La formation et l'activité des objets compacts, tels que Trous Noirs ou étoiles à Neutrons, s'accompagne couramment d'éjection de matière ionisée sous forme de jets à la vitesse proche de celle de la lumière (vitesses relativistes). Se propageant dans le milieu environnant, par exemple Milieu Interstellaire, ces jets conduisent inéluctablement à la formation d'ondes de choc relativistes. Une forte turbulence magnétique et une population d'électrons accélérés sont requises afin de tenir compte de l'émission radiative non-thermique de ces chocs. L'approche naturelle de ce problème, décrivant de manière auto-consistante la structure du choc non-collisionnel, est celle de la physique cinétique des plasmas en régime relativiste. L'aspect essentiel de cette approche est l'étude du précurseur du choc, sous forme d'un faisceau de protons très énergétiques. Un ensemble d'instabilites plasma y prend lieu et dissipe l'energie du choc sous forme de micro-turbulence électromagnétique, électrons chauffés et particules accélérées. Ce cadre conceptuel emmène à reconsidérer le processus de transport de particules charges autour du choc. Deux études indépendantes, effectuées pendant la thèse, montrent que les lois de diffusion en aval et amont du choc se mettent sous une forme concise, en loi de puissance en fonction de l'énergie des particules et de l'intensité de la micro-turbulence magnétique. Les lois de diffusion, dérivées à l'aide des simulations Monte-Carlo et analytiquement, chiffrent l'énergie maximale des protons accélérés au choc à 10^15 eV, si le facteur de Lorentz du choc est très grand devant 1. Cette limite se situe loin de l'énergie maximale des Rayons Cosmique et rend les chocs relativistes comme accélérateurs de particules inefficaces aux énergies les plus extrêmes. Le rayonnement, issu de l'accélération des électrons, atteint plusieurs GeV et corrobore l'idée que les chocs externes des Sursauts Gamma peuvent émettre à de telles énergies. L'approche alternative de l'étude des chocs, simulations Particle-In-Cell, m'as permis d'étudier la formation, structuration et évolution des chocs modérément relativistes dans une géométrie spatiale 1D. L'auto-reformation du front d'un choc perpendiculaire, connue dans le régime non-relativiste, persiste dans le régime moyennement relativiste et exhibe un front de choc non-stationnaire. A magnétisation basse, les électrons sont préchauffés dans le pied du choc par l'instabilité de Buneman entre protons réfléchis et électrons incidents, mais leur température en aval du choc reste plus faible que celle des protons. A magnétisation croissante, l'instabilité Maser Synchrotron devient essentielle dans la structuration du front de choc, avec émission d'un fort précurseur électromagnétique a partir du front de choc. Dans ce cas les électrons se mettent en équipartition avec les protons. Ces simulations 1D ne montrent pas d'évidence d'accélération des particules et des simulations 2D (3D) sont nécessaires.

In this thesis, the properties of the Bose-Einstein condensation (BEC) in low dimensions are reviewed. Three dimensional weakly interacting Bose systems are examined by the variational method. The effects of both the attractive and the repulsive interatomic forces are studied. Thomas-Fermi approximation is applied to find the ground state energy and the chemical potential. The occurrence of the BEC in low dimensional systems, is studied for ideal gases confined by both harmonic and power-law potentials. The properties of BEC in highly anisotropic trap are investigated and the conditions for reduced dimensionality are derived.

This thesis addresses the isotropic diffuse gamma-ray background, as measured by the Fermi gamma ray space telescope, and its implications for indirect detection of dark matter. We describe the measurement of the isotropic background, including also an alternative analysis method besides the one published by the Fermi-LAT collaboration. The measured isotropic diffuse background is compatible with a power law differential energy spectrum with a spectral index of  -2.41 ± 0.05 and -2.39 ± 0.08, for the two analysis methods respectively. This is a softer spectrum than previously reported by the EGRET experiment. This rules out any dominant contribution with a significantly different shape, e.g. from dark matter, in the energy range 20 MeV to 102.4 GeV. Instead we present upper limits on a signal originating from annihilating dark matter of extragalactic origin. The uncertainty in the dark matter signal is primarily dependent on the cosmological evolution of the dark matter distribution. We use recent N-body simulations of structure formation, as well as a semi-analytical calculation, to assess this uncertainty. We investigate three main annihilation channels and find that in some, but not in all, of our scenarios we can start to probe, and sometimes rule out, interesting parameter spaces of particle physics models beyond the standard model.We also investigate the possibility to use the angular anisotropies of the annihilation signal to separate it from a background originating from conventional sources, e.g. from active galactic nuclei. By carefully modelling the performance of the Fermi gamma-ray space telescope and galactic foregrounds we find that this method could be as sensitive as using information from the energy spectrum only. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript.</p>

Thesis; Thesis information not provided; 1 Feb 2003. / Published through the Information Bridge: DOE Scientific and Technical Information. "JLAB-PHY-03-38" "DOE/ER/40150-2762" Bin Zhang. 02/01/2003. Report is also available in paper and microfiche from NTIS.

