<p> A number of questions are examined concerning many body correlations in connection with electron gas at metallic densities (2 < = rs <= 5.7) and the annihilation of a positron in simple metals, by means of a technique involving the two particle correlation Green's function. Estimates are made of low temperature contributions to angular correlation data, which describe the momentum distribution of annihilating electron positron pairs, in the form of smearing at the sharp cutoff corresponding to the Fermi momentum from electron- and positron- photon interactions, and in the form of broad tails beyond the cutoff resulting from the high-momentum components introduced into the electron wave function by the presence of a periodic crystal lattice. </p> <p> Phonon effects are introduced into the perturbation expansion of the two particle Green's function describing an electron positron pair. A calculation of the lowest order phonon contribution seems to indicate that such effects do not explain the smearing at the Fermi momentum. </p> <p> A Green's function calculation of the first-order enhancement of the lattice tails, due to the positron-electron correlation, is made by introducing particle-lattice interactions explicitly in a model based on a simple metal such as sodium. It considers a weak potential and treats as zero the lattice components corresponding to other than nearest-neighbours points in reciprocal lattice space. The enhancement for rs =4, which is almost a constant, is very similar to that for the main part of angular correlation data. This indicates that, for simple metals at least, angular correlation data can be interpreted directly from a free-particle model. </p> <p> Short-range correlations among opposite-spin electrons are examined by field-theoretic techniques as a step to obtaining a fundamental understanding of the correlations among electrons at metallic densities. A calculation of the p.d.f. for opposite-spin electrons is positive over a wide range of metallic densities and seems to account for short-range correlations of the Coulomb hole through the multiple scattering of particle-particle ladders. </p> / Thesis / Doctor of Philosophy (PhD)

<p> Although the electronic pairing hypothesis of Bardeen, Cooper and Schrieffer is verified by experiment, the lattice polarisation mechanism for over-screening the repulsion between mates in a pair need not be the sole mechanism. </p> <p> Alternatives to the lattice polarisation mechanism are discussed and a new approach to electronic polarisation is developed. Propagator formalism is reworked using a charge density propagator to avoid the Dyson equation for the screened potential. The fermi liquid approach to electrons in a metal is avoided, too, because it misses the correlations which give a solid its atomicity. An atomic picture is contrived instead and used to predict what observable features of a superconductor are influenced by atomic correlations. </p> / Thesis / Doctor of Philosophy (PhD)

No description available.

This thesis is a sandwich thesis composed of three papers that are published in refereed journals or conferences. The first paper is a systematic experimental study conducted to identify the first phase to form during cobalt germanidation. Hexagonal β-Co5Ge3 was the first phase to form at temperatures as low as 227°C followed by monoclinic CoGe as the second phase at the same temperature. We also report for the first time that both phases that formed were highly ordered partial epitaxial crystal orientations suggesting that both of those low-temperature phases could potentially serve as high quality contacts for germanium based devices with a very low thermal budget which is advantageous for the process design. Those results contributed to a better understanding of cobalt germanidation leading to the first multiphase technology computer aided design model presented in the second paper. This kinetic model for cobalt germanide growth can predict the resulting phase based on anneal time, temperature, and ambient. The model has been calibrated to experimental results. This predictive model can help in the design of cobalt germanide contacts with low resistance and can serve as a general modeling framework for multiphase solid state reaction binary systems. A comprehensive survey of the experimental results for formation of cobalt germanides is discussed and the data are reconciled in the third paper. Factors affecting the resulting phases and their quality are identified and some optimum choices for the experimental parameters are pointed based on the survey. The role of germanium crystal orientation in ohmic and Schottky properties of the contact is analyzed. Fermi level pinning plays a role mainly on metal/(100) n-type Ge interfaces and its role is minimal on p-type Ge and other crystalline orientations. Schottky Barrier Heights for cobalt germanide contacts reported in the literature are surveyed. Crystalline cobalt germanides, forming when Co is deposited at high temperatures, are expected to have lower interface resistivities compared to those reported. The work is important because contact resistance has become one of the most important factors in advanced complementary metal oxide semiconductor (CMOS) technology and advanced devices already include germanium (Ge) in the source/drain regions of devices. It is also important because heating at the interface due to contact resistance is one of the key challenges in power devices and cobalt germanide can be used both for Si and Ge based devices as well as for gallium nitride (GaN) devices. The latter application is possible because cobalt germanide is lattice-matched to GaN. / Thesis / Doctor of Philosophy (PhD) / The main goal of this thesis is to create predictive empirical, mathematical, and physical models to help the designer of the semiconductor process technology to design high quality electric contacts, namely cobalt germanides, to their semiconductor devices, germanium based. The choice of cobalt germanides is motivated by their expected superior quality given the possibility of growing them in crystalline form. We settled a theoretical and experimental controversy regarding the first phase to form by conducting experiments demonstrating that low-temperature forming cobalt germanide phases are highly ordered and could serve as high quality contacts. A predictive physical based mathematical model was developed to assist the designer in obtaining the desired cobalt germanide phase for its needed electrical properties by design. Factors affecting the quality of the germanide were identified based on an extensive survey and the optimum choices for the parameters to obtain high quality contact were pointed.

