubband THz emission was observed with linewidth narrower than 0.65 THz, and center frequency at the designed value of 3.8 THz. Different triple-quantum-well structures were designed, grown, and tested. The measured emission power levels were one order of magnitude lower than calculated values, and possible extra cavity loss mechanisms were discussed. To verify the triple-quantum-well structure design, a mid-infrared absorption measurement was performed on a sample grown on semi-insulating substrate. Information such as subband energy separations, dipole moments, and linewidth broadening, was extracted from the absorption spectrum and gave a good confirmation on numerical simulations and MBE growth quality of the MQW structures.
by Bin Xu.
Ph.D.
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Weiss, Bernard Lawson. "Modelling and characterisation III-V semiconductor quantum well structures and Si based structures for optoelectronic applications." Thesis, University of Surrey, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.267875.
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