Dans ce travail de thèse, nous nous intéressons à l'étude mathématique du transport dans les systèmes de Hall quantiques en milieu désordonné. Plus précisément nous commençons par étudier la théorie de la réponse linéaire dans le cas continu pour un opérateur de Schrödinger magnétique aléatoire. Nous exploitons le formalisme de l'intégration non commutative pour développer une théorie de la réponse linéaire adaptée au problème et obtenir une formule de Kubo-Středa. Dans un deuxième temps nous nous intéressons à la quantification des courants de bord créés par un mur magnétique modélisé par un Hamiltonien d'Iwatsuka. Nous démontrons la stabilité de cette quantification sous certaines perturbations magnétiques. Enfin nous achevons ce travail de thèse par une discussion plus approfondie sur le formalisme développé dans la première partie, de manière à permettre une généralisation future de la théorie de la réponse linéaire.

Quantum theory is a probabilistic theory, where certain variables are hidden or non-accessible. It results in lack of representation of systems under study. However, I deduce system's representation in probabilistic manner, introducing probability of existence w, and quantize it exploiting Schrödinger's quantization rule. The formalism enriches probabilistic quantum theory, and enables system's representation in probabilistic manner.

<p>Abstract: Considering the Schrödinger equation $\Delta_x u = i\partial{u}/\partial{t}$, we have a solution $u$ on the form $$u(x, t)= (2\pi)^{-n} \int_{\RR} {e^{i x\cdot \xi}e^{it|\xi|^2}\widehat{f}(\xi)}\, d \xi, x \in \RR, t \in \mathbf{R}$$ where $f$ belongs to the Sobolev space. It was shown by Sjögren and Sjölin, that assuming $\gamma : \mathbf{R}_+ \rightarrow \mathbf{R}_+ $ being a strictly increasing function, with $\gamma(0) = 0$ and $u$ and $f$ as above, there exists an $f \in H^{n/2} (\RR)$ such that $u$ is continuous in $\{ (x, t); t>0 \}$ and $$\limsup_{(y,t)\rightarrow (x,0),|y-x|<\gamma (t), t>0} |u(y,t)|= + \infty$$ for all $x \in \RR$. This theorem was proved by choosing $$\widehat{f}(\xi )=\widehat{f_a}(\xi )= | \xi | ^{-n} (\log | \xi |)^{-3/4} \sum_{j=1}^{\infty} \chi _j(\xi)e^{- i( x_{n_j} \cdot \xi + t_j | \xi | ^a)}, \, a=2,$$ where $\chi_j$ is the characteristic function of shells $S_j$ with the inner radius rapidly increasing with respect to $j$. The purpose of this essay is to explain the proof given by Sjögren and Sjölin, by first showing that the theorem is true for $\gamma (t)=t$, and to investigate the result when we use $$S^a f_a (x, t)= (2 \pi)^{-n}\int_{\RR} {e^{i x\cdot \xi}e^{it |\xi|^a}\widehat{f_a}(\xi)}\, d \xi$$ instead of $u$.</p>

On étudie les propriétés dynamiques et spectrales de deux types d'opérateurs de Schrödinger à valeurs matricielles. Le premier est un modèle d'Anderson, le second un modèle d'interactions ponctuelles. On prouve l'absence de spectre absolument continu pour ces deux opérateurs en prouvant la séparabilité de leurs exposants de Lyapounov, puis on étudie la régularité des exposants de Lyapounov et de la Densité d'Etats Intégrée associées à ces opérateurs. On prouve que ces deux quantités sont Hölder continues.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of Master of Science. Johannesburg, 2012. / Unable to load abstract.