The electron energy loss spectrometer at Virginia Tech has been modified with new momentum deflectors and various other hardware improvements. In addition to some hardware changes all the software to run the spectrometer and analyze the data has been rewritten. It was suggested by Tsuei (1990) that a nested Fermi surface could be used to explain both the fact that these materials have high superconducting transition temperatures and have linear relaxation rates. It was suggested in that same paper that EELS might be able to confirm the presence of Fermi surface nesting. We have attempted to use transmission electron energy loss spectroscopy in a novel way to search for evidence of Fermi surface nesting in the high T_c superconductor Bi₂Sr₂Ca₁Cu₂O₈. High quality single crystal samples were obtained and thinned for use in the EELS. Data was taken at low energy (<1.0eV) and momentum was scanned along three different momentum transfer directions for |q| =0.0 to 3.0Å⁻¹. The data was scaled to be Im(-1/ε). A calculation of Im(-1/ε) was performed using a tight binding model and it was found that the features associated with the Fermi surface (from this TB calculation) were too small to be seen in our spectra. We interpret this problem to be associated with the presence of thermal diffuse scattering. / Ph. D.

Cette thèse traite de la structure électronique de supraconducteurs telle que déterminée par la théorie de la fonctionnelle de la densité. Une brève explication de cette théorie est faite dans l’introduction. Le modèle de Hubbard est présenté pour pallier à des problèmes de cette théorie face à certains matériaux, dont les cuprates. L’union de deux théories donne la DFT+U, une méthode permettant de bien représenter certains systèmes ayant des électrons fortement corrélés. Par la suite, un article traitant du couplage électron- phonon dans le supraconducteur NbC1−xNx est présenté. Les résultats illustrent bien le rôle de la surface de Fermi dans le mécanisme d’appariement électronique menant à la supraconductivité. Grâce à ces résultats, un modèle est développé qui permet d’expliquer comment la température de transition critique est influencée par le changement des fré- quences de vibration du cristal. Ensuite, des résultats de calcul d’oscillations quantiques obtenus par une analyse approfondie de surfaces de Fermi, permettant une comparaison directe avec des données expérimentales, sont présentés dans deux articles. Le premier traite d’un matériau dans la famille des pnictures de fer, le LaFe2P2. L’absence de su- praconductivité dans ce matériau s’explique par la différence entre sa surface de Fermi obtenue et celle du supraconducteur BaFe2As2. Le second article traite du matériau à fermions lourds, le YbCoIn5. Pour ce faire, une nouvelle méthode efficace de calcul des fréquences de Haas-van Alphen est développée. Finalement, un dernier article traitant du cuprate supraconducteur à haute température critique YBa2Cu3O6.5 est présenté. À l’aide de la DFT+U, le rôle de plusieurs ordres magnétiques sur la surface de Fermi est étudié. Ces résultats permettent de mieux comprendre les mesures d’oscillations quan- tiques mesurées dans ce matériau. / In this thesis, the electronic structure of different kinds of superconductors is explored with the density functional theory. A brief explanation of this theory is done in the in- troduction. The Hubbard model is also presented as it can be used to solve shortcomings of the theory in some materials such as cuprates. The blend of the two theories is the DFT+U which is used to describe materials with strongly correlated electrons. After- ward, a paper describing the electron-phonon coupling in the superconductor NbC1−xNx is presented. Results from this work show the role of the Fermi surface in the electron pairing mechanism leading to superconductivity. Based on these results, a model is de- veloped explaining how the critical temperature is influenced by the change in frequency of the vibration modes. Then, quantum oscillation results based on a detailed analysis of Fermi surfaces, allowing a direct comparison with experimental data, are presented within two papers. The first one is about a material in the iron pnictide family, the LaFe2P2. Our calculations show that the Fermi surface of this material is different from the superconducting doped BaFe2As2 which explains why this material shows no sign of superconductivity. The second paper is about the heavy fermion system YbCoIn5. To do this, a new efficient method to calculate de Haas-van Alphen frequencies is developed. Finally, a paper on superconducting YBa2Cu3O6.5 is presented. Using DFT+U, the role of various magnetic orders on the Fermi surface are studied. The results allow a better understanding of the measured quantum oscillations in this material.

