The topological insulator Bi2Te3 has been grown on Si(111)-(7 × 7) surface by molecular beam epitaxy. Reflection high energy electron diffraction, in situ scanning tunnelling microscopy, x-ray photoelectron spectroscopy and ex situ x-ray diffraction studies have been performed to analyze the quality of the growth. These analyses suggest a very good layer-by-layer epitaxial growth of Bi2Te3 on the atomically at Si surface. The magnetoresistance of the samples has been studied with magnetic field perpendicular and parallel to the sample surface, up to 9 T, over a temperature range of 2 K to 20 K. A sharp dip at low fields (0 T - 1 T) and near-linear behavior for high fields (> 4 T) have been observed in the perpendicular field magnetoresistance. The low field dip is due to weak antilocalization that agrees well with the simplified Hikami-Larkin-Nagaoka model. It has been demonstrated that both the low field dip and the high field near-linear behavior can be explained by the original Hikami-Larkin-Nagaoka formula alone in a system with strong spin-orbit coupling. From the fitting of the perpendicular field magnetoresistance the phase coherence length, the mean free path and the spin-orbit relaxation time have been estimated. The phase coherence length shows power law dependence with temperature indicating two dimensional nature of the transport. The power law also suggests electron electron interaction as the prominent dephasing mechanism. The out-of-plane spin-orbit relaxation time is determined to be small and the in-plane spin-orbit relaxation time is found to be comparable to the momentum relaxation time. The estimation of these charge and spin transport parameters is useful for topological insulator based magneto electric device applications. It also has been shown that the strong spin-orbit coupling suppresses the Zeeman contribution in perpendicular field magnetoresistance. The logarithmic divergence of perpendicular field magnetoresistance with temperature for low temperature range (2 K - 20 K) at high fields shows the presence of Coulomb interaction in the spin singlet channel. For magnetoresistance with the field parallel to the sample surface, the observed magnetoresistance has parabolic dependence for small fields (0 T - 0.6 T) and logarithmic dependence for large fields (> 3 T), which is due to the Zeeman effect. It is found that the data are inconsistent with only the Maekawa and Fukuyama theory of non interacting electrons with Zeeman contributions to the transport, but are consistent with theory if one also takes into account the electron electron interaction and the Zeeman splitting term in the electron electron interaction theory of Lee and Ramakrishnan. The Zeeman g-factor and the strength of Coulomb scattering due to electron electron interaction have been estimated from fitting of the parallel field magnetoresistance. The magnetoresistance also shows anisotropy with respect to the field directions. The angle dependent anisotropic magnetoresistance can be fitted well by the original HLN theory alone. The anisotropy can have potential application in anisotropic magnetic sensors. / text

This thesis presents a theory of electron-electron interaction effects and optical properties of nanostructures of two-dimensional (2D) honeycomb crystals - graphene and transition metal dichalcogenides (TMDC). Graphene, a semimetallic hexagonal lattice of carbon atoms can be described by a massless Dirac fermion model, with the conduction band (CB) and valence band (VB) touching in the corners of a hexagonal Brillouin zone, valleys K and -K. TMDC crystals sites host either a transition metal atom or a chalcogen dimer, which opens the energy gap and allows for describing their low-energy nature with massive Dirac fermion (mDf) model. The metal atom in TMDC crystals causes strong spin-orbit (SO) coupling, resulting in large SO splitting in bands at both valleys. For TMDCs it is possible to excite carriers in each valley with oppositely circularly polarised light, which offers promising prospects for devices based on electrons valley index, i.e. valleytronic devices. Additionally, the optical response of TMDCs is enhanced by the presence