The Schrödinger equations have had a profound impact on a wide range of fields of modern science, including quantum mechanics, superfluidity, geometrical optics, Bose-Einstein condensates, and the analysis of dispersive phenomena in the theory of PDE. The main purpose of this thesis is to explore two Schrödinger-type equations appearing in the so-called Bohmian formulation of quantum mechanics and in the study of exciton-polariton condensates. For the first topic, the linear Schrödinger equation is the starting point in the formulation of a phase-space model proposed in [1] for the Bohmian interpretation of quantum mechanics. We analyze this model, a nonlinear Vlasov-type equation, as a Hamiltonian system defined on an appropriate Poisson manifold built on Wasserstein spaces, the aim being to establish its existence theory. For this purpose, we employ results from the theory of PDE, optimal transportation, differential geometry and algebraic topology. The second topic of the thesis is the study of a nonlinear Schrödinger equation, called the complex Gross-Pitaevskii equation, appearing in the context of Bose-Einstein condensation of exciton-polaritons. This model can be roughly described as a driven-damped Gross-Pitaevskii equation which shares some similarities with the complex Ginzburg-Landau equation. The difficulties in the analysis of this equation stem from the fact that, unlike the complex Ginzburg-Landau equation, the complex Gross-Pitaevskii equation does not include a viscous dissipation term. Our approach to this equation will be in the framework of numerical computations, using two main tools: collocation methods and numerical continuation for the stationary solutions and a time-splitting spectral method for the dynamics. After performing a linear stability analysis on the computed stationary solutions, we are led to postulate the existence of radially symmetric stationary ground state solutions only for certain values of the parameters in the equation; these parameters represent the “strength” of the driving and damping terms. Moreover, numerical continuation allows us to show, for fixed parameters, the ground and some of the excited state solutions of this equation. Finally, for the values of the parameters that do not produce a stable radially symmetric solution, our dynamical computations show the emergence of rotating vortex lattices.

Nous nous intéressons dans ce travail à la dynamique des opérateurs de Schrödinger unidimensionnels, discrets, associés à un potentiel sturmien quasi-périodique. Le résultat principal de cette thèse est une borne supérieure pour les exposants de transport qui mesurent la vitesse de propagation du système. Cette borne, valide pour presque tous les potentiels sturmiens, est sous balistique pour une force de couplage suffisante. La validité de la borne est couplée à une condition diophantienne liée au nombre irrationnel qui définit le potentiel. Cette condition est vraie presque sûrement. Nous exhibons par ailleurs un exemple d’irrationnel pour lequel une borne supérieure sous balistique est impossible indépendamment de la force de couplage. Nous faisons l’étude de la dimension fractale du spectre de l’opérateur qui minore sous certaines conditions les exposants de transport. Nous obtenons une nouvelle borne inférieure pour la dimension de boîte du spectre grâce aux propriétés connues sur la forme du pseudo spectre. Les restrictions pour obtenir une borne dynamique à partir de notre résultat sont d’avoir une condition initiale cyclique standard et que le potentiel soit associé à un irrationnel à densité bornée. Enfin dans la dernière partie de ce travail, nous démontrons que le spectre de l’opérateur associé au nombre d’argent ß = [2, 2, . . . ] possède une structure hyperbolique. L’expression du pseudo spectre peut être vu comme un système dynamique. Nous conjuguons ledit système à une dynamique symbolique abstraite selon la méthode dite des partitions de Markov. Le système se comporte comme un fer à cheval de Smale. Nous dérivons de l’hyperbolicité des propriétés pour les dimensions fractales du spectre. Dimensions dont l’attrait dynamique a été rappelé dans la partie précédente. Nous déduisons notamment l’égalité des dimensions de Hausdorff et de boîte pour cet opérateur. / In this thesis, we study the dynamics of discrete, one-dimensional, sturmian Schrödinger operators. The main result is a dynamical bound from above for transport exponents that valuate speed of the wavepacket spreading. This bound is true for almost every sturmian potential and is sub-ballistic for a coupling constant big enough. This bound is valid with respect to a diophantine condition on the irrational number that define the potential. This condition is true for almost every irrational numbers. We show an example of irrational number with ballistic motion at any coupling constant. We study the fractal dimension of the spectrum of these operators which can bound from below, under more restrictive assumptions, transport exponents.We get a new bound from below for the box dimension of the spectrum. Assumptions needed to use this bound on dynamical purpose are the initial condition to be cyclic and the potential associated to a bounded means irrational number. In the last part of the thesis, we show that the spectrum of the operator associated to the so-called silver mean ß = [2, 2, . . . ] has a hyperbolic structure. The spectrum can be express as the non wandering set of a dynamical system. Using Markov partition method, we conjugate its dynamics to a symbolic one. The dynamical system behave like a Smale horseshoe. We derive from hyperbolicity spectral information, especially on fractal dimension. For example, we get that Hausdorff and box dimensions coincide for this operator.

