The Schrödinger equation is nearly a century old, yet we are still in the midst of uncovering the remarkable phenomena emerging in many-body quantum systems. From superconductivity to anyonic quasiparticles, nature consistently surprises with its rich self-organization. To elucidate and grasp this variety, it is paramount to understand the phases of matter that can occur in many-body ground states, as well as their emergent collective excitations. Of particular interest are topological phases of matter, characterized by exotic excitations or edge phenomena. There exist by now several universal frameworks for gapped systems, i.e., those with an energy gap above the ground state. However, in the last decade, a multitude of gapless quantum wires---effectively one-dimensional systems---have been reported to be topologically non-trivial. A framework for their understanding and classification is missing. In addition to ground state order---topological or otherwise---a more complete picture involves the properties of excitations above the ground state. Alas, little is known about excitations beyond the universal low-energy regime. In part, this is due to a lack of analytical and numerical methods able to describe excitations at finite energies, especially in strongly-interacting systems beyond one dimension. In this thesis, we address these issues: firstly, we build a general understanding of topological phases in one dimension, including both gapped and gapless cases. In particular, we unify previously studied examples into a single framework. Secondly, we develop a novel numerical method for obtaining spectral functions in two dimensions---these give direct insight into the properties of excitations and are moreover experimentally measurable. Using this numerical method, we uncover a variety of robust properties of excitations at finite energies. Part I of this thesis concerns gapped and gapless topological phases in one dimension. In Chapter 2, we first treat the case of non-interacting fermions. Therein, we review the known classification of gapped phases before extending it to the gapless case, showing that exponentially-localized Majorana zero modes can still emerge at the edge when the bulk is gapless. Interacting gapped phases are discussed in Chapter 3, with a focus on symmetry-protected topological order. These have already been classified; our contribution is to provide a non-technical review of this classification as well as showing that many paradigmatic model Hamiltonians can be related to one another. Finally, Chapter 4 introduces the notion of symmetry-enriched quantum criticality, which we propose as a framework for classifying gapless phases. The key message is that in the presence of symmetries, a universality class can divide into distinct phases, characterized by the symmetry action on the low-energy scaling operators. This includes gapless topological phases, with examples hiding in plain sight; we clarify their stability and reinterpret previously studied examples. Part II studies the excitations above the ground states of two-dimensional quantum spin models. The main object of our study is the dynamic spin structure factor; this type of spectral function is reviewed in the first part of Chapter 5. The second part of this chapter introduces a novel matrix-product-state-based algorithm to efficiently compute it, opening a new window on the dynamics of two-dimensional quantum systems. We benchmark this numerical method in Chapter 6 on the exactly-solvable Kitaev model---a paradigmatic topological model realizing a quantum spin liquid. By adding non-integrable Heisenberg perturbations, we identify the first unequivocal theoretical realization of a proximate spin liquid: the ground state becomes conventionally ordered, yet the high-energy spectral properties are structurally similar to those of the nearby Kitaev spin liquid. The latter agrees with aspects of recent inelastic neutron scattering experiments on alpha-RuCl3. In Chapter 7, we turn to one of the oldest models in many-body quantum physics: the spin-1/2 Heisenberg antiferromagnet on the square lattice. Despite its venerable history, there is still disagreement about the physical origin of high-energy spectral features which low-order spin wave theory cannot account for. We provide a simple picture for this strongly-interacting-magnon feature by connecting it to a simple Ising limit. Lastly, Chapter 8 discusses the stability of quasiparticles---collective excitations behaving like a single emergent entity, of which magnons are a prime example. These are often known to be stable at the lowest energies and are presumed to decay whenever this is seemingly allowed by energy and momentum conservation. However, we show that strong interactions can prevent this from happening. We numerically confirm this principle of avoided decay in the (slightly-detuned) Heisenberg antiferromagnet on the triangular lattice. Moreover, we can even identify its fingerprints in existing experimental data on Ba3CoSb2O9 and superfluid helium. In this thesis, we thus enlarge our understanding of quantum phases and their excitations. The identification of the key principles of gapless topological phases in one dimension calls for direct analogues in higher dimensions, waiting to be uncovered. With regard to the robust properties of the excitations identified in this thesis, we are hopeful that these can be extended into a theory of quasiparticle properties away from the universal low-energy regime.

