Les phonons contribuent à la diffusion inélastique résonante des rayons X (RIXS) du fait du couplage entre les degrés de liberté électronique et ceux du réseau. Contrairement à d'autres techniques sensibles aux interactions électron-phonon, la technique RIXS peut donner accès aux constantes de couplage dépendantes du moment. Des informations sur la dispersion de l'interaction électron-phonon sont très précieuses dans le contexte de la supraconductivité anisotrope conventionnelle et non conventionnelle.Nous avons considéré la contribution des phonons sur la diffusion RIXS d’un point de vue théorique. Contrairement aux études précédentes nous soulignons le rôle du couplage du réseau avec les trous de cœur. Notre modèle, avec les paramètres obtenus ab-initio, montre que même dans le cas d'un trou de coeur profond, la technique RIXS sonde le couplage exciton-phonon plutôt qu’un couplage direct électron-phonon.Cette différence conduit à des écarts quantitatifs et qualitatifs pour le couplage électron-phonon implicite par rapport à l'interprétation standard dans la littérature. Ainsi, notre objectif est de développer une approche rigoureuse pour quantifier le couplage électron-phonon dans le contexte des mesures de diffusion RIXS. La possibilité de reproduire avec précision les résultats expérimentaux à partir des calculs ab-initio, sans recourir à des paramètres ajustés, doit être considérée comme le test ultime d'une compréhension correcte de la contribution des phonons sur la diffusion RIXS.Nous commençons notre travail en considérant uniquement l’interaction trou de coeur-phonon dans le contexte de la spectroscopie par photoémission de rayons X. Nous combinons un calcul ab-initio de la fonction de réponse en espace réel avec des techniques de fonctions de Green à plusieurs corps pour reproduire les bandes latérales vibrationnelles dans les molécules SiX4 (X = H, F). L'approche que nous avons développée peut être appliquée aux matériaux cristallins.Nous examinons ensuite la contribution des phonons aux spectres d'absorption des rayons X. Contrairement aux excitations chargées générées par la photoémission par rayons X, l'absorption des rayons X crée une excitation neutre que nous approchons en tant que trou de cœur et électron excité. Nous résolvons d’abord la partie électronique du problème au niveau de l’équation de Bethe-Salpeter, puis nous habillons la quasi-particule excitonique à 2 particules résultante avec les interactions exciton-phonon en utilisant l’Ansatz des cumulants. La viabilité de cette méthode a été testée en calculant le seuil K XAS de la molécule N2 et le seuil K d’Oxygène de l’acétone. Les spectres vibrationnels obtenus concordent avec les résultats expérimentaux.Enfin, nous construisons une formulation hybride de la section transversale RIXS qui préserve la sommation explicite sur un petit nombre d'états finals, mais remplace la sommation sur les états intermédiaires, ce qui pourrait être extrêmement coûteux, par une fonction de Green. Nous avons obtenu un développement de la fonction de Green et dérivé des solutions analytiques exactes (dans la limite de non-recul) et approximatives. Le formalisme a de nouveau été testé sur le seuil K de l'acétone et est bien en accord avec l'expérience. En perspectives des travaux futurs, nous discutons de l’applicabilité de notre formalisme aux matériaux cristallins. / Phonons contribute to resonant inelastic X-ray scattering (RIXS) as a consequence of the coupling between electronic and lattice degrees of freedom. Unlike other techniques that are sensitive to electron-phonon interactions, RIXS can give access to momentum dependent coupling constants. Information about the dispersion of the electron-phonon interaction is highly desirable in the context of understanding anisotropic conventional and unconventional superconductivity.We considered the phonon contribution to RIXS from the theoretical point of view. In contrast to previous studies, we emphasize the role of the core-hole lattice coupling. Our model, with parameters obtained from first principles, shows that even in the case of a deep core-hole, RIXS probes exciton-phonon coupling rather than a direct electron-phonon coupling.This difference leads to quantitative and qualitative deviations from the interpretation of the implied electron-phonon coupling from the standard