Orientadores: Alex Eduardo de Bernardini, Marcelo Moraes Guzzo / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataglin / Made available in DSpace on 2018-08-17T16:24:26Z (GMT). No. of bitstreams: 1 DuartePerico_EderLeonardo_M.pdf: 2896271 bytes, checksum: 3596fcdea9450fe936e36e96d5ffbad3 (MD5) Previous issue date: 2011 / Resumo: O assunto deste estudo é formação de estruturas em grandes escalas em um universo plano dominado por radiação, matéria escura fria e constante cosmológica como modelo de energia escura no caso particular de presença de um gás degenerado de férmions (GDF) não interagentes como fluído de teste. Nosso modelo admite uma evolução linear das perturbações cosmológicas como também se limita a perturbações escalares, responsáveis pela formação de estruturas. O objetivo principal é analisar a contribuição do GDF no espectro de potências da matéria no presente após uma evolução isentrópica das perturbações primordiais, e compará-la com resultados obtidos para neutrinos cosmológicos. Neste caso, teremos uma mudança contínua de comportamento do GDF de gás ultra-relativístico para não relativístico, o qual aconteceria durante o período de domínio da matéria. Com o objetivo de obter expressões analíticas para a evolução temporal das perturbações do GDF tivemos que fazer o estudo destas em quatro casos diferentes: no período de domínio da radiação, no período de domínio da matéria escura, na escala de super-horizonte durante a transição entre estes dois períodos, e finalmente no período de domínio da constante cosmológica. Fomos bem sucedidos ao chegar a resultados consistentes utilizando dois caminhos diferentes: no primeiro, usando a equação de conservação do tensor momento energia para um gás ideal de férmions totalmente degenerado e não interagente, e no segundo, usando a equação de Boltzmann para um gás de férmions fortemente degenerado e também não interagente. Os dois métodos analíticos levam à mesma solução para as perturbações do GDF quanto escritas até primeira ordem em teoria de perturbações. De forma complementar, os nossos resultados numérico mostram um aumento no espectro de potências da matéria para escalas intermediárias se comparado com a contribuição dos neutrinos massivos. Finalmente estendemos nossa análise numérica ao substituirmos a matéria escura fria CDM e a constante cosmológica por um gás generalizado de Chaplygin GCG como modelo efetivo para o setor escuro do universo, mantendo as condições de contorno que envolvem as densidades médias, e as condições iniciais para as perturbações / Abstract: The subject of this study is the formation of large scale structures (LSS) in a at universe dominated by radiation, cold dark matter and cosmological constant - as a dark energy model - in presence of a degenerate fermionic gas (GDF) as non-interacting test fluid. Our model assumes a linear evolution of cosmological perturbations as well as merely scalar perturbations responsible for structure formation. Our main objective is to analyze the contribution of the GDF in the matter power spectrum today, after an isentropic evolution of primordial perturbations and a continuous change of behavior of ultra-relativistic for non-relativistic GDF, which occurs during the matter domination era in our model. To obtaining analytical expressions for temporal evolution of the GDF perturbations we did study them in four different cases: during the radiation domination era, the dark matter domination, the super-horizon scale limit during the transition between these first two periods and finally during the cosmological constant domination era. We get these results using two different approaches: first, using the conservation equation of the stress-energy tensor for a perfect and non-interacting and fully degenerated fermionic gas, and second, using the Boltzmann equation for a non-interacting and strongly degenerated fermionic gas. Both methods lead to the same analytical solution for GDF perturbations at first order on perturbation theory. On the other hand, our numerical results show an increase in the power spectrum of matter for intermediate scales if compared it with the contribution of massive neutrinos. Finally, we show the change on the results of the standard model of cosmology (CDM) when we exchanging the cold dark matter CDM and the cosmological constant for a generalized Chaplygin gas GCG how effective model of twice old fluids with the same mean density of energy / Mestrado / Cosmologia / Mestre em Física