of secondary CB minima, at Q-points. The dimensionality of 2D crystals can be further reduced to form quantum dots (QDs) - nanostructures con ned in all dimensions. This thesis first discusses hexagonal graphene QDs, which exhibit energy gap oscillation as a function of size, due to the edge type: zigzag or armchair. These QDs are divided into concentric rings, analysed with tight-binding (TB) model. An armchair edged QD is built from a zigzag edged QD by adding a 1D Lieb lattice of carbon atoms on its edge. The energy gap is formed differently for both edges: from the outer ring states for zigzag edge and from the 1D Lieb lattice zero-energy states for armchair edge, which causes the energy gap. The remaining portion of the thesis focuses on TMDC materials. First a TB model is presented for a member of TMDC group, MoS2, using three d orbitals of Mo atom and three p orbitals of the S2 dimers. The tunneling matrix elements between nearest-neighbor and next-nearest-neighbour sites are explicitly derived at K and -K to form a six band TB Hamiltonian. Its solutions are fitted to the bands obtained from the density functional theory ab initio calculations to obtain the correct behaviour of bands around K and additional minima at Q-points, which explains the role of d orbitals in TMDCs. Close to K the TB model is reduced to mDf model, which is then studied in response to light, yielding the valley-dependent selection rules for absorption. The interaction of mDf with light is further studied in the presence of strong external magnetic eld, which leads to the formation of Landau levels (LLs), asymmetric between both valleys, and valley Zeeman splitting. These LLs are populated with electrons to form a Hartree-Fock ground state (GS), which can exhibit valley polarisation due to the LL asymmetry. Quasi-electron-hole excitations out of the GS are then formed and their self-energy, vertex corrections and scattering energy is calculated. The effect of electron-electron interactions on valley Zeeman splitting is demonstrated and the Bethe-Salpeter equation is numerically solved to give magnetoexciton spectrum for both valleys. The results include a valley-dependent absorption spectrum for mDf magnetoexcitons that vary with the valley polarisation. The final part of this thesis discusses the single particle and interacting effects in gated MoS2 QDs. First, I perform a single electron atomistic calculation for a million-atom computational box with periodic boundary conditions based on a TB model developed from ab initio methods for bulk MoS2. Electrons are then con ned with a parabolic electrostatic potential from top metallic gates. They exhibit twofold degenerate harmonic oscillator energy spectrum with shell spacing ω associated with valleys K as well as a sixfold degenerate energy spectrum derived from the Q-points. The degeneracy of electronic shells is broken due to valley contrasting Berry curvature,which acts as an effective magnetic eld splitting opposite angular momentum states in both valleys. I populate up to ve K-derived harmonic oscillator shells with up to six electrons and turn on the electron-electron interactions. The resulting GS phases form two regimes dependent on ω, which are dominated each by a broken-symmetry phase, i.e. valley and spin polarised GS for low ω and valley and spin unpolarised but spin intervalley antiferromagnetic GS for higher ω. This behaviour is explained as an effect of the strong SO splitting, weak intervalley exchange interaction and strong correlations. Means of detecting these effects in experiment based on the spin and valley blockade are proposed. These results advance the understanding of interaction-driven breaking of symmetry for valley systems, crucial for designing of valleytronic devices in the future.

The first part of this thesis deals with the study of mesoscopic fluctuations of the Coulomb drag resistance in double-layer GaAs/AlGaAs heterostructures, both in weak magnetic fields and strong magnetic fields. In the second part, measurements are made in a monolayer graphene structure, specifically of the quantum lifetime, and the mesoscopic resistance fluctuations at quantising magnetic fields.