view expressed in the literature. Thus, our objective is to develop a rigorous approach to quantify electron-phonon coupling within the context of RIXS measurements. The ability to accurately reproduce experimental results from first-principles calculations, without recourse to adjustable parameters, should be viewed as the ultimate test of a proper understanding of the phonon contribution to RIXS.We start by considering only the core-hole--phonon interaction within the context of X-ray photoemission spectroscopy. We combine an ab initio calculation of the real-space response function with many-body Green's functions techniques to reproduce the vibrational side-bands in SiX4 (X=H, F) molecules. The approach we developed is suitable for application to crystalline materials.We next consider the phonon contribution to X-ray absorption spectra. Unlike the charged excitations generated by X-ray photoemission, X-ray absorption creates a neutral excitation that we approximate as a core-hole and an excited electron. We first solved the electronic part of the problem on the level of the Bethe-Salpeter equation and then dressed the resulting 2-particle excitonic quasiparticle with the exciton-phonon interactions using the cumulant ansatz. The viability of this methodology was tested by calculating the N K-edge XAS of the N2 molecule and the O K-edge of acetone. The resulting vibronic spectra agreed favorably with experimental results.Finally, we construct a hybrid formulation of the RIXS cross section that preserves explicit summation over a small number of final states, but replaces the summation over intermediate states, which might be enormously expensive, with a Green's function. We develop an expansion of the Green's function and derive both analytically exact (in the no-recoil limit) and approximate solutions. The formalism was again tested on the O K-edge of acetone and agrees well with the experiment. To provide an outlook towards future work, we discuss application of the developed formalism to crystalline materials.

<div>Metallic components and metal-dielectric interfaces appear widely in modern electronics and the thermal management is an important issue. A very important feature that has been overlooked in the conventional Fourier's equations analyses is the nonequilibrium thermal transport induced by selective electron-phonon (e-p) coupling and phonon-phonon (p-p) coupling. It signicantly affects many processes such as laser heating and ignoring this phenomenon can lead to wrong or misleading predictions. On the other hand, as devices shrink into nano-scale, heat generation and dissipation at the interfaces between different components start to dominate the thermal process and present a challenge for thermal mitigations. Many unresolved issues also arise from interfaces, such as the unexpected high interfacial thermal conductance (ITC) at metal-diamond interfaces. Both of these require a deep understanding of the physics at interfaces.</div><div><br></div><div><div>Therefore in this work, I present multi-scale simulations in metals/dielectrics and interfaces based on two-temperature model (TTM) and establish the new multitemperature model (MTM). The methods are combined with Fourier's Law, molecular dynamics (MD), Boltzmann transport equations (BTE) and implemented to predict the thermal transport in several materials and interfaces where e-p coupling and p-p coupling are important. First-principles studies based on density functional theory (DFT) are also presented as predictive approaches to acquire the properties, as well as investigating the new physical phenomenon of non-local e-p coupling in metals. This research seeks to provide general, sophisticated but also simple simulation approaches which can help people accurately predict the thermal transport process. It also seeks to explore new physics which cannot be captured and predicted by conventional analyses based on Fourier's Law and can advance our understanding as well as providing new insights in the current thermal analysis paradigm.