Abstract The problem of describing a system of many interacting particles is one of the most fundamental questions in physics. One of the central theories used in condensed matter physics to address the problem is the Fermi liquid theory developed by L. D. Landau in the 1956. The theory describes interacting fermions, and can be used to explain transport phenomena of electrons in metals and dynamics of helium three. Even when the theory is not directly applicable, it forms a basis against which other, more sophisticated theories can be compared. this thesis the Fermi liquid theory is applied to 3He-4He-mixtures at temperatures where the bosonic 4He part is superfluid, and the mechanical properties of the system are largely determined by the 3He component, treated as a degenerate normal Fermi liquid. The dynamics of strongly interacting liquid 3He can be described as a collection of quasiparticles, elementary excitations of the system, which interact only weakly. In 3He-4He mixtures the interactions can be continuously tuned by changing the temperature and the concentration of the mixture. The scattering time of quasiparticles depends on temperature, and thus the transition from the hydrodynamic limit of continuous collisions at higher temperatures to the collisionless ballistic limit at low temperatures can be studied. This gives invaluable information on the role of the interactions in the dynamics of the system. In this work, by using the Fermi liquid theory and Boltzmann transport equation, the dynamics of helium mixture disturbed by a mechanical oscillator is described in the full temperature range. The solution necessarily is numeric, but new analytical results in the low temperature limit are obtained as well. The numerical approach enables one to study various boundary conditions thoroughly, and allows application of the theory to a specic geometry. It is shown that in order to explain the experimental observations, it is necessary to take into account the reflection of quasiparticles from the walls of the container. For suitable choice of oscillator frequency and container size, second sound resonances are observed at higher temperatures, while in the ballistic limit quasiparticle interference can be seen. The numerical results are in quantitative agreement with experiments, thus attesting the accuracy of Fermi liquid theory. In particular, the previously observed decrease of inertia of a mechanical oscillator immersed in helium at low temperatures is reproduced in the calculations, and is explained by elasticity of the fluid due to Fermi liquid interactions.

Les progrès techniques réalisés dans le cadre des expériences sur les gaz de fermions ultrafroids ont engendré une émulation particulièrement importante ces dernières années. En effet, ces dispositifs expérimentaux permettent de produire des systèmes gazeux ≪ à la carte ≫, notamment grâce au phénomène de résonances de Feshbach qui permet de contrôler le signe de la longueur de diffusion a par application d'un champ magnétique extérieur. Il est alors possible de générer aussi bien une interaction attractive (a < 0) que répulsive (a > 0). La résonance de Feshbach en elle-même se trouve en a → ±∞, cette limite correspondant à un régime de fortes corrélations entre les particules. De plus, dans la région où a est positive, des états lies moléculaires (bosoniques car formés de deux fermions) peuvent se former. En-dessous d'une certaine température, une phase superfluide peut alors apparaitre, et une transition de phase continue entre l'état bosonique et l'état fermionique peut être observée (BEC-BCS crossover). En fonction de la position dans le diagramme de phases, les modes collectifs possèderont des caractéristiques (fréquence, amortissement) différentes. En ce sens, ils constituent une sonde de l'état de la matière et une connaissance précise de ces modes est par conséquent très importante. Le travail présenté dans cette thèse comporte une caractérisation détaillée de plusieurs modes collectifs dans la phase normale du système atomique. L'étude repose principalement sur l'équation de Boltzmann, que nous résolvons de deux façons différentes. La première consiste à utiliser une méthode des moments ≪ améliorée ≫ (c'est-à-dire d'ordre supérieur). La seconde est numérique et a nécessité l'écriture d'un programme de simulation permettant l'incorporation de tous les effets de milieu (potentiel de champ moyen et section efficace). Une attention toute particulière a été apportée à la mise en place des simulations afin de reproduire le plus fidèlement possible les conditions expérimentales. Les techniques expérimentales permettent également désormais la création de gaz polarisés. Nous présenterons donc dans ce travail une étude de ces gaz utilisant notre programme de simulation (mise en évidence des différents régimes de collision), puis une étude plus théorique ayant pour principal objectif d'établir le diagramme de phase encore méconnu de ces gaz particuliers, et enfin de proposer une méthode de calcul des effets de milieu, les techniques habituelles utilisées pour les gaz non polarisés n'étant plus valables / Technical progress on ultra-cold Fermi gases experiments induced numerous studies for the last few years. Using these experimental setups, it is effectively possible to generate ultra-cold gases with selected properties, in particular through the Feshbach resonances phenomenon. This allows us to set the sign of the scattering length a using an external magnetic field. It is then possible to have an attractive interaction (a < 0) as well as a repulsive one (a > 0). The Feshbach resonance itself is defined for infinite values of a (positive or negative), which corresponds to a strongly interacting regime. Moreover, when a > 0, molecular bound states (bosonic because they are made with two fermionic atoms) can appear. Thus, below a critical temperature, a superfluid phase can emerge and a crossover can be observed (from the BEC to BCS superfluid states). Depending on the position on the phase diagram, frequency and damping of collective modes will be different. This is why the collective modes are good probes of the system phase. A precise extensive knowledge of their characteristics is thus very important. This thesis presents a complete study of some of these collective modes in the normal phase. This work mainly relies on the Boltzmann equation which will be solved in two different ways: firstly, with an improved (higher order) version of the so-called moments method; secondly with a numerical solution that has required to write a numerical code in order to take into account the in-medium effects (mean field potential and in-medium cross section). Particular attention has been paid to numerical simulations in order to reproduce as closely as possible the experimental conditions. Moreover, experimental procedures now allow to create spin unbalanced gases. We have shown in this work a study of these systems using the numerical resolution of the Boltzmann equation. Moreover, we have developed a theoretical approach in order to build the phase diagram of these polarized gases, which is not fully described yet. Finally, we have suggested a method to determine the in-medium effects, with the aim to solve the problem emerging with the usual method used in the balanced case