Esta tese é composta por duas partes. Na primeira parte, os efeitos da interação elétron-elétron nas configurações do estado fundamental de dois anéis quânticos acoplados (CQRs) são estudados. Os CQRs podem formar um novo tipo de molécula artificial, onde o raio dos anéis juntamente com a distância entre os anéis, são novos parâmetros ajustáveis que fornecem novos graus de liberdade para controlar a estrutura eletrônica destas moléculas. Através da bem estabelecida teoria do funcional densidade dependente de spin, as configurações ou fases do estado fundamental dos CQRs com alguns elétrons são determinadas. Uma rica variedade de fases para o estado fundamental destas novas moléculas artificiais é encontrada para sistemas contendo até N=13 elétrons. Para CQRs com N menor ou igual a 8 são obtidas qualitativamente configurações para o estado fundamental similares às dos pontos quânticos acoplados (CQDs). As novas configurações eletrônicas aparecem para N maior ou igual a 9. Na segunda parte desta tese é desenvolvido um método numérico para estudar o espalhamento eletrônico através de um ponto quântico com N-elétrons confinados. Considera-se que o ponto quântico está imerso num sistema bidimensional ou confinado em um canal unidimensional. As taxas de espalhamento são obtidas resolvendo iterativamente a equação de Lippmann-Schwinger incluindo a interação elétron-elétron entre o elétron incidente e os N-elétrons confinados dentro do QD. Para exemplificar, este método é aplicado para um elétron externamente injetado sobre um QD contendo um único elétron. As taxas de espalhamento elástico, inelástico e de spin-flip são obtidas. Os efeitos da interação de troca no espalhamento eletrônico e transporte através do QD são analisados.Também são considerados os processos do espalhamento multi-canal neste sistema e suas influências nas propriedades de transporte. / This thesis is composed of two parts. In the first part, we study the effects of electron-electron interactions on the ground state configurations of two vertically coupled quantum rings (CQRs). The CQRs can form a new type of artificial molecule (AM) where the ring radius together with the inter-ring distance are new tunable parameters providing new degrees of freedom to modulate and control the electronic structure of the artificial ring shaped molecules. In this work, we apply the well established spin-density functional theory to study the ground state configurations or phases of few-electron CQRs. A rich range of ground state phases of these new quantum ring AMs is uncovered for systems containing up to N = 13 electrons. For CQRs with N less or equal to 8 we found qualitatively similar ground state phases as for coupled quantum dots (CQDs). Novel phases appear for N greater or equal to 9. In the second part of this thesis, we develop a numerical method to study the electron scattering through an occupied quantum dot (QD) with a few electrons. The QD is considered embedded in a two-dimensional system or confined in a one-dimemsional channel. An external electron is injected and scattered through the QD. The scattering rates are obtained by solving iteratively the Lippmann-Schwinger equation including the electron-electron interactions between the incident electron and the N-electrons confined in the QD. As an example, we apply this model for an externally injected electron through a QD with one electron inside. The elastic, inelastic, and spin-flip scattering rates are obtained. The effects of electron exchange interaction on the electron scattering and transport through the QD are analyzed. We also show the multi-channel scattering processes in such systems and their influences on the electron transport properties.

Esta tese é composta por duas partes. Na primeira parte, os efeitos da interação elétron-elétron nas configurações do estado fundamental de dois anéis quânticos acoplados (CQRs) são estudados. Os CQRs podem formar um novo tipo de molécula artificial, onde o raio dos anéis juntamente com a distância entre os anéis, são novos parâmetros ajustáveis que fornecem novos graus de liberdade para controlar a estrutura eletrônica destas moléculas. Através da bem estabelecida teoria do funcional densidade dependente de spin, as configurações ou fases do estado fundamental dos CQRs com alguns elétrons são determinadas. Uma rica variedade de fases para o estado fundamental destas novas moléculas artificiais é encontrada para sistemas contendo até N=13 elétrons. Para CQRs com N menor ou igual a 8 são obtidas qualitativamente configurações para o estado fundamental similares às dos pontos quânticos acoplados (CQDs). As novas configurações eletrônicas aparecem para N maior ou igual a 9. Na segunda parte desta tese é desenvolvido um método numérico para estudar o espalhamento eletrônico através de um ponto quântico com N-elétrons confinados. Considera-se que o ponto quântico está imerso num sistema bidimensional ou confinado em um canal unidimensional. As taxas de espalhamento são obtidas resolvendo iterativamente a equação de Lippmann-Schwinger incluindo a interação elétron-elétron entre o elétron incidente e os N-elétrons confinados dentro do QD. Para exemplificar, este método é aplicado para um elétron externamente injetado sobre um QD contendo um único elétron. As taxas de espalhamento elástico, inelástico e de spin-flip são