Neutron-rich rare-earth nuclei around the maximum of collectivity are predicted to exist with an extremely stable intrinsic configuration in their ground-state structure. Due to the high degree of axial symmetry and large deformation, these nuclei are also excellent candidates for having long-lived high-K isomers. The present work concerns a study of the structure of the yrast bands and a search for isomers in the neutron-rich 68Er isotopes. Excited states of 68Er isotopes were populated via multi-nucleon transfer reactions. A 859 MeV 136Xe-beam was used to bombard a 170Er-target. The experimental setup consisted of the high-resolution γ-ray spectrometer AGATA coupled to the heavy-ion magnetic spectrometer PRISMA. The experiment collected 2 TB of useful data corresponding to 3 days of effective beam time. Beam-like fragments were identified by the PRISMA specrometer placed at the grazing angle of 44 degrees. PRISMA allows for Z, A and q identification as well as TOF and velocity vector determination. This is required for the Doppler correction of the emitted γ rays detected in time coincidence with AGATA. A good Z and very clean A separation has been achieved in PRISMA. Making use of two-body kinematics, Doppler corrected γ-ray spectra for target-like fragments were obtained as well. Due to the novel techniques of PSA and γ-ray tracking, AGATA provided high-quality γ-ray spectra for both beam-like xenon and target-like erbium isotopes. Known yrast bands and isomeric states in neutron-rich erbium isotopes were observed. A candidate for the decay of an isomeric state with Eγ=184 keV  in 173Er, which has no previously known excited states, was identified.

The vehicle manufacturing sector is constantly evolving, and corporations are fully aware of increased consumer expectations for both driver and passenger´s comfort. SCANIA CV AB, as one of the largest Swedish manufacturers of commercial vehicles, has put an emphasis on this area. To guarantee these high-quality standards, several tests are conducted daily. Within this framework, this project aims to gain a better understanding of the phenomena associated with steering wheel vibrations. This project has an experimental focus on recreating sensitive driving conditions and addressing the vibration transfer paths to the main user interface such as the steering wheel. As widely known, the main problems related to vibrations come from resonance excitations. The most obvious solution would be to simply avoid matching any system´s eigenmodes with external excitations. Considering broadband excitations such as bumpy roads or engine vibrations, it is very unlikely that none of the critical frequencies is triggered. A better and more realistic idea would be minimizing the effects of these resonances by structural optimization. However, to do so, the eigenmodes should first be addressed. For this purpose, this project focused on identifying the annoying frequencies triggered while recreating sensitive driving scenarios. These sensitive scenarios were identified by Scania as circumstances in which the steering wheel feel gets altered. Specifically, it was decided to focus on road-induced vibrations, wheel-induced vibrations and engine-induced vibrations. The main findings show that during these tests, some resonances are triggered and interesting features are captured on the steering wheel. / Fordonstillverkningssektorn växer ständigt och företag är fullt medvetna om ökade konsumentförväntningar på både förarens och passagerarnas komfort. SCANIA CV AB, som en av de största svenska tillverkarna av kommersiella fordon, har lagt vikt vid detta område. För att garantera dessa högkvalitativa standarder genomförs flera tester dagligen.Inom denna ram syftar detta projekt till att få en bättre förståelse för de fenomen som är förknippade med rattvibrationer. Detta projekt har ett experimentellt fokus på att återskapa känsliga körförhållanden och adressera vibrationsöverföringsvägarna till huvudanvändargränssnittet, såsom ratten. Som allmänt känt kommer de största problemen relaterade till vibrationer från resonansexcitationer. Den mest uppenbara lösningen skulle vara att helt enkelt undvika att matcha något systems egenmoder med externa excitationer. Med tanke på bredbandsexcitationer som gropiga vägar eller motorvibrationer är det mycket osannolikt att ingen av de kritiska frekvenserna utlöses. En bättre och mer realistisk idé skulle vara att minimera effekterna av dessa resonanser genom strukturell optimering. För att göra det bör egenmoden först behandlas. För detta ändamål fokuserade detta projekt på att identifiera de irriterande frekvenser som triggades samtidigt som känsliga körscenarier återskapades. Dessa känsliga scenarier identifierades av Scania som omständigheter där rattkänslan förändras. Specifikt beslutades att fokusera på väginducerade vibrationer, hjulinducerade vibrationer och motorinducerade vibrationer. Huvudfynden visar att under dessa tester triggas vissa resonanser och intressanta funktioner fångas på ratten.