</div></div><div><br></div><div><div>The rst part of this thesis focuses on the non-equilibrium thermal transport in metals and across metal-dielectric interfaces based on TTM. First of all, nonequilibrium thermal transport in metal matrix composites (MMC) is investigated. Metal particle is usually added to polymer matrix for enhanced thermal performance. Here we apply TTM calculations and manifest a \critical particle size" above which the thermal conductivity of the composite material can be enhanced. MD simulations are performed to predict the thermal properties. TTM-Fourier and TTM-BTE calculations are conducted as comparisons. The widely used Au-SAM (self-assemblymonolayers) material pair is chosen to demonstrate our models. For a 1-D SAMAu-SAM sandwich system, the two calculation approaches present almost identical results, and the critical particle size is 10.7 nm. A general interpretation of thermal transport in sandwiched metal thin lms between two dielectric materials is also presented. It is found that when the lm thickness is on the order of several nanometers, due to strong e-p non-equilibrium the thermal transport is dominated by phonons</div><div>and electrons hardly contribute.</div></div><div><br></div><div><div>Then the e-p non-equilibrium thermal transport across metal-dielectric interfaces is investigated using TTM-MD. One possible explanation to the unexpected ITC at metal-diamond interfaces is the cross-interface e-p coupling mechanism, which is based on the hypothesis that electrons can couple to phonons within a certain distance rather than just those at the same location. Therefore we extend TTM-MD by modifying its governing equation to a non-local integral form. Two models are proposed to describe the coupling distance: the \joint-phonon-modes" model and the \phonon-wavelength" model. A case study of thermal transport across Cu-Si interfaces is presented, and both models predict similar coupling distances of 0.5 nm in Cu and 1.4 nm in Si near the interfaces. The cross-interface e-p coupling can increase the ITC by 20% based on our models. Based on the results, we construct a new mixed series-parallel thermal circuit. It is shown that such a thermal circuit is essential for understanding metal-nonmetal interfacial transport, while calculating a single resistance without solving temperature proles as done in most previous studies is generally incomplete.</div></div><div><br></div><div><div>Inspired by the previous work, we investigate further into the physics of nonlocal e-p coupling. First-principles calculations based on DFT is used due to their predictive feature without assumptions or adjustable parameters. By calculating the e-p coupling in metal lms of different sizes, we nd that e-p coupling has size effect which can only be explained by a non-local coupling picture. Results show that in Al, electrons and phonons can couple to each other in a range of up to 2 lattice-constants, or 0.8 nm. The coupling strength between electrons and phonons in adjacent atomic layers still has 75% of that in the same layer. Comparative studies are also performed on Cu and Ag. Results show that their non-local e-p coupling is not as signicant as in Al, with coupling distances of 0.37 nm for Cu and 0.49 nm for Ag. Similar results in Cu and Ag also indicate that materials with similar electronic structures have similar non-local e-p coupling properties.</div></div><div><br></div><div><div>In TTM, it is assumed that phonons are in thermal equilibrium and have a common temperature. In the second part of this thesis we go beyond TTM to investigate the non-equilibrium between phonons as well. TTM is extended to a general MTM with e-p coupling strength for each phonon branch. An averaged scattering lattice reservoir is dened to represent p-p scattering. The thermal transport process in single-layer graphene under constant and pulse laser irradiation is investigated. Results show that the phonon branches are in strong non-equilibrium. A comparison with TTM reveals that MTM can increase the thermal conductivity prediction by 50% and the hot electron relaxation time by 60 times. We also perform MTM simulations on Si-Ge interfaces to investigate the effect of non-equilibrium thermal transport on ITC. Results show that thermal non-equilibrium between phonons will introduce additional resistance at the interfaces, which is similar with e-p non-equilibrium's impact on ITC at metal-dielectric interfaces.</div></div>

Thesis (Ph.D.)--Mississippi State University. Department of Physics and Astronomy. / Title from title screen. Includes bibliographical references.