obtidas. Os efeitos da interação de troca no espalhamento eletrônico e transporte através do QD são analisados.Também são considerados os processos do espalhamento multi-canal neste sistema e suas influências nas propriedades de transporte. / This thesis is composed of two parts. In the first part, we study the effects of electron-electron interactions on the ground state configurations of two vertically coupled quantum rings (CQRs). The CQRs can form a new type of artificial molecule (AM) where the ring radius together with the inter-ring distance are new tunable parameters providing new degrees of freedom to modulate and control the electronic structure of the artificial ring shaped molecules. In this work, we apply the well established spin-density functional theory to study the ground state configurations or phases of few-electron CQRs. A rich range of ground state phases of these new quantum ring AMs is uncovered for systems containing up to N = 13 electrons. For CQRs with N less or equal to 8 we found qualitatively similar ground state phases as for coupled quantum dots (CQDs). Novel phases appear for N greater or equal to 9. In the second part of this thesis, we develop a numerical method to study the electron scattering through an occupied quantum dot (QD) with a few electrons. The QD is considered embedded in a two-dimensional system or confined in a one-dimemsional channel. An external electron is injected and scattered through the QD. The scattering rates are obtained by solving iteratively the Lippmann-Schwinger equation including the electron-electron interactions between the incident electron and the N-electrons confined in the QD. As an example, we apply this model for an externally injected electron through a QD with one electron inside. The elastic, inelastic, and spin-flip scattering rates are obtained. The effects of electron exchange interaction on the electron scattering and transport through the QD are analyzed. We also show the multi-channel scattering processes in such systems and their influences on the electron transport properties.

Cette thèse expérimentale traite de l'électroluminescence de contacts atomiques en or. Les contacts métalliques ponctuels sont formés et pilotés à l'aide d'un dispositif de jonction brisée contrôlée mécaniquement. Les contacts sont formés à partir d'un fil d'or et sont étudiés à la température ambiante.L'électroluminescence est observée dans le visible au travers d'un microscope optique. Le détecteur est une caméra sensible en silicium. Pour l'analyse du spectre émis, un dispositif dispersif en ligne a été spécifiquement développé. Pour l'infrarouge, le détecteur photovoltaïque monocanal est en InAsSb.Nos mesures électriques et optiques simultanées permettent de sonder la physique des interactions entre électrons et photons à l'échelle nanométrique. L'électroluminescence est attribuée à l'émission spontanée d'un nanogaz à haute température d'électrons chauds, conséquence des fortes densités de courant. Cette haute température électronique est fonction des conditions opératoires. Pour ces nanojonctions d'or, nos expériences nous permettent d’en proposer une expression analytique simple.Ces travaux complètent des expériences similaires menées depuis le début des années 2000. Ils sont discutés dans le cadre d'un modèle développé pour expliquer l'émission d'électrons chauds à partir de films métalliques granulaires. Nous discutons de la physique d’échauffement du gaz d’électron en rapprochant nos résultats d'expériences pompe/sonde femtoseconde interrogeant la dynamique des électrons hors équilibre dans des nanobilles d'or ainsi que d'expériences de transport en physique mésoscopique menées à très basse température. / This experimental thesis deals with electroluminescence from gold atomic point contacts. Metallic point contacts are formed and driven with a home-made mechanically controlled break junction device. The nanojunctions are made from gold wires. Experiments are performed at room temperature and in air.Electroluminescence is observed in the visible range with an infinity corrected inverted optical microscope. The detector is a high sensitivity silicon camera. To perform spectral analysis, a dispersive on-line device has been developed to be inserted directly within the microscope. A reflective objective collects infrared photons and focuses them onto an InAsSb photovoltaic cooled detector.Our simultaneous electrical and optical measurements allow us to investigate the physics of electrons and photons interactions at the nanometric scale. Electroluminescence is explained by the spontaneous emission of a hot electron nano-gas favoured by huge current densities. This high electron temperature depends on operating conditions. For gold ballistic nanojunctions, our results lead us to propose a simple expression of this temperature. This work extends similar electroluminescence studies performed since the early 2000’s. The results are discussed in this context and in the framework of a model first introduced to account for hot electron emission from thin granular metallic films. Moreover, we discuss the physics leading to the hot electron gaz with the support of pump/probe femtosecond experiments probing the nonequilibrium electron dynamics in gold nanosphere and with the support of low temperature mesoscopic transport experiments.