Ce mémoire présente une étude théorique des propriétés de localisation collective dans les superfluides de Bose désordonnés ou quasipériodiques. S'il est connu depuis Anderson que le désordre peut localiser les particules libres, comprendre ses effets dans les systèmes quantiques en interaction, où il est à l'origine de transitions de phase et d'effets de localisation non-Triviaux, représente aujourd'hui un défi majeur. En nous focalisant sur le cas d'un gaz de Bose dans le régime de faibles interactions, bien décrit par la théorie de Bogoliubov, nous étudions les transitions de localisation de ses excitations collectives dans différents contextes. Dans le cas d'un vrai désordre dans l'espace continu tout d'abord, nous développons un formalisme de désordre fort allant au-Delà des études antérieures, aboutissant à une description complète des propriétés de localisation des excitations en dimension arbitraire. Nous présentons un diagramme de localisation générique, et une interprétation microscopique de la propagation des excitations dans le désordre. Dans un second temps, nous considérons le cas d'un potentiel quasipériodique unidimensionel, aux propriétés intermédiaires entre un vrai désordre et un potentiel périodique. Notre traitement analytique et numérique du problème révèle une transition de localisation collective, que nous caractérisons et interprétons en termes de localisation dans un potentiel effectif multiharmonique. Pour finir, nous considérons le cas d'un gaz de Bose à deux composants. Nous développons le formalisme général pour étudier ces questions et décrivons la physique de base de ces systèmes qui présentent leurs propres spécificités. / In this thesis, we theoretically investigate the collective localization properties of weakly-Interacting Bose superfluids subjected to disordered or quasiperiodic potentials. While disorder has been recognized since Anderson to induce single-Particle localization, the interplay between disorder and interactions in quantum systems is today among the most challenging questions in the field, and underlies fascinating phase transitions and non-Trivial localization effetcs. Focusing on Bose gases in the weakly-Interacting regime for which the Bogoliubov theory proves a successful tool, we study the localization transitions of collective excitations in several contexts. First, in the case of a continuous true disorder, we develop a strong-Disorder formalism going beyond previous studies, providing us with a complete description of the localization behaviour of collective excitations in arbitrary dimension. A generic localization diagram is obtained and the transport of excitations in the disorder is microscopically interpreted. Secondly, we consider the case of one-Dimensional quasiperiodic potentials, which are known to display intermediate properties between periodic and disordered ones. We perform a numerical and analytical treatment of the localization problem of collective excitations, allowing us to quantitatively characterize and interpret the localization transition in terms of an effective multiharmonic problem. Finally, we set up the general inhomogeneous formalism to address such issues in multicomponent Bose gases, and enlighten the basic physic of such systems, which are known to exhibit their own specific features.