Comprendre le transport électronique dans les cristaux bidimensionnels est un enjeu conceptuel majeur pour la nanoélectornique de demain. Dans cette thèse, on dévelloppe des méthodes ab initio pour étudier l'interaction électron-phonon, l'écrantage et le transport dans le graphène. Pour surpasser les limites des méthodes ab initio en ondes planes, à l'origine destinées aux matériaux périodiques en trois dimensions, on tronque l'interaction coulombienne dans la troisième dimension, isolant ainsi le système bidimensionnel de ses images périodiques. Ceci est réalisé au sein de la théorie de la fonctionnelle de la densité en perturbation, afin de calculer la réponse de la densité de charge et le spectre des phonons dans un cadre bidimensionnel. On utilise ces méthodes pour obtenir un modèle quantitatif du couplage électron-phonon dans le graphène pour une configuration de type transistor à effet de champ. Le couplage aux phonons acoustiques est dominé par le champ de jauge non-écranté, que nous calculons en incluant l'effet des interactions électron-électron au niveau GW. Nos simulations des propriétés d'écrantage statiques du graphene valident les modèles analytiques et montrent que le potentiel de déformation est fortement écranté, de sorte que sa contribution à la diffusion des électrons par les phonons acoustiques est négligeable. On montre également que le couplage avec les phonons hors-plan est faible mais fini. On obtient la contribution de la diffusion par les phonons à la résistivité en résolvant l'équation de Boltzmann pour le transport. En dessous de la température ambiante, nos résultats confirment le rôle des phonons acoustiques et une augmentation de 15% du paramètre de jauge it ab initio permet un excellent accord avec l'expérience. Au dessus de la température ambiante, on dénote l'importance des phonons optiques intrinsèques. / Understanding the transport properties of two-dimensional crystals doped by field effect is a conceptual milestone for tomorrow's nanoelectronics. In this thesis we develop first-principles methods to investigate electron-phonon interactions, screening and phonon-limited transport in graphene. To overcome the limitations of existing plane-wave ab initio packages, originally devised for three-dimensional periodic solids, we truncate the Coulomb interaction in the third direction and isolate the 2D system from its periodic images. This is implemented in density-functional perturbation theory to calculate charge density responses and phonon spectra in a two-dimensional framework. We use those methods to develop a quantitative model of electron-phonon coupling for graphene in the field effect transistor configuration. We find that the coupling of electrons to acoustic phonons is dominated by the unscreened gauge field, which we compute with full inclusion of electron-electron interactions at the GW level. Our simulations of the static screening properties of graphene validate analytical models and reveal that the deformation potential is strongly screened, such that its contribution to acoustic phonon scattering is negligible. We find a small but finite linear coupling with out-of-plane phonons. By solving the Boltzmann transport equation we obtain the phonon-limited resistivity. Below room temperature, our results confirm the role of acoustic phonons and a 15% increase of the ab initio gauge field parameter leads to an excellent quantitative agreement with experiment. Above room-temperature, we point to the importance of the coupling with intrinsic optical phonons.

A series of experimental results about unusual heating of carbon nanotubes by microwaves is analyzed in this dissertation. Two of vibration types, cantilever type (one end is fixed and the other one end is free), the second type is both ends are fixed, have been studied by other people. A third type of forced vibration of carbon nanotubes under an alternating electromagnetic field is examined in this paper. Heating of carbon nanotubes (CNTs) by microwaves is described in terms of nonlinear dynamics of a vibrating nanotube. Results from the model provide a way to understand several observations that have been made. It is shown that transverse vibrations of CNTs during microwave irradiation can be attributed to transverse parametric resonance, as occurs in the analysis of Melde's experiment on forced longitudinal vibrations of a stretched elastic string. For many kinds of carbon nanotubes (SWNT, DWNT, MWNT, ropes and strands) the resonant parameters are found to be located in an unstable region of the parameter space of Mathieu's equation. Third order wave equations are used to qualitatively describe the effects of phonon-phonon interactions and energy transfer from microwaves to CNTs. This result provides another way to input energy from microwaves to carbon nanotubes besides the usual Joule heating via electron-phonon interaction. This model appears to be the first to point out the role of nonlinear dynamics in the heating of CNTs by microwaves.