In this work we investigate some aspects of the physics of strongly correlated systems by taking into account both electron-electron and electron-phonon interactions as basic mechanisms for reproducing electronic correlations in real materials. The relevance of the electron-electron interactions is discussed in the first part of this thesis in the framework of a self-consistent theoretical approach, named Composite Operator Method (COM), which accounts for the relevant quasi-particle excitations in terms of a set of composite operators that appear as a result of the modification imposed by the interactions on the canonical electronic fields. We show that the COM allows the calculation of all the relevant Green s and correlation functions in terms of a number of unknown internal parameters to be determined self-consistently. Therefore, depending on the balance between unknown parameters and self-consistent equations, exact and approximate solutions can be obtained. By way of example, we discuss the application of the COM to the extended t-U-J-h model in the atomic limit, and to the two-dimensional single-band Hubbard model. In the former case, we show that the COM provides the exact solution of the model in one dimension. We study the effects of electronic correlations as responsible for the formation of a plethora of different charge and/or spin orderings. We report the phase diagram of the model, as well as a detailed analysis of both zero and finite temperature single-particle and thermodynamic properties. As far as the single-band Hubbard model is concerned, we illustrate an approximated self-consistent scheme based on the choice of a two-field basis. We report a detailed analysis of many unconventional features that arise in single-particle properties, thermodynamics and system's response functions. We emphasize that the accuracy of the COM in describing the effects of electronic correlations strongly relies on the choice of the basis, paving the way for possible multi-pole extensions to the two-field theory. To this purpose, we also study a three-field approach to the single-band Hubbard model, showing a significant step forward in the agreements with numerical data with respect to the two-pole results. The role of the electron-phonon interaction in the physics of strongly correlated systems is discussed in the second part of this thesis. We show that in highly polarizable lattices the competition between unscreened Coulomb and Fröhlich interactions results in a short-range polaronic exchange term Jp that favours the formation of local and light pairs of bosonic nature, named bipolarons, which condense with a critical temperature well in excess of hundred kelvins. These findings, discussed in the framework of the so-called polaronic t-Jp model, are further investigated in the presence of a finite on-site potential U, coming from the competition between on-site Coulomb and Fröhlich interactions. We discuss the role of U as the driving parameter for a small-to-large bipolaron transition, providing a possible explanation of the BEC-BCS crossover in terms of the properties of the bipolaronic ground state. Finally, we show that a hard-core bipolarons gas, studied as a charged Bose-Fermi mixture, allows for the description of many non Fermi liquid behaviours, allowing also for a microscopic explanation of pseudogap features in terms of a thermal-induced recombination of polarons and bipolarons, without any assumption on preexisting order or broken symmetries.

