La supraconductivité est un ordre électronique fascinant dans lequel les électrons s'apparient par le biais d'une interaction attractive et condensent dans un état quantique macroscopique pouvant porter un courant non dissipatif, i. E. Un supercourant. Dans les structures hybrides où des supraconducteurs (S) sont mis en contact avec des matériaux non supraconducteurs (X), les paires se propageant de S " contaminent " X lui conférant des propriétés supraconductrices à proximité de l'interface, parmi lesquels la possibilité de porter un supercourant. La transmission d'un supercourant à travers n'importe quelle structure S-X-S s'explique par l'interférence constructive de paires d'électrons traversant X. En effet, à la manière d'un résonateur Fabry-Perot, une telle interférence a seulement lieu pour certains états électroniques résonants appelés Etats Liés d'Andreev (ELA). Récemment, il est devenu possible de fabriquer une variété de nanostructures dans lesquelles X peut être par exemple un nanofil, un nanotube de carbone ou même une molécule. Ces dispositifs ont en commun que leur X contient seulement quelques électrons de conduction ce qui implique que les ELA sont aussi en petit nombre. Dans ce cas, pour comprendre quantitativement l'effet de proximité dans ces systèmes, il est nécessaire de comprendre en détail la formation des ELA individuellement. Ceci peut être vu comme la question centrale du développement d'électronique supraconductrice à l'échelle nanométrique. Dans cette thèse, nous avons observé des ELA résolus individuellement par spectroscopie tunnel dans un nanotube de carbone.

This thesis discusses the elastic and inelastic scattering in monolayer graphene, investigated by means of microwave carrier dynamics and noise. We study in a first part the high frequency properties of graphene field‐effect transistors on different substrates. Particular interest lies in the figures of merit like e.g. the transit frequency fT, defining the transistor's current amplification capabilities, and the transconductance gm representing its gate sensitivity. High values are obtained for both parameters in GHz measurements. We find in particular that these figures remain substantial even in miniaturized devices. We introduce top‐gated graphene field‐effect capacitors as a probe of the elastic scattering mechanisms in graphene. Employing similar techniques as in the transistor experiments, we are able to directly access the diffusion constant D and its dependence on carrier density. The latter is the signature of the scattering mechanism present in the graphene sheet. Our novel GHz experiments reveal a constant transport scattering time as a function of energy which is in disagreement with conventional theoretical predictions, but supports the random Dirac mass disorder mechanism. Furthermore, we study inelastic scattering of charge carriers by acoustic phonons in graphene which is among the first realizations of such an experiment in a genuine two‐dimensional geometry. A broadband cryogenic noise thermometry setup is used to detect the electronic fluctuations, the current noise, from which we extract the average electron temperature Te as a function of Joule power P. At high bias we find P∝ΣTe^4 as predicted by theory and which is the tell‐tale sign of a 2D phonon cooling mechanism. From a heat equation analysis of data in a broad bias range, we extract accurate values of the electron‐acoustic phonon coupling constant Σ. Our measurements point to an important effect of lattice disorder in the electron‐phonon energy relaxation.

Dans cette thèse, on s'intéresse au transport quantique des électrons dans des nano-systèmes et des cavités chaotiques . En particulier, on apporte dans un premier temps la base théorique qui permet d'expliquer les expériences de microscopie à effet de grille dans des contacts quantiques ponctuels.

This dissertation discusses the Landau-Zener (LZ) theory and its application in noisy environments and in many-body systems. The first project considers the effect of fast quantum noise on LZ transitions. There are two important time intervals separated by the characteristic LZ time. For each interval we derive and solve the evolution equation, and match the solutions at the boundaries to get a complete solution. Outside the LZ time interval, we derive the master equation, which differs from the classical equation by a quantum commutation term. Inside the LZ time interval, the mixed longitudinal-transverse noise correlation renormalizes the LZ gap and the system evolves according to the renormalized LZ gap. In the extreme quantum regime at zero temperature our theory gives a beautiful result which coincides with that of other authors. Our initial attempts to solve two experimental puzzles - an isotope effect and the quantized hysteresis curve of a single molecular magnet - are also discussed. The second project considers an ultracold dilute Fermi gas in a magnetic field sweeping across the broad Feshbach resonance. The broad resonance condition allows us to use the single mode approximation and to neglect the energy dispersion of the fermions. We then propose the Global Spin Model Hamiltonian, whose ground state we solve exactly, which yields the static limit properties of the BEC-BCS crossover. We also study the dynamics of the Global Spin Model by converting it to a LZ problem. The resulting molecular production from the initial fermions is described by a LZ-like formula with a strongly renormalized LZ gap that is independent of the initial fermion density. We predict that molecular production during a field-sweep strongly depends on the initial value of magnetic field. We predict that in the inverse process of molecular dissociation, immediately after the sweeping stops there appear Cooper pairs with parallel electronic spins and opposite momenta.