Cet exposé est scindé en deux grandes parties. Dans la première, nous estimons des constantes photo-physiques du saphir dopé au titane à partir d'un modèle analytique simple exploitant une théorie de Huang-Rhys pour la détermination du profil spectral des bandes simples et une hypothèse réaliste de superposition de ces dernières. Nous déterminons une formule pour l'indice de réfraction total du Ti:saphir en fonction de la concentration de dopant. Dans une seconde partie, nous évaluons, selon la vérification d'un concept, la capacité de laser sana inversion de populations pour un cristal dopé possédant une basse symétrie. Nous appuyons notre démonstration en établissant une condition de seuil généralisée d'effet laser. Ce concept pourrait être une rupture technologique dans le domaine des grands cristaux dopés et n'a pas encore été investigué par la communauté. / This presentation is split into two main parts. In the first, we estimate photo-physical constants of titanium doped sapphire from a simple analytical model using a Huang-Rhys theory for the determination of the spectral profile of simple bands and from a realistic hypothesis of superposition of the latter. We define a formula for the total refractive index of Ti:sapphire as a function of dopant concentration. In a second part, we evaluate, according to the verification of a concept, the laser capability without population inversion for a doped crystal with low symmetry. We support our demonstration by establishing a generalized laser threshold condition. This concept would be a technological breakthrough in the field of large doped crystals and has not yet been investigated by the community.

No description available.

Com o rápido avanço das técnicas experimentais observado nas últimas décadas, a fabricação de sistemas nanoestruturados se tornou uma realidade. Nessa escala de grandeza, as interações entre elétrons e vibrações nucleares têm um papel importante no transporte eletrônico, podendo causar a perda de coerência de fase dos elétrons, a abertura de novos canais de condução e a supressão de canais puramente elásticos. Neste trabalho, o problema do transporte eletrônico em escala nanométrica foi tratado considerando as interações elétron-fônon, o que resultou na implementação de ferramentas computacionais para simulação realística de materiais. O transporte eletrônico foi abordado por meio do formalismo das Funções de Green Fora do Equilíbrio, onde as interações elétron-fônon foram tratadas por diferentes modelos. Para considerar o efeito dessas interações no transporte, é necessário, em princípio, incluir um termo de autoenergia de espalhamento na Hamiltoniana do sistema. Contudo, a forma exata dessa autoenergia é desconhecida e aproximações são necessárias. O primeiro efeito da interação elétron-fônon estudado foi a perda de coerência de fase, o que foi abordado pelo modelo fenomenológico das sondas de Büttiker [1]. Foram realizadas duas implementações diferentes deste modelo, a primeira na forma usual, onde se considera uma aproximação elástica para o cálculo da corrente, e a segunda por meio de uma nova proposta sem a aproximação elástica. Entretanto, como a autoenergia de interação utilizada não contém informação a respeito da estrutura dos fônons, o modelo produz somente um alargamento do canal de condutância, simulando apenas o efeito de perda de coerência de fase dos elétrons devido à interação com fônons do material. Para poder incluir as informações sobre a estrutura dos fônons, foi desenvolvido o programa PhOnonS ITeratIVE VIBRATIONS, para o cálculo das frequências e dos modos vibracionais de materiais e para calcular a matriz de acoplamento elétron-fônon, a partir de métodos de primeiros princípios. No cálculo da matriz de acoplamento elétron-fônon, além da implementação do código algumas intervenções foram realizadas no programa SIESTA [2,3] (uma implementação da Teoria do Funcional da Densidade). Outra abordagem para a interação elétron-fônon consiste em expandir a autoenergia de interação perturbativamente em diagramas de Feynman até a primeira ordem, o que é convencionalmente chamado de primeira aproximação de Born. Essa aproximação, assim como a sua versão autoconsistente, no qual uma classe mais ampla de diagramas é considerada, foram incorporadas ao programa SMEAGOL [4], um código de transporte eletrônico ab initio baseado na combinação DFT-NEGF e que utiliza como plataforma do cálculo da estrutura eletrônica o código SIESTA. Essas implementações, em conjunto com diversas mudanças realizadas no código SMEAGOL, deram origem