Στην εργασία αυτή, μελετάμε, τόσο με αναλυτικό όσο και με υπολογιστικό τρόπο, γραμμικά και μη γραμμικά οπτικά φαινόμενα σε συστήματα ημιαγώγιμων κβαντικών πηγαδιών GaAs/AlGaAs δύο ενεργειακών υποζωνών, όπου επάγονται διαϋποζωνικές μεταβάσεις, υπό την επίδραση ηλεκτρομαγνητικών πεδίων. Στο πρώτο κεφάλαιο, γίνεται μια θεωρητική περιγραφή των ημιαγώγιμων ετεροεπαφών. Ακολουθούν βασικά στοιχεία της στατιστικής και κβαντικής μηχανικής. Στο δεύτερο κεφάλαιο, εξάγονται οι γενικευμένες εξισώσεις Bloch για τις διαϋποζωνικές μεταβάσεις σε ημιαγώγιμα κβαντικά πηγάδια, στις οποίες ενυπάρχουν όροι που υπεισάγουν τις μη αμελητέες, λόγω εμπλουτισμού, αλληλεπιδράσεις μεταξύ των ηλεκτρονίων. Οι εξισώσεις αυτές αποτελούν τη βάση της μελέτης που ακολουθεί. Στα δύο επόμενα κεφάλαια, μελετούμε την αλληλεπίδραση μιας δομής διπλών συζευγμένων ημιαγώγιμων κβαντικών πηγαδιών με ένα ηλεκτρομαγνητικό πεδίο μεταβλητής γωνιακής συχνότητας, καταλήγουμε σε αναλυτικές εκφράσεις για τη οπτική επιδεκτικότητα πρώτης, τρίτης και πέμπτης τάξεως και αναλύουμε τα φάσματα διαφόρων οπτικών φαινομένων, ως προς τη γωνιακή συχνότητα του εξωτερικού πεδίου, για διάφορες τιμές της επιφανειακής ηλεκτρονιακής πυκνότητας της κβαντικής δομής. Επιπλέον, προσδιορίζουμε τις περιοχές όπου λαμβάνουν τιμή οι διάφορες παράμετροι, ούτος ώστε στο σύστημά μας να αναδυθεί η οπτική διστάθεια. Στα τρία τελευταία κεφάλαια, θεωρούμε ότι η ημιαγώγιμη κβαντική δομή αλληλεπιδρά ταυτόχρονα με ένα ισχυρό ηλεκτρομαγνητικό πεδίο (πεδίο άντλησης) καθορισμένης γωνιακής συχνότητας και ένα ασθενές (πεδίο ανίχνευσης) μεταβλητής συχνότητας και μελετούμε τα φάσματα γραμμικών και μη γραμμικών φαινομένων του πεδίου ανίχνευσης (μίξη τεσσάρων κυμάτων, απορρόφηση, διασπορά, μη γραμμικό οπτικό φαινόμενο Kerr), σε στάσιμη κατάσταση, καθώς και τη χρονική εξέλιξη αυτών. Περιγράφουμε τα φαινόμενα τόσο με αναλυτικές εκφράσεις που εξάγουμε, όσο και με την αριθμητική επίλυση των μη-γραμμικών διαφορικών εξισώσεων του πίνακα πυκνότητας που διέπουν τη δυναμική. Στη μελέτη των φαινομένων αυτών, εξετάζεται η επίδραση της επιφανειακής ηλεκτρονιακής πυκνότητας της κβαντικής δομής στις οπτικές ιδιότητες των κβαντικών πηγαδιών. / In this PhD thesis, we study analytically and numerically linear and nonlinear optical phenomena in intersubband transitions of a symmetric GaAs/AlGaAs double quantum well structure, with two energy subbands. In the first chapter, a theoretical description of the semiconductor heterostructures is presented. This is accompanied with a brief analysis of the basic elements of statistical and quantum mechanics follows, as far as this kind of structures is concerned. In the second chapter, we derive the generalised optical Bloch equations in intersubband transitions of semiconductor quantum well structures, which constitute the basis of the analysis that follows. These equations contain terms which owe their presence to the electron-electron interactions, because the quantum structure is doped with electron carriers. In the two following chapters, we consider the interaction of intersubband transitions of a double quantum well structure with an electromagnetic field of varying frequency, we derive analytical expressions for the first, third and fifth order optical susceptibility and, at last, we analyze the corresponding spectra, with respect to the frequency of the external field, for different values of electron sheet density of the structure. Furthermore, we identify the areas of values of the parameters used, in which the phenomenon of optical bistability arises. In the last three chapters, we consider the two quantum well subbands to be coupled to a strong pump electromagnetic field with fixed frequency and a weak probe electromagnetic field of varying frequency and study the spectra of various linear and nonlinear optical phenomena, which are due to the existence of the probe field. More specifically, we examine the spectra of four-wave mixing, absorption, dispersion and the nonlinear optical Kerr effect of the probe field as they evolve in time and in the steady state. Both analytical expressions are derived and numerical results are presented by solving the nonlinear differential density matrix equations that govern the dynamics of the system. In the study of the different kinds of optical phenomena, the influence of the electron sheet density on the spectral shapes is carefully examined.