Cette thèse traite de la structure électronique de supraconducteurs telle que déterminée par la théorie de la fonctionnelle de la densité. Une brève explication de cette théorie est faite dans l’introduction. Le modèle de Hubbard est présenté pour pallier à des problèmes de cette théorie face à certains matériaux, dont les cuprates. L’union de deux théories donne la DFT+U, une méthode permettant de bien représenter certains systèmes ayant des électrons fortement corrélés. Par la suite, un article traitant du couplage électron- phonon dans le supraconducteur NbC1−xNx est présenté. Les résultats illustrent bien le rôle de la surface de Fermi dans le mécanisme d’appariement électronique menant à la supraconductivité. Grâce à ces résultats, un modèle est développé qui permet d’expliquer comment la température de transition critique est influencée par le changement des fré- quences de vibration du cristal. Ensuite, des résultats de calcul d’oscillations quantiques obtenus par une analyse approfondie de surfaces de Fermi, permettant une comparaison directe avec des données expérimentales, sont présentés dans deux articles. Le premier traite d’un matériau dans la famille des pnictures de fer, le LaFe2P2. L’absence de su- praconductivité dans ce matériau s’explique par la différence entre sa surface de Fermi obtenue et celle du supraconducteur BaFe2As2. Le second article traite du matériau à fermions lourds, le YbCoIn5. Pour ce faire, une nouvelle méthode efficace de calcul des fréquences de Haas-van Alphen est développée. Finalement, un dernier article traitant du cuprate supraconducteur à haute température critique YBa2Cu3O6.5 est présenté. À l’aide de la DFT+U, le rôle de plusieurs ordres magnétiques sur la surface de Fermi est étudié. Ces résultats permettent de mieux comprendre les mesures d’oscillations quan- tiques mesurées dans ce matériau. / In this thesis, the electronic structure of different kinds of superconductors is explored with the density functional theory. A brief explanation of this theory is done in the in- troduction. The Hubbard model is also presented as it can be used to solve shortcomings of the theory in some materials such as cuprates. The blend of the two theories is the DFT+U which is used to describe materials with strongly correlated electrons. After- ward, a paper describing the electron-phonon coupling in the superconductor NbC1−xNx is presented. Results from this work show the role of the Fermi surface in the electron pairing mechanism leading to superconductivity. Based on these results, a model is de- veloped explaining how the critical temperature is influenced by the change in frequency of the vibration modes. Then, quantum oscillation results based on a detailed analysis of Fermi surfaces, allowing a direct comparison with experimental data, are presented within two papers. The first one is about a material in the iron pnictide family, the LaFe2P2. Our calculations show that the Fermi surface of this material is different from the superconducting doped BaFe2As2 which explains why this material shows no sign of superconductivity. The second paper is about the heavy fermion system YbCoIn5. To do this, a new efficient method to calculate de Haas-van Alphen frequencies is developed. Finally, a paper on superconducting YBa2Cu3O6.5 is presented. Using DFT+U, the role of various magnetic orders on the Fermi surface are studied. The results allow a better understanding of the measured quantum oscillations in this material.