Les désintégrations semileptoniques du méson $B$ participent à la détermination de certains paramètres fondamentaux du Modèle Standard. Ce travail décrit essentiellement l'étude des deux canaux de désintégrations $B_s \to D_s \ell \bar\nu_\ell$ et $B\to D^{**} \ell\bar\nu_\ell$ (où les $D^{**}$ sont les premières excitations orbitales des mésons $D$ ayant une parité positive). Le cadre théorique est celui de la QCD sur réseau qui, en discrétisant l'espace-temps, permet de calculer non perturbativement les fonctions de Green de la théorie. En utilisant l'action à masse twistée avec deux saveurs dégénérées de quarks dynamiques ($N_f=2$), nous avons commencé par étudier la spectroscopie des états charmés scalaires $D_0^*$ et tenseurs $D_2^*$. Ensuite, nous avons réalisé la détermination du facteur de forme $G_s(1)$ décrivant le processus $B_s\to D_s \ell \bar\nu_\ell$ dans le Modèle Standard. Ce paramètre offre un moyen d'extraire l'élément de la matrice CKM $V_{cb}$. Par ailleurs, et pour la première fois en QCD sur réseau, nous avons déterminé les rapports $F_0(q^2)/F_+(q^2)$ et $F_T(q^2)/F_+(q^2)$ dans la région proche du recul nul: ces contributions sont en effet nécessaires afin de discuter ce canal de désintégration dans certains modèles au-delà du Modèle Standard. Enfin, une étude préliminaire du canal de désintégration $B\to D^{**}$ a été abordée où nous avons trouvé une valeur non nulle de l'élément de matrice décrivant la désintégration $B\to D_0^*$ à recul nul contrairement de ce qui est connu à la limite des quarks lourds. Dans le cas du $B \to D_2^*$, nos résultats ont montré un signal indiquant une différence par rapport aux prédictions de masse infinie. Ces calculs sont indispensables afin de tirer une conclusion plus solide concernant le ``puzzle 1/2 vs 3/2''. / Semileptonic decays of $B$ mesons provide a rich source of knowledge for determining fundamental parameters of the Standard Model. This work reports mainly on the study of two semileptonic decay channels: the $B_s \to D_s \ell\bar\nu_\ell$ and $B\to D^{**} \ell\bar\nu_\ell$ (where the $D^{**}$ are the first orbitally excited states of the $D$ mesons having a positive parity). The theoretical framework is Lattice QCD which is considered as the only satisfying approach which calculates in a non perturbative way the transition amplitudes from first principles. By using the twisted mass QCD on the lattice with $N_f = 2$ dynamical flavors we studied, first, the spectroscopy of the scalar $D_0^*$ and the tensor $D_2^*$ states. Then, we determined the normalization $G_s(1)$ of the form factor dominating $B_s \to D_s \ell \bar\nu_\ell$ in the Standard Model which provides a means of extracting the CKM matrix element $V_{cb}$. Next, we make the first lattice determination of $F_0(q^2)/F_+(q^2)$ and $F_T(q^2)/F_+(q^2)$ near the zero recoil. The obtained results are important for the discussion of this decay in various scenarios of physics beyond the Standard Model. Finally, we did a preliminary study of $B\to D^{**}$ where we have obtained a non vanishing matrix element corresponding to the decay of $B$ into the $D_0^*$ at zero recoil contrary to what was known in the heavy quark limit. Moreover, the computations corresponding to $B\to D_2^*$ show a signal indicating a difference with respect to the infinite mass limit prediction. These results are important to draw a firm conclusion on the ``1/2 vs 3/2 puzzle''.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-06-09T12:29:52Z No. of bitstreams: 1 leonardedonascimentorodrigues.pdf: 2214371 bytes, checksum: 63b390c0d1258662578ebfbb5c79419b (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-06-26T20:28:58Z (GMT) No. of bitstreams: 1 leonardedonascimentorodrigues.pdf: 2214371 bytes, checksum: 63b390c0d1258662578ebfbb5c79419b (MD5) / Made available in DSpace on 2017-06-26T20:28:58Z (GMT). No. of bitstreams: 1 leonardedonascimentorodrigues.pdf: 2214371 bytes, checksum: 63b390c0d1258662578ebfbb5c79419b (MD5) Previous issue date: 2016-09-16 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O espalhamento inelástico de luz tem sido extensivamente utilizado no estudo de materiais semicondutores e se tornou uma técnica indispensável para o entendimento de processos físicos fundamentais. Os efeitos das interações em sistemas eletrônicos quantizados como o gás de elétrons bidimensional foram investigados por meio do espalhamento inelástico de luz, o qual permite entender a natureza das excitações coletivas que são conhecidas como excitações de densidade de carga (CDE) e excitações de densidade de spin (SDE). CDE são oscilações plasmônicas resultantes do acoplamento entre as flutuações de carga via interações de Coulomb e correlação e troca, enquanto SDE ocorrem apenas em decorrência da presença dos efeitos de correlação e troca. CDE são ativas quando a energia do laser é ressonante com o gap óptico do semicondutor (regime de ressonância