Ce travail de thèse consiste à visualiser sur un détecteur sensible en position les oscillations spatiales des électrons lents (~ meV) émis par photoionisation au seuil en présence d’un champ électrique extérieur. La figure d’interférence obtenue représente quantiquement le module carré de la fonction d’onde électronique. Ce travail fondamental nous permet d’avoir accès à la dynamique électronique quelques µm autour de l’atome et donc de mettre en évidence plusieurs mécanismes quantiques (champ coulombien, interaction électron/électron..) se déroulant à l’échelle atomique. Malgré la présence d’un cœur électronique quoique limité dans Li, nous avons réussi, expérimentalement et pour la première fois, à visualiser la fonction d’onde associée aux états Stark quasi-discrets couplés au continuum d’ionisation. En outre, à l’aide des simulations quantiques de propagation du paquet d’ondes, basées sur la méthode de « Split-operator », nous avons réalisé une étude complète sur les atomes H, Li et Cs tout en dévoilant les effets significatifs des résonances Stark. Un très bon accord, sur et hors résonances, a été obtenu entre les résultats simulés et les résultats expérimentaux. Par ailleurs, nous avons développé un modèle analytique généralisable permettant de comprendre profondément le fonctionnement d’un spectromètre de VMI. Ce modèle repose sur l’approximation paraxiale, il est basé sur un calcul d’optique matricielle en faisant une analogie entre la trajectoire électronique et le rayon lumineux. Un excellent accord a été obtenu entre les prédictions du modèle et les résultats expérimentaux. / This work of thesis aims to visualize, on a position sensitive detector, the spatial oscillations of slow electrons (~meV) emitted by a threshold photoionization in the presence of an external electric field. The interference figure obtained represents the square magnitude of electronic wavefunction. This fundamental work allows us to have access to the electronic dynamics and thus to highlight several quantum mechanisms that occur at the atomic scale (field Coulomb, electron/electron interaction..). Despite the presence an electronic core in Li atom, we have succeeded, experimentally and for the first time, to visualize the wave function associated with the quasi-discrete Stark states coupled to the ionization continuum. Besides, using simulations of wave packet propagation, based on the "Split-operator” method, we have conducted a comprehensive study of the H, Li and Cs atoms while revealing the significant effects of the Stark resonances. A very good agreement, on and off resonances, was obtained between simulated and experimental results. In addition, we have developed a generalized analytical model to understand deeply the function of VMI spectrometer. This model is based on the paraxial approximation; it is based on matrix optics calculation by making an analogy between the electronic trajectory and the light beam. An excellent agreement was obtained between the model predictions and the experimental results.