Ultrasonic wave transport in strongly scattering, disordered media is investigated via analysis of the multiply-scattered transmitted field. Measurements of transverse confinement, statistics, and correlations of the intensity were performed on an aluminum mesoglass, where aluminum beads were brazed together to form a porous slab sample. Comparison of the transverse confinement measurements with the self-consistent theory of localization was used to identify and locate a mobility edge in the sample at f = 1.1011 MHz, enabling a measurement of the critical exponent nu characterizing the Anderson transition, nu ~ 1.6–2. Infinite-range C0 correlations were observed, and observed to grow dramatically near the mobility edge, along with the C2 and C3 correlations. Measurements of the multifractal exponent Delta_2 were able to confirm the link between C0 correlations and Anderson localization. Experiments using the aluminum mesoglass with ethanol-filled pores showed evidence of two nearly-independent propagating modes, one of which appears to be characterized by a strongly renormalized diffusion coefficient. The density of states and level spacing statistics were investigated using a different mesoglass, constructed by sintering glass beads percolated on a random lattice. Direct measurements of these quantities were obtained by cutting small samples of this mesoglass, allowing individual vibrational modes to be resolved. The density of states showed a plateau extending well into the expected Debye regime, and evidence of a Boson peak was observed at low frequencies. The level spacing statistics indicated that transport in the frequency ranges measured was on the diffusive side of the mobility edge, showing agreement with the predictions of the GOE from random matrix theory. The dynamics of a suspension of bubbles were investigated using phase-based Diffusing Acoustic Wave Spectroscopy, where phase correlations were found to give additional information beyond traditional field- and intensity-based correlation measurements. / October 2015

This Thesis is concerned with the quasiclassical theory of meso-scopic superconductivity. The aim of the Thesis is to introduce the boundary conditions for a quasiclassical Green’s function on partially transparent interfaces in mesoscopic superconducting structures and to analyze the range of applicability of the quasiclassical theory. The linear boundary conditions for Andreev amplitudes, factoring the quasiclassical Green’s function, are presented.  The quasiclassical theory on classical trajectories is reviewed and then generalized to include knots with paths intersections.  The main focus of the Thesis is on the range of validity of the quasiclassical theory. This goal is achieved by comparison of quasiclassical and exact Green’s functions.  The exact Gor’kov Greens function cannot be directly used for the comparison because of its strong microscopic variations on the length-scale of λF. It is the coarse-grain averaged exact Green’s function which is appropriate for the comparison. In most of the typical cases the calculations show very good agreement between both theories. Only for certain special situations, where the classical trajectory contains loops, one encounters discrepancies. The numerical and analytical analysis of the role of the loop-like structures and their influence on discrepancies between both exact and quasiclassical approaches is one of the main results of the Thesis. It is shown that the terms missing in the quasiclassical theory can be attributed to the loops formed by the interfering paths.  In typical real samples any imperfection on the scale larger than the Fermi wavelength disconnects the loops and the path is transformed into the tree-like graph. It is concluded that the quasiclassical theory is fully applicable in most of real mesoscopic samples. In the situations where the conventional quasiclassical theory is inapplicable due to contribution of the interfering path, one can use the modification of the quasiclassical technique suggested in the Thesis.

L'objectif de cette thèse est d'étendre le champ d'application d'une méthode de simulation mésoscopique, appelée " Stochastic Rotation Dynamics " (SRD), au cas des électrolytes asymétriques en taille et en charge, tels que les suspensions de nanoparticules chargées. La modélisation de ces systèmes est difficile d'une part à cause des interactions à longue portée entre les solutés (interactions électrostatiques et hydrodynamiques), et d'autre part à cause de la différence entre les échelles de taille et de temps des espèces chargées. Nous avons adapté les algorithmes existants et développé de nouveaux algorithmes afin d'étudier les propriétés dynamiques des solutés tels que l'autodiffusion et la conductivité électrique, en gardant avec un bon compromis entre la précision et l'efficacité. Ce travail est financé par le projet ANR « Celadyct ». / The objective of this thesis is to extend the scope of the mesoscopic simulation technique called “Stochastic Rotation Dynamics” (SRD), for asymmetric electrolytes, such as suspensions of charged nanoparticles. The modeling of these systems is difficult, firstly because of long-range interactions between solutes (electrostatic and hydrodynamic interactions), and secondly due to the difference between the size and time scales of charged species. We have adapted the existing algorithms, and developed new ones in order to study the dynamic properties of solutes, such as self-diffusion and electrical conductivity, keeping up with a good compromise between accuracy and efficiency. This work is funded by the ANR project "Celadyct".