próxima) e as polarizações da luz incidente e espalhada são paralelas entre si. SDE é também ativa em condições de ressonância próxima e possui polarizações da luz incidente e espalhada perpendiculares entre si. Todavia, tal cenário rompe quando a luz do laser coincide com as energia das transições interbandas do material (regime de ressonância extrema). Em adição as excitações coletivas emergem transições anômalas de gás de elétrons tipo não interagente conhecidas como excitações de partícula independente (SPE). A física envolvida em tais transições ainda parece não ser completamente entendida. Neste trabalho, são apresentados resultados experimentais e teóricos via espalhamento Raman eletrônico oriundos de gás de elétrons bidimensional realizados a partir de poços quânticos simples com o intuito de compreender a natureza das excitações de partícula independente. A medida experimental e os cálculos presentes na tese consistem de um poço quântico simples de 250Å de GaAs (arseneto de gálio) com dopagem modular e densidade eletrônica total de 8.81 x 1011cm-2. Também foi considerado o acoplamento das flutuações de densidade de carga com o fônon óptico longitudinal do GaAs. Em adição, é realizado um cálculo teórico de uma estrutura de poço quântico simples considerado estreito de 100Å de GaAs com dopagem modular e densidade eletrônica total de 1.2 x 1012cm-2. Portanto, o objetivo deste trabalho é mostrar em um caminho claro e transparente a situação física da existência das excitações eletrônicas em gás de elétrons. O trabalho mostrou que, no regime de extrema ressonância, as oscilações de plasma se dividem em duas contribuições: um conjunto de excitações coletivas renormalizadas (plasmons) e transições eletrônicas não renormalizadas (SPE). A fim de alcançar esse propósito, o trabalho evidenciou que o espalhamento Raman eletrônico pertence a uma mesma classe de problemas como o oscilador harmônico amortecido, acoplado e forçado assim como o estado supercondutor na teoria BCS de metais normais. A comparação entre os dados experimentais e teóricos mostrou um excelente acordo. / The inelastic light scattering has been widely used in the study of the semiconductor materials and it has become an indispensable technique for the understanding of fundamental physical processes. The effects of the electronic interactions on quantized electronic systems as two-dimensional electron gas are investigated through means of Raman scattering which allows understand the nature of collective excitations which are known as charge density excitations (CDE) and spin density excitations (SDE). CDE are plasmonic oscillations arising from the coupling between charge fluctuations via Coulombian and exchange-correlation interactions while SDE occur only when exchange-correlation effects are present. CDE is active when the laser energy is resonant with a semiconductor optical gap (near resonance regime) and the incoming and outgoing light polarizations are parallel to each other and SDE has incoming and outgoing light polarizations perpendicular to each other. Nevertheless, such a picture breaks down when the laser matches interband transitions energies of the material (extreme resonance regime). In addition to the collective excitations emerge anomalous transitions of the electron gas noninteracting-like known as single-particle excitations (SPE). The physics of such transitions is still not completely understood. In this work, were present experimental and theoretical results via electronic Raman scattering originating from the two-dimensional electron gas carried out from single quantum wells with the aim of understand the nature of the single-particle excitations. The experimental measures and calculations present in the thesis consists of the GaAs (gallium arsenide) single quantum well of a 250Å wide with modulation-doped and total electronic density of 8.81 x 1011cm2. It was also considered the coupling of the charge fluctuations with the longitudinal optical phonon of GaAs. In addition, it is performed a theoretical calculation of a structure of the GaAs single quantum well considered narrow of a 100Å wide with modulation-doped and and total electronic density of 1.2 x 1012cm2. However, the goal of this work is to show in a clear and transparent way the physical situation of the existence of electronic excitaions in electron gas. The work showed that, in extreme resonance regime, the plasma oscillations splits into two contributions: a set of renormalized collective excitations (plasmons) and unrenormalized electronic transitions (SPE). In order to accomplish this purpose, the study showed that electronic Raman scattering belongs to the same class of problems such as a set of forced, coupled and damped harmonic oscillators or formation of the superconducting state in BCS theory of normal metals. Comparison between experiment and theory shows an excellent agreement.