ao programa Inelastic SMEAGOL para cálculos de transporte inelástico ab initio. Nessa busca por uma descrição mais realista dos dispositivos eletrônicos, outro aspecto que deve ser considerado é o fato de que os dispositivos muitas vezes podem alcançar escalas de comprimento da ordem de 100 nm com um grande número de defeitos aleatoriamente distribuídos, o que pode levar a um novo regime fundamental de transporte, a saber, o de localização de Anderson [5]. Neste trabalho, foi desenvolvido o programa Inelastic DISORDER, que permite calcular, por primeiros princípios, as propriedades de transporte elástico e inelástico de sistemas com dezenas de milhares de átomos com um grande número de defeitos posicionados aleatoriamente. O método combina cálculos de estrutura eletrônica via DFT com o formalismo NEGF para o transporte, onde as interações elétron-fônon são incluídas por meio de teoria de perturbação com relação à matriz de acoplamento elétron-fônon (Lowest Order Expansion). O método desenvolvido foi aplicado ao estudo de nanofitas de grafeno com impurezas hidroxílicas. Observou-se que, ao incluir a interação elétron-fônon, as propriedades de transporte sofrem mudanças significativas, indicando que estas interações podem influenciar nos efeitos de localização por desordem. [1] M. Büttiker. Phys. Rev. B 33(5), 30203026 (1986). [2] E. Artacho, D. Sánchez-Portal, P. Ordejón, A. García e J. M. Soler. Phys. Stat. Sol. (b) 215, 809817 (1999). [3] J. M. Soler, E. Artacho, J. D. Gale, A. García, J. Junquera, P. Ordejón e D. Sánchez- Portal. J. Phys. Cond. Mat. 14, 27452779 (2002). [4] A. R. Rocha, V. M. García-Suárez, S. W. Bailey, C. J. Lambert, J. Ferrer e S. Sanvito. Phys. Rev. B 73, 085414 (2006). [5] P. W. Anderson. Phys. Rev. 109, 1492 (1958). / With the fast improvement of experimental techniques over the past decades, the synthesis of nanoscale systems has become a reality. At this length scales, the interaction between electrons and ionic vibrations plays an important role in electronic transport, and may cause the loss of the electron\'s phase coherence, the opening of new conductance channels and the suppression of purely elastic ones. In this work the electronic transport problem at nanoscale was addressed considering the electron-phonon interactions, resulting on the development of computational tools for realistic simulations of materials. The electronic transport was approached with the Non-Equilibrium Green\'s Function formalism, where electron-phonon interactions were addressed by different models. To take into account the interaction\'s effects, one needs in principle to include a self-energy scattering term in the system Hamiltonian. Nevertheless, the exact form of this self-energy is unknown and approximations are required. The first effect from electron-phonon interactions dealt was the loss of phase coherence, which was approached by the Büttiker\'s probes phenomenological model [1]. Two different implementations of this model were performed, the first in the standard form, where an elastic approximation is considered in order to compute the current, and the second by a new method without the elastic approximation. However, since the interaction self-energy used doesn\'t contains any information about the phonon\'s structure, this model only produces a broadening at the conducting channels, simulating just the effect of loss of phase coherence from the electrons due to their interactions with the phonons. In order to be able to include information about the phonon\'s structure, the computational code PhOnonS ITeratIVE VIBRATIONS was developed, for calculating the frequencies and vibrational modes of the materials and to compute the electron-phonon coupling matrix, from first principles methods. In the calculation of the electron-phonon coupling matrix, besides the code implementation some changes were performed at the SIESTA program [2,3] (a Density Functional Theory implementation). Another approach for the electron-phonon interactions consists of expanding the interaction self-energy perturbatively in Feynman diagrams until the first order, what is conventionally called the first Born approximation. This approximation, together with its self-consistent version, where a wider class of diagrams are regarded, have been incorporated into the SMEAGOL program [4], an ab initio electronic transport code based on the combination DFT-NEGF which uses the SIESTA code as a platform for electronic structure calculations. The implementations, together with many changes performed on SMEAGOL code, gave rise