Nous présentons des travaux autour de trois axes : 1- Phénomène de focalisation en un point en optique géométrique non linéaire. Les équations considérées sont principalement des équations d'ondes et de Schrödinger non linéaires. 2- Rôle des oscillations quadratiques dans les équations de Schrödinger non linéaires. 3- Equations de Schrödinger non linéaire en présence d'un potentiel extérieur.

In this thesis, which consists of four papers, we study the discrete spectrum of Schrödinger operators in waveguides. In these domains the quadratic form of the Dirichlet Laplacian operator does not satisfy any Hardy inequality. If we include an attractive electric potential in the model or curve the domain, then bound states will always occur with energy below the bottom of the essential spectrum. We prove that a magnetic field stabilises the threshold of the essential spectrum against small perturbations. We deduce this fact from a magnetic Hardy inequality, which has many interesting applications in itself. In Paper I we prove the magnetic Hardy inequality in a two-dimensional waveguide. As an application, we establish that when a magnetic field is present, a small local deformation or a small local bending of the waveguide will not create bound states below the essential spectrum. In Paper II we study the Dirichlet Laplacian operator in a three-dimensional waveguide, whose cross-section is not rotationally invariant. We prove that if the waveguide is locally twisted, then the lower edge of the spectrum becomes stable. We deduce this from a Hardy inequality. In Paper III we consider the magnetic Schrödinger operator in a three-dimensional waveguide with circular cross-section. If we include an attractive potential, eigenvalues may occur below the bottom of the essential spectrum. We prove a magnetic Lieb-Thirring inequality for these eigenvalues. In the same paper we give a lower bound on the ground state of the magnetic Schrödinger operator in a disc. This lower bound is used to prove a Hardy inequality for the magnetic Schrödinger operator in the original waveguide setting. In Paper IV we again study the two-dimensional waveguide. It is known that if the boundary condition is changed locally from Dirichlet to magnetic Neumann, then without a magnetic field bound states will occur with energies below the essential spectrum. We however prove that in the presence of a magnetic field, there is a critical minimal length of the magnetic Neumann boundary condition above which the system exhibits bound states below the threshold of the essential spectrum. We also give explicit bounds on the critical length from above and below. / QC 20101007

The theme of this thesis is combinatorics, complex analysis and algebraic geometry. The thesis consists of six articles divided into four parts. Part A: Spectral properties of the Schrödinger equation This part consists of Papers I-II, where we study a univariate Schrödinger equation with a complex polynomial potential. We prove that the set of polynomial potentials that admit solutions to the Schrödingerequation is connected, under certain boundary conditions. We also study a similar result for even polynomial potentials, where a similar result is obtained. Part B: Graph monomials and sums of squares In this part, consisting of Paper III, we study natural bases for the space of homogeneous, symmetric and translation-invariant polynomials in terms of multigraphs. We find all multigraphs with at most six edges that give rise to non-negative polynomials, and which of these that can be expressed as a sum of squares. Such polynomials appear naturally in connection to expressing certain non-negative polynomials as sums of squares. Part C: Eigenvalue asymptotics of banded Toeplitz matrices This part consists of Papers IV-V. We give a new and generalized proof of a theorem by P. Schmidt and F. Spitzer concerning asymptotics of eigenvalues of Toeplitz matrices. We also generalize the notion of eigenvalues to rectangular matrices, and partially prove the a multivariate analogue of the above. Part D: Stretched Schur polynomials This part consists of Paper VI, where we give a combinatorial proof that certain sequences of skew Schur polynomials satisfy linear recurrences with polynomial coefficients. / <p>At the time of doctoral defence the following papers were unpublished and had a status as follows: Paper 5: Manuscript; Paper 6: Manuscript</p>