Une étude expérimentale de gaz d'électrons bidimensionnel confinés dans des puits quantiques de CdTe et de CdMnTe est présentée. L'analyse de données est soutenue par des calculs numériques de la structure de bande des états confinés, utilisant l'approximation de densité locale et de fonction enveloppe. Un calcul de type k.p a été utilisé pour justifier l'approximation parabolique appliquée pour les bandes valence. Les échantillons ont été caractérisés par spectroscopie Raman et par spectroscopie d'absorption de la résonance cyclotron infrarouge. Le magnéto-transport à bas champ est dominé par la contribution semi-classique de Drude et révèle trois contributions plus faibles, qui sont la localisation faible, l'interaction électron-électron et les oscillations Shubnikov-De Haas. La contribution des interactions électron-électron est expliquée dans un modèle semi-classique à trajectoire circulaire. La forme des niveaux de Landau, leurs élargissement, les temps de vie transport et quantique de la diffusion et le mécanisme (long-portée) de la diffusion dominant ont été déterminés. Le magnéto-transport sous champs magnétiques intenses révèle la présence d'états Hall quantique fractionnaires dans les niveaux de Landau N=0 et N=1. Nous avons montré, que les états 5/3 et 4/3 étaient complètement polarisés en spin, en accord avec l'approche des fermions composites pour l'effet Hall quantique fractionnaire. La forme de la photoluminescence à champ magnétique nul et son évolution avec la température sont décrites par un modèle analytique simple. La dépendance en champ magnétique et en température de la photoluminescence indique que le gap de spin est amplifié dans les niveaux de landau entièrement occupés. Ces effets multi-corps de l'amplification du gap du spin ont été décrits avec succès par un modèle numérique simple. L'intensité de la photoluminescence a mise en évidence l'importance des processus non-radiatifs pendant la recombinaison, la dégénérescence des niveaux de Landau, leur taux d'occupation, les règles de sélection et l'influence de l'écrantage. Le mécanisme de la relaxation parallèle de spin d'électron et de trou a été identifié et attribué au mécanisme Bir-Aharonov-Pikus, assistée par les phonons acoustiques. Les spectres de photoluminescence d'excitation reflètent la densité des états caractéristique des systèmes bidimensionnels. Les résonances excitoniques, qui sont observées aux bords des sous-bandes électriques inoccupées, illustrent l'importance de l'écrantage et des champs électriques intrinsèques dans les puits asymétriquement dopés. / Experimental studies of two-dimensional electron gases confined in CdTe and CdMnTe quantum wells are presented. The data analysis is supported by numerical calculations of the band structure of confined states, using the local density and envelope function approximations. Four by four, k.p calculations have been performed to justify the parabolic approximation of valence bands. Samples were characterized by Raman scattering spectroscopy and far infrared cyclotron resonance absorption measurements. Low-field magneto-transport shows the dominant contribution of the semi-classical Drude conductivity and ten times weaker contributions of weak localization, electron-electron interaction and Shubnikov-de Haas oscillations. The contribution of electron-electron interactions is explained within a semi-classical model of circling electrons. The shape of Landau levels, broadening, transport and quantum lifetimes and dominant long-range scattering mechanism have been determined. High-field magneto-transport displays fractional quantum Hall states at Landau levels N=0 and N=1. The ground states 5/3 and 4/3 have been determined to be fully spin polarized, in agreement with the approach of composite fermions for the fractional quantum Hall effect. The form of the photoluminescence at zero magnetic field and its evolution with temperature have been described by simple analytical model. Magnetic field and temperature dependence of the photoluminescence has been found to display the enhanced spin splitting of fully occupied Landau levels. This many body enhanced spin gap has been successfully described by a numerical model. The intensity of the photoluminescence demonstrated the importance of the non-radiative recombination channel, degeneracy of Landau levels, their occupation, selection rules and screening. The mechanism of the simultaneous electron and hole spin-flip was recognized and attributed to the longitudinal acoustical phonon assisted Bir-Aharonov-Pikus spin relaxation mechanism. Photoluminescence excitation spectra embody the characteristic density of states of two-dimensional systems. The excitonic resonances, which are observed at the edges of unoccupied electric subbands, illustrate the importance of screening and internal electric fields in asymmetrically doped quantum wells.