This thesis is concerned with the growth, electronic properties and modification of hydrogenated amorphous carbon films of a thickess range of 50-300 nm, which have been deposited using rf plasma-enhanced chemical vapour deposition. These films may be subdivided into two types according to the electrode on which they are grown and the resulting film properties. These are polymer-like amorphous carbon or PAC, and diamond-like amorphous carbon or DAC. PAC possesses a wide optical band gap (2.7 eV), high resistivity (1014 - 10 15 Ocm) and low density of paramagnetic defects (~ 10 17 spins cm-3). The dominant current transport mechanism at room temperature has been observed to be hopping conduction at low electric fields and space-charge-limited current at high electric fields. The addition of nitrogen gas to the plasma to incorporate nitrogen within the film has been shown to move the Fermi level by 1 eV, towards midgap. A mechanism of doping due to the introduction of aromatic nitrogen-containing sites has been postulated. The boron, carbon and nitrogen ion implantation of PAC has resulted in the controllable increase in conductivity from 1015 to 106 O cm as a function of ion dose, from 2 x 1012 to 2 X 1016 ions cm-2. At low ion doses (up to 6 x 1014 ions cm-2) this occurs without any change in band gap; however, at higher doses the band gap collapses as a result of graphitisation. The dependence on the implant ion shows that it is possible to move the Fermi level towards the valence band with the implantation of boron, and towards midgap with the implantation of nitrogen. A hysteresis effect is observed at intermediate ion doses, which is attributed to the trapping of holes resulting in an increase in electron current. Implanting part of the thickness of the film at this ion dose has resulted in rectification, which has not previously been reported for this type of structure in amorphous carbon. DAC has been shown to possess a smaller band gap (0.7 eV), higher density of defects (~ 1020 spins cm-3) and lower resistivity (~ 1013 O cm) than PAC. The room-temperature current transport is governed by band-tail conduction at fields below 105 V cm-1, and the Poole-Frenkel effect at higher fields. The addition of nitrogen of up to 8 at. % has been observed to increase the band gap from 0.7 to 1.0 eV and therefore decrease the magnitude of the Poole-Frenkel conductivity. The Fermi level remains pinned at midgap, however. Therefore, it appears that PAC shows advantages over DAC in terms of future device applications.