to the Inelastic SMEAGOL program for inelastic ab initio transport calculations. In this search for a more realistic description of electronic devices, another feature that should be taken into account is the fact that these devices most often can reach the 100 nm length scale with a large number of randomly distributed defects, which can lead to a fundamentally new transport regime, namely the Anderson localization regime [5]. In this work, the program Inelastic DISORDER was developed, which allows one to compute, by first principles, the elastic and inelastic transport properties from systems with tens of thousands of atoms with a large number of randomly positioned defects. The method combines electronic structure calculations via DFT with the NEGF formalism for transport, where the electron-phonon interactions are included with perturbation theory on the electron-phonon coupling matrix (Lowest Order Expansion). The developed method was applied to the study of graphene nanoribbons with joint attachment of hydroxyl impurities. It was observed that, by including the electron-phonon interaction, the transport properties experience significant changes, indicating that these interactions can influence the effects of localization by disorder. [1] M. Büttiker. Phys. Rev. B 33(5), 30203026 (1986). [2] E. Artacho, D. Sánchez-Portal, P. Ordejón, A. García, and J. M. Soler. Phys. Stat. Sol. (b) 215, 809817 (1999). [3] J. M. Soler, E. Artacho, J. D. Gale, A. García, J. Junquera, P. Ordejón, and D. Sánchez- Portal. J. Phys. Cond. Mat. 14, 27452779 (2002). [4] A. R. Rocha, V. M. García-Suárez, S. W. Bailey, C. J. Lambert, J. Ferrer, and S. Sanvito. Phys. Rev. B 73, 085414 (2006). [5] P. W. Anderson. Phys. Rev. 109, 1492 (1958).

O objetivo deste trabalho é examinar os efeitos causados pela interação elétron-fônon em pontos quânticos semicondutores polares. Primeiramente, nós apresentamos cálculos detalhados da taxa de espalhamento e do tempo de relaxação eletrônico em pontos quânticos simples (Single Quantum Dot - SQD) e em dois pontos quânticos acoplados (Coupled Quantum Dots - CQDs) devido à interação entre o elétron e os fônons longitudinais acústicos (LA) na presença e na ausência de campos externos, magnético ou elétrico. O regime de energia usado no cálculo do espalhamento eletrônico foi escolhido de forma que os fônons LA dominam o processo de espalhamento. Nós verificamos que na ausência de campo externo, a taxa de espalhamento do elétron por fônons LA entre dois níveis específicos é essencialmente determinada pela diferença de energia entre estes dois níveis. Observamos que um campo magnético modula fortemente a taxa de espalhamento. Verificamos que o processo de relaxação via multicanais desempenha um papel essencial no mecanismo de relaxação do elétron de estados excitados para o estado fundamental. Um campo magnético externo aumenta ainda mais a relaxação através de transições indiretas. Também fizemos um estudo teórico dos efeitos da interação elétron-fônons longitudinais ópticos (LO) em dois pontos quânticos acoplados compostos de InAs/AlInAs. Fizemos cálculos para o polaron ressonante num regime onde a energia de confinamento do elétron é comparável a energia do fônon L0 utilizando o formalismo da função de Green e teoria de perturbação considerando temperatura zero e finita. Observamos uma renormalização do estado fundamental obtida devido a absorção de fônons virtuais para uma temperatura T > O. Discutimos os efeitos do tunelamento entre os pontos quânticos e a sua influência nas propriedades eletrônicas e analisamos o espectro de absorção óptica neste sistema. Verificamos modificações nos orbitais eletrônicos como resultado direto do tunelamento assistido por fônons. Finalmente, avaliamos os efeitos da interação elétron-fônons L0 na densidade de estados do elétron confinado em pontos quânticos utilizando dois modelos distintos: Um modelo não-perturbativo e o formalismo da função de Green. Estudamos cada método separadamente e avaliamos a densidade de estados como função da temperatura e do confinamento lateral. Consideramos um sistema com apenas dois níveis eletrônicos de energia e comparamos os dois métodos avaliando as suas diferenças básicas. Utilizando o método não-perturbativo fizemos cálculos da densidade de estados para um regime de acoplamento forte entre o elétron e os fônons LO / The purpose of this work is to study effects of electron-phonons interactions in polar semiconductor quantum dots. Firstly, we present a detailed calculation on the electron-LA-phonon scattering rates and electron