SOUSA, Duarte José Pereira de. Transporte eletrônico em anéis quânticos de grafeno. 2015. 83 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2015. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2016-01-05T19:55:02Z No. of bitstreams: 1 2015_dis_djpsousa.pdf: 9267324 bytes, checksum: 1dab83c7a9473498a415cb6cb9e5bf4b (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2016-01-05T19:55:40Z (GMT) No. of bitstreams: 1 2015_dis_djpsousa.pdf: 9267324 bytes, checksum: 1dab83c7a9473498a415cb6cb9e5bf4b (MD5) / Made available in DSpace on 2016-01-05T19:55:40Z (GMT). No. of bitstreams: 1 2015_dis_djpsousa.pdf: 9267324 bytes, checksum: 1dab83c7a9473498a415cb6cb9e5bf4b (MD5) Previous issue date: 2015 / In this work, we propose a current switch device that exploits the phase acquired by a charge carrier as it tunnels through a potential barrier in graphene in the ballistic regime without the need of the presence of a gap in the spectrum. The system acts as an interferometer based on an armchair graphene quantum ring, where the phase difference between interfering electronic wave functions for each path can be controlled by tuning the height of a potential barrier in the ring arms. By varying the parameters of the potential barriers the interference can become completely destructive. We demonstrate how this interference effect can be used for developing a simple graphene-based logic gate. / Neste trabalho, é proposto um dispositivo de controle de corrente que explora a fase adquirida por um portador de carga quando este tunela através de uma barreira de potencial no grafeno no regime balístico sem a necessidade da presença de um gap no espectro de energias. O sistema atua como um interferômetro baseado em um anel quântico de grafeno com bordas armchair, onde a diferença de fase entre as funções de onda para elétrons que tomam diferentes caminhos pode ser controlada através da intensidade das barreiras de potencial nos braços do anel. Variando os parâmetros das barreiras a interferência pode tornar-se completamente destrutiva. É demonstrado como esse efeito de interferência pode ser utilizado para o desenvolvimento de portas lógicas simples baseadas em grafeno.

Ce mémoire de thèse s'intéresse au comportement du béton sous chargement triaxial sévère. L'étude est réalisée dans le contexte plus général de la compréhension du comportement du béton sous impact, ce qui induit des états de contraintes triaxiales de très haute intensité. Afin de reproduire des niveaux de contraintes très élevés avec des chemins de chargement bien contrôlés, des essais triaxiaux statiques sont réalisés sur des échantillons de béton au moyen d'une presse triaxiale de très grande capacité. Nous nous sommes intéressés à l'influence des granulats, qui occupent environ 40% du volume du béton. Plus précisément, les effets de la forme et de la composition chimique des granulats sont étudiés pour des compression triaxiale entre 0 et 650 MPa de confinement . On considère à la fois la réponse macroscopique et les modes de rupture. La forme des granulats semble peu influencer le comportement du béton, alors que les effets de leur composition sont assez importants, quelque soit le confinement. La deuxième partie de la thèse est consacrée à une modélisation numérique du béton. L'objectif principal de ce travail numérique est de développer un modèle très simple en termes de lois d'interaction et d'introduire l'hétérogénéité du béton au niveau mésoscopique. Un modèle mésoscopique du béton est ainsi établi dans le cadre des éléments discrets. On étudie l'influence des propriétés des granulats et de l'interface graulats/mortier sur les courbes contrainte-déformation et les mécanismes d'endommagement. D'autre part, l'influence des vides est étudiée en utilisant des échantillons mono-phasiques avec différentes caractéristiques des vides. Un comportement complexe non linéaire, résultant de la présence, des vides est observé. Par ailleurs, les résultats numériques complètent, et permettent de mieux comprendre, les résultats expérimentaux. / This PhD thesis focuses on identifying concrete behavior under high triaxial loading. The study is carried out within a more general context of understanding the behavior of concrete under impact, which induces very high-intensity triaxial stress states. In order to reproduce very high stress levels with well-controlled loading paths, static triaxial tests are carried out on concrete samples by means of a very high-capacity triaxial press. We are interested in the influence of coarse aggregates, which occupy approximately 40% of the concrete volume. More specifically, the effects of coarse aggregate shape and composition on concrete behavior for triaxial compression tests ranging from 0 to 650 MPa of confinement are investigated. Both macroscopic response and failure patterns are concerned. Coarse aggregate shape seems to have only a slight influence on concrete behavior while the effects of coarse aggregate composition are quite important over whole range of confining pressures. The second part of the thesis is devoted to a numerical modeling of concrete. The main objective of this numerical work is to develop a very simple model in terms of interaction laws and introduce concrete heterogeneity at the mesoscopic level. A mesoscopic model of concrete is thus established within the discrete element framework. The effects of coarse aggregate and aggregate/mortar interface properties on the stress-strain curves and damage mechanisms in a concrete sample are investigated. On the other hand, the influence of voids is studied using one-phase samples having different void properties. A complex, nonlinear behavior of a numerical specimen arising from the void structuring is observed. The numerical results complete and allow better understanding of experimental observations.