Pendant sa mission, un lanceur est soumis à des excitations large bande, sévères, aérodynamiques, de provenances diverses, qui peuvent mettre en danger la survivabilité de la charge utile et de l’équipement électronique du véhicule, et par conséquent le succès de la mission. Les structures aérospatiales sont généralement caractérisées par l’utilisation de matériaux composites exotiques des configurations et des épaisseurs variantes, ainsi que par leurs géométries largement complexes. Il est donc d’une importance cruciale pour l’industrie aérospatiale moderne, le développement d’outils analytiques et numériques qui peuvent prédire avec précision la réponse vibroacoustique des structures larges, composites de différentes géométries et soumis à une combinaison des excitations aéroacoustiques. Récemment, un grand nombre de recherches ont été menées sur la modélisation des caractéristiques de propagation des ondes au sein des structures composites. Dans cette étude, la méthode des éléments finis ondulatoires (WFEM) est utilisée afin de prédire les caractéristiques de dispersion des ondes dans des structures composites orthotropes de géométries variables, nommément des plaques plates, des panneaux simplement courbés, des panneaux doublement courbés et des coques cylindriques. Ces caractéristiques sont initialement utilisées pour prédire la densité modale et le facteur de perte par couplage des structures connectées au milieu acoustique. Par la suite, la perte de transmission (TL) à large bande des structures modélisées dans le cadre d’une analyse statistique énergétique (SEA) dans un contexte ondulatoire est calculée. Principalement en raison de la complexité géométrique importante de structures, l’utilisation des éléments finis (FE) au sein de l’industrie aérospatiale est souvent inévitable. L’utilisation de ces modèles est limitée principalement à cause du temps de calcul exigé, même pour les calculs dans la bande basses fréquences. Au cours des dernières années, beaucoup de chercheurs travaillent sur la réduction de modèles FE, afin de rendre leur application possible pour des systèmes larges. Dans cette étude, l’approche de SOAR est adoptée, afin de minimiser le temps de calcul pour un système couplé de type structurel-acoustique, tout en conservant une précision satisfaisante de la prédiction dans un sens large bande. Le système est modélisé sous diverses excitations aéroacoustiques, nommément un champ acoustique diffus et une couche limite turbulente (TBL).La validation expérimentale des outils développés est réalisée sur un ensemble de structures sandwich composites orthotropes. Ces derniers sont utilisés afin de formuler une approche couche équivalente unique (ESL) pour la modélisation de la réponse spatiale du panneau dans le contexte d’une approche de matrice de raideur dynamique. L’effet de la température de la structure ainsi que du milieu acoustique sur la réponse du système vibroacoustique est examiné et analysé. Par la suite, un modèle de la structure SYLDA, également fait d’un matériau sandwich orthotrope, est testé principalement dans le but d’enquêter sur la nature de couplage entre ses divers sous-systèmes. La modélisation ESL précédemment développée est utilisé pour un calcul efficace de la réponse de la structure dans la gamme des basses et moyennes fréquences, tandis que pour des fréquences plus élevées, une hybridisation WFEM / FEM pour la