relaxation processes in single and coupled quantum dots in the absence and in the presence of external magnetic or electric fields. In the absence of external field, interplay among the effective confinement lengths in different directions as well as the phonon wavelength leads to a strong oscillation of the LA-phonon scattering rate between two levels. In other words, the scattering depends strongly on the geometry and confinement potential of the quantum dot. An external magnetic field also strongly modulates the scattering rate in severa1 orders of magnitude. The magnetic field induced effects are very similar in single quantum dot (SQD) and coupled quantum dots (CQDs) where the effective confinement strength in the x-direction affects strongly the scattering rate. However, we find that the multiple relaxation process plays an essential role for electron relaxing from the excited states to ground state both in single and coupled quantum dots. Including all possible relaxation channels, an external magnetic field enhances the relaxation through indirect transitions. Secondly, we present a theoretical study on the effects of electron-LO-phonon interaction in two coupled stacked InAs/InAIAs quantum dots. The contribution of resonant and nonresonant electron-LO-phonon coupling to the polaron states are obtained in the framework of t he Green function formalism and the perturbation approach at zero and finite temperatures. Ground state renormalization is found due to virtual phonon absorption at T > O. Tunneling effects between the dots have been addressed and their influente on the electronic properties and optical absorption are analyzed. Topological modifications of electronic orbitals are found as a result of phonon-assisted tunneling. Finally, we investigate the effects of electron-LO-phonon interaction on the electron density of states in quantum dots using two distinct models. A non-perturbative model and the Green function formalism. Within the non-perturbative model, we consider only two electronic levels in a quantum dot interacting to LO-phonons. An exact solution is obtained for the polaron states and spectral function. We evaluate the density of states in the regime at zero and finite temperature for severa1 values of the lateral confinement. We compare the density of states obtained within the two models. Furthermore, we study the polaron effects in strong electron-LO-phonon coupling regime based on the non-perturbative model

Neste trabalho estudamos os efeitos polarônicos sobre um gás de elétrons quase bidimensional presente em heteroestruturas semicondutoras (heterojunções e poços quânticos de GaAs-AlGaAs) sob a ação de um campo magnético uniforme aplicado na direção perpendicular a interface, através de teoria de perturbação de segunda ordem. Calculamos a massa ciclotrônica considerando a interação elétron-fonon LO e os efeitos de blindagem e não parabolicidade da banda de condução do GaAs. Os resultados obtidos são comparados com recentes dados experimentais de ressonância ciclotronica e apresentam ótima concordância. Estudamos também a energia de ligação do estado fundamental de uma impureza hidrogenóide localizada no interior de um fio quântico retangular de GaAs envolvido por AlGaAs, como função das dimensões do fio para varias alturas das barreiras de variacional, usando várias formas para a função de onda tentativa do sistema. Consideramos também a contribuição polarônica a energia de ligação. Comparamos nossos resultados com recentes cálculos da energia de ligação, efetuados por outros autores. / In this work we study the polaronic effects on the two dimensional electron gas present in semiconductor heteroestructures (GaAs-AlGaAs heterojunctions and quantum wells) when a uniform magnetic field is applied perpendicular to the interface, using second order perturbation theory. By taking into account the effect of nonparabolicity and screening of the electron-fonon LO interaction the calculated effective mass is compared to the recent experimental date. Good agreement is found with available date. The binding energies of a hydrogenic impurity located in quantum well wires of GaAs surrounded by AlGaAs are calculated as a functions of the size of the wire for several values of the heights of the potential barriers and diferent positions of the impurity inside the wire. We follow a variational approach choosing several trial wave functions for the ground state. The polaronic contribution to the binding energy is considered. We compare our results with those previously obtained by other authors.