Dédié à l'étude du ciel en rayons gamma, le satellite Fermi comporte à son bord le Large Area Telescope (LAT), sensible au rayonnement gamma de 20 MeV à 300 GeV. Les données recueillies par le LAT depuis son lancement en 2008 ont permis de multiplier par 10 le nombre de noyaux actifs de galaxie (NAG) détectés dans le domaine du GeV. Les rayons gamma observés dans les NAGs proviennent de processus énergétiques faisant intervenir des particules chargées de très haute énergie. Ces particules sont confinées dans un jet de plasma magnétisé qui prend sa source dans une région proche du trou noir supermassif habitant la zone centrale de la galaxie hôte. Ce jet s’éloigne à des vitesses aussi élevées que 0.9999c, formant dans de nombreux cas des lobes radio sur des échelles du kiloparsec voire du mégaparsec. Les NAGs dont le jet fait un angle faible avec la ligne de visée sont appelés blazars. La combinaison de cette très faible inclinaison du jet par rapport à la ligne de visée et de vitesses d’éjection relativistes donne lieu à des effets relativistes : mouvements apparents superluminiques, amplification de la luminosité et modification des échelles de temps. Les blazars sont caractérisés par une extrême variabilité à toutes les longueurs d’onde, sur des échelles de temps allant de quelques minutes à plusieurs mois. Une étude temporelle et spectrale du plus brillant d'entre ceux détectés par le LAT, 3C 454.3, a été réalisée afin de contraindre les modèles d'émission. Une nouvelle méthode de génération de courbes de lumière à échantillonnage adaptatif est également proposée dans cette thèse. Celle-ci permet d'extraire le maximum d'information des données du LAT quel que soit l'état de flux de la source. / The Fermi Gamma-ray Space Telescope was launched on 2008 June 11, carrying the Large Area Telescope(LAT), sensitive to gamma-rays in the 20 MeV – 300 GeV energy range. The data collected since then allowed to multiply by a factor of 10 the number of Active Galactic Nuclei (AGN) detected in the GeV range. Gamma-rays observed in AGNs come from energetic precesses bringing into play very high energy charged particles. These particles are confined in a magnetized plasma jet rising in a region close to the supermassive black hole in the center of the host galaxy. This jet moves away with velocities as high as 0.9999c, forming in many cases radio lobes on kiloparsec or even megaparsec scales. Among the AGNs, those whose jet inclination angle to the line of sight is small are called blazars. The combination of this small inclination angle with relativistic ejection speeds leds to relativistic effects : apparent superluminal motions, amplification of the luminosity and modification of the time scales. Blazars are characterized by extreme variability at all wavelengths, on time scales from a few minutes to several months. A temporal and spectral study of the most luminous of those detected by the LAT, 3C 454.3, was done so as to constrain emission models. A new method for generating adaptive-binning lightcurves is also suggested in this thesis. It allows to extract the maximum of information from the LAT data whatever the flux state of the source.

Dédié à l’étude du ciel en rayons gamma, le satellite Fermi comporte à son bord le Large AreaTelescope (LAT), sensible au rayonnement de 20 MeV à plus de 300 GeV. Sa sensibilité et ses performances grandissantes apportent de plus en plus de contraintes sur les nébuleuses à vent de pulsar. Ces nébuleuses sont alimentées par l’injection permanente d’un vent d’électrons et de positrons relativistes accélérés par l’onde de choc délimitant la nébuleuse. Ces particules chargées rayonnent dans les différents domaines du spectre électromagnétique et notamment dans le domaine des rayons gamma à l’aide du processus de diffusion Compton inverse. Cette thèse présente l’étude morphologique et spectrale détaillée de deux sources potentiellement associées à des nébuleuses : MSH 11-62 et HESS J1857+026, afin de déterminer si l’émission gamma est produite par des nébuleuses de pulsar. Dans un deuxième temps, l’analyse de toutes les sources reliées à des contreparties détectées par les télescopes Cerenkov permet la détection de six nouvelles nébuleuses de pulsars et candidats dans la gamme de Fermi. Ceci permet d’apporter de