modélisation des structures discontinues est utilisé. / During its mission, a launch vehicle is subject to broadband, severe, aeroacoustic and structure-borne excitations of various provenances, which can endanger the survivability of the payload and the vehicles electronic equipment, and consequently the success of the mission. Aerospace structures are generally characterized by the use of exotic composite materials of various configurations and thicknesses, as well as by their extensively complex geometries and connections between different subsystems. It is therefore of crucial importance for the modern aerospace industry, the development of analytical and numerical tools that can accurately predict the vibroacoustic response of large, composite structures of various geometries and subject to a combination of aeroacoustic excitations. Recently, a lot of research has been conducted on the modelling of wave propagation characteristics within composite structures. In this study, the Wave Finite Element Method (WFEM) is used in order to predict the wave dispersion characteristics within orthotropic composite structures of various geometries, namely flat panels, singly curved panels, doubly curved panels and cylindrical shells. These characteristics are initially used for predicting the modal density and the coupling loss factor of the structures connected to the acoustic medium. Subsequently the broad-band Transmission Loss (TL) of the modelled structures within a Statistical Energy Analysis (SEA) wave-context approach is calculated. Mainly due to the extensive geometric complexity of structures, the use of Finite Element(FE) modelling within the aerospace industry is frequently inevitable. The use of such models is limited mainly because of the large computation time demanded even for calculations in the low frequency range. During the last years, a lot of researchers focus on the model reduction of large FE models, in order to make their application feasible. In this study, the Second Order ARnoldi (SOAR) reduction approach is adopted, in order to minimize the computation time for a fully coupled composite structural-acoustic system, while at the same time retaining a satisfactory accuracy of the prediction in a broadband sense. The system is modelled under various aeroacoustic excitations, namely a diffused acoustic field and a Turbulent Boundary Layer (TBL) excitation. Experimental validation of the developed tools is conducted on a set of orthotropic sandwich composite structures. Initially, the wave propagation characteristics of a flat panel are measured and the experimental results are compared to the WFEM predictions. The later are used in order to formulate an Equivalent Single Layer (ESL) approach for the modelling of the spatial response of the panel within a dynamic stiffness matrix approach. The effect of the temperature of the structure as well as of the acoustic medium on the vibroacoustic response of the system is examined and analyzed. Subsequently, a model of the SYLDA structure, also made of an orthotropic sandwich material, is tested mainly in order to investigate the coupling nature between its various subsystems. The developed ESL modelling is used for an efficient calculation of the response of the structure in the lower frequency range, while for higher frequencies a hybrid WFEM/FEM formulation for modelling discontinuous structures is used.