Dans cette thèse, nous avons étudié expérimentalement les propriétés physiques des fermions ultra-froids grâce à une machine conçue pour refroidir un mélange fermionique de 6Li et 40K. Après une courte description concernant la construction de l'expérience et quelques améliorations que j'ai implémentées pendant ma thèse (telles que la désorption atomique par lumière ultraviolette dans le 2D-MOT), l'exposé se concentre sur deux observations principales de l'origine fermionique des gaz de potassium et de lithium.La première partie présente la quasithermalization du 6Li dans un potentiel quadrupolaire, créé par un piège magnétique. Malgré l'absence de collisions dans un gaz fermionique polarisé en dessous d'une température donnée, nous observons une redistribution d'énergie dans l'ensemble statistique après une excitation dans le piège linéaire. Une étude expérimentale détaillée ainsi qu'une analyse théorique du phénomène sont présentées. De plus, une transformation canonique de l'hamiltonien du système permet la description de particules sans masses dans un piège harmonique. Les résultats expérimentaux du système réel (gaz 6Li dans un potentiel quadrupolaire) sont donc réinterprétés pour décrire ces particules non massiques, difficiles à observer. Un développement supplémentaire de notre système expérimental permet également la réalisation d'un couplage spin-orbite non-abélien dans le gaz fermionique sans interactions.Dans la deuxième partie, on décrit la réalisation d'un gaz dégénéré de 40K à l'aide du refroidissement évaporatif. Une succession d'étapes d'évaporation, utilisant différentes technologies de piégeage, nous permet d'obtenir 1.5e5 atomes dans l'état fondamental à une température de 62nK, température équivalente à 17% de la température de Fermi. / In this thesis we investigate experimentally the physics of a cold fermionic mixture consisting of 6Li and 40K. After a short description of the experimental apparatus and of a few technical particularities implemented during my PhD, for example the light-induced atomic desorption in the 2D-MOT by UV-light, we focus on two main observations of the fermionic nature of the gas.The first part describes the quasithermalization of 6Li in a magnetic quadrupole potential. Even though collisions are absent in a spin-polarized fermionic gas below a given temperature, the statistical ensemble undergoes energy redistribution after an excitation within the linear potential. We present an extensive experimental study as well as a comprehensive theoretical analysis. Moreover, the studied Hamiltonian can be canonically mapped onto a system of massless, harmonically trapped particles and the previously developed results are re-interpreted in order to describe this experimentally inaccessible system. A further development of the realized experiment allows even for the implementation of spin-orbit coupling in a gas of non-interacting fermions.In the second part, we describe the evaporative cooling of 40K to quantum degeneracy. Through different evaporative cooling stages we reach with a final number of 1.5e5 atoms in the ground-state a temperature of 62nK, which corresponds to 17% of the Fermi temperature.

Au cours de ma thèse, j'ai travaillé à la construction de l'expérience Fermix, consacrée à l'étude d'un mélange de fermions (6Li-40K) à très basses températures où les effets quantiques sont prédominants. Nous présentons ici deux résultats principaux. Premièrement, nous avons développé une nouvelle méthode de refroidissement sub-Doppler qui tire parti de l'existence d'états noirs dans la raie optique D1. Cette mélasse grise permet d'atteindre une densité de l'espace des phases de 10^-4, la valeur la plus élevée rapportée dans la littérature pour le refroidissement laser simultané des deux espèces. Grâce à cette étape, nous avons pu réaliser un gaz fortement dégénéré de 3x10^5 atomes de 40K, répartis dans deux états de spins, à une température de 62 nK, soit 17% de la température de Fermi. D'autre part, nous introduisons une transformation canonique pour montrer l'équivalence formelle entre le comportement de particules ultra-relativistes sans interactions (particules de Weyl) dans un potentiel harmonique et celui de fermions froids confinés dans un piège quadrupolaire. Nous étudions expérimentalement, numériquement et théoriquement la relaxation de tels systèmes vers un état stationnaire, non-Botlzmanien, caractérisé par des températures effectives fortement anisotropes. Cette analogie permet également d'interpréter des propriétés caractéristiques des particules relativistes. Ainsi, nous montrons que le paradoxe de Klein est analogue aux pertes Majorana. Pour finir, nous proposons une étude théorique des interactions médiées à longue distance par un système en dimensions mixtes. / During my PhD, I contributed to the design and construction of the Fermix experiment, dedicated to the study of a 6Li-40K fermionic mixture at ultra low temperatures. Our main results are twofold. First, we developed a new sub-Doppler laser cooling scheme, taking advantage of the existence of dark states in the D1 line of alkali atoms. This so-called \emph{grey molasses} allows for a phase space density up to $10^{-4}$, the highest value reported for the simultaneous laser cooling of those two species. The improvement due to this cooling step enabled the production of a quantum degenerate 40K gas in a dipole trap, with 3x10^5 atoms in two spin states at 62 nK, corresponding to 17% of the Fermi temperature. Second, introducing a canonical mapping, we showed that non-interacting ultra-relativistic particles (Weyl fermions) in a harmonic trap can be simulated by cold fermions confined in a quadrupole potential. We study experimentally, numerically and theoretically the relaxation of these systems towards a steady state which can not be described by a Boltzman distribution, but rather presents strongly anisotropic effective temperatures. This analogy also allows us to translate fundamental properties of relativistic particles in the language of cold atoms. In particular, we demonstrate that the Klein paradox is equivalent to Majorana losses.Finally, we present a theoretical study of the long range interactions between particles confined in two 2D layers immersed in a 3D atomic cloud.