Submitted by Vasti Diniz (vastijpa@hotmail.com) on 2017-09-19T12:43:14Z No. of bitstreams: 1 arquivototal.pdf: 1876360 bytes, checksum: 1b9ec239b2428bd64558f9f7bc7ec228 (MD5) / Made available in DSpace on 2017-09-19T12:43:14Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 1876360 bytes, checksum: 1b9ec239b2428bd64558f9f7bc7ec228 (MD5) Previous issue date: 2016-03-18 / The Landauer-Büttiker matrix formulation is one of the main tools used on the mesoscopic physics to describe systems whose behavior is the same like the nuclear ones, specially concerns to the ressonances presentes on the decays processes when they are bombarded. In this work, we study the random matrix theory to justify continuity conditions presents on the mesoscopic systems in charge transport phenomena. Next, we are take the systems boundary conditions in apropriate scale and we use the ideas of matricial scattering formulation to derive the Mahaux-Weindermüller formula and nd the structure of the scattering matrix to systems in the lenght scale under consideration. With possession of this scattering matrix, we have analyzed the quantum transport conditions of eletric charge for systems at null temperature. Next, we have expand our analysis investigating this systems at not null temperature, leading to emergence of unconventional e ects, as happens in the spins Seebeck e ect, for instance. Later, we use a random matrix numerical implementation to represents the Wigner's ensembles to take back speci cs results of the same papers that presents results related to this work. This was the way to test the method and demonstrate their e ectiveness. Finally, also through numerical implementation where we used an ensemble with 4998 random matrix, we have analyzed the channels quantum transport for spins, investigating their correlations to note that, starting of the generating functions, it is possible to nd the statistical cumulants that allow us analyze transport observables, both in the semi classical regime (where there are larger number of the open scattering channels), as in the extreme quantum regime (where the number of the open scattering channels is small). / A formulação matricial de Landauer-Büttiker é uma das principais ferramentas utilizadas na física mesoscópica para descrever sistemas que guardam consigo semelhanças com os sistemas nucleares, no que diz respeito às possíveis ressonâncias apresentadas nos decaimentos desses sistemas quando bombardeados. Neste trabalho, estudamos a teoria de matrizes aleatórias como forma de justi car as condições de continuidade e de contorno que os sistemas mesoscópicos apresentam no transporte de carga elétrica. A seguir, levamos as condições do sistema à escala adequada e aplicamos as ideias da formulação matricial do espalhamento para deduzir a fórmula de Mahaux-Weindermüller e encontrar a estrutura da matriz de espalhamento de sistemas que se encontram na escala de comprimento em quest ão. De posse da matriz de espalhamento, analisamos as condições do transporte quântico da carga elétrica para sistemas à temperatura nula. A seguir, podemos ampliar o alcance de nossa análise investigando os sistemas onde a temperatura deixa de ser nula, acarretando o surgimento de efeitos não convencionais de transporte, como o que acontece no efeito Seebeeck de spins, por exemplo. Na sequência, utilizamos uma implementação numérica de matrizes aleatórias que representam os ensembles de Wigner para recuperar resultados espec í cos de referências que apresentam resultados relacionados com este trabalho. Esta foi a forma de testar o método aqui utilizado e demonstrar a e cácia do mesmo. Por m, também por meio de implementações numéricas, onde foi utilizado um ensemble de 4998 matrizes aleatórias, analisamos o transporte quântico de canais de spins, fazendo a correlação dos mesmos, para constatar que, a partir de uma função geratriz, é possível encontrar os cumulantes estatísticos que nos permitem analisar os observáveis de transporte, tanto no regime semi-clássico (para o qual há um número grande de canais de espalhamento abertos), quanto no regime quântico extremo (onde número de canais de espalhamento abertos é pequeno).