La rationalisation des processus chimiques élémentaires aux surfacesest d'intérêt primordial pour de nombreux phénomènes naturels ou d'intérêttechnologique. D'un point de vue fondamental, la façon dont l'énergie, concomitanteà toute réaction chimique, est distribuée parmi les degrés de liberté des moléculesformées et/ou transférée à la surface est loin d'être systématisée. Dans ce travail,des simulations, reposant sur la méthode des trajectoires quasi-classiques (QCT),sont réalisées pour examiner cette problématique lors de recombinaisons demolécules d'hydrogène (H2) et d'azote (N2) résultant de l'abstraction d'atomesadsorbés via collision par un atome provenant de la phase gazeuse sur des surfacesde Tungstène - W(100) et W(110) - à taux de couverture non nul. Ces processussont ici étudiés pour leur intérêt en physique des interactions plasma-paroi. Dessurfaces d'énergie potentielle, construites à partir de calculs de structure électroniquebasés sur la théorie de la fonctionnelle densité (DFT), sont utilisées pour simuler,dans le cadre de la mécanique classique - incluant les corrections semi-classiquespertinentes - les processus ultrarapides dit de "Eley-Rideal" et par "atomes-chauds"(sub-picoseconde). La mise en place de modèle effectifs, pour tenir compte de ladissipation de l'énergie aux phonons de la surface et aux excitations électroniques(paires électron-trou), permet de rationaliser la dynamique non-adiabatique del'abstraction atomique aux surfaces métalliques. / The rationalization of elementary processes at surfaces is of prime importance for numerous natural and technological areas. From a fundamental pointof view, the way the energy concomitant to any chemical reaction is distributed among the desorbing molecules degrees-of-freedom and the surface is far frombeing fully pictured. In this work, quasiclassical molecular dynamics (QCT)simulations have been carried out to investigate this issue for the recombination ofH2 and N2 resulting from atomic adsorbate abstraction by atom scattering off theW(100) and W(110) covered surfaces, these processes being of relevance inplasma-wall interactions. Potential energy surfaces, built from density functional(DFT) theory calculations, have been used to simulate, within the framework ofclassical dynamics (including semi-classical corrections), the subpicosecond Eley-Rideal and Hot-Atom processes. The implementation of effective models to accountfor energy dissipation to surface phonons and electron-hole pair excitations, have allowed to rationalize the non-adidabatic dynamics of atom abstraction at metalsurfaces.

This thesis addresses the investigation of doubly excited 2l´nl states in helium atoms and double core excitations in solid lithium compounds. Measurements on He are made in field free environments and under the influence of electric and magnetic fields, using synchrotron based inelastic photon scattering. Cross sections for scattering to singly excited final states are directly determined and compared to theoretical results and are found to be in excellent agreement. Radiative and spin-orbit effects are quantified and are shown to play an important role in the overall characterization of highly excited He states below the N =2 threshold. A dramatic electric field dependence is also observed in the flourecence yield already for relatively weak fields. This signal increase, induced by electric as well as magnetic fields, is interpreted in terms of mixing with states of higher fluorescence branching ratios. Double core excitations at the lithium site in solid lithium compounds are investigated using resonant inelastic x-ray scattering (RIXS). The lithium halides LiF, LiCl, LiBr and LiI are studied as well as the molecular compounds Li2O, Li2CO3 and LiBF4. States with one, as well as both, of the excited electrons localized at the site of the bare lithium nucleus are identified, and transitions which involve additional band excitations are observed. A strong influence of the chemical surrounding is found, and it is discussed in terms of the ionic character of the chemical bond.

String theory is currently the best-known candidate for a theory of quantum gravity, having the necessary ingredients to describe all known elementary particles and interactions. It also includes several novel features, arising, for instance, from the additional six compact dimensions required for its internal consistency, making it the natural arena to construct extensions of the Standard Model. In this thesis, we analyze some of the new phenomenological aspects introduced by string theory within the framework of low energy effective theories, focusing on their applications to cosmology, astrophysics and collider experiments. We first consider a particular realization of the brane-world scenario in branonium bound states, showing that the orbital motion of a probe antibrane about a central brane stack leads to a resonant amplification of its world-volume scalar modes. We analyze the cosmological development of this process and also its potential relevance for either dark or baryonic matter generation in the early universe. We then focus on the spectrum of quark and lepton string excitations in warped compactifications, modeled by an effective 5-dimensional Randall- Sundrum throat. Motivated by the observed fermion mass hierarchy, we show that the spin-3/2 Regge excitation of the right-handed top quark is the lightest of such resonances in a significant region of parameter space, possibly lying below the TeV scale, and discuss its potential signatures at the Tevatron and at the LHC. Finally, we study the emission of sub-eV scalar particles by maximally rotating Kerr black holes, motivated by the recent string axiverse proposal. We focus on the spectrum of unstable scalar bound states in the superradiant regime, leading to an exponentially large axion cloud around astrophysical black holes, and analyze two semi-analytical methods for computing the growth rate of this instability, comparing the obtained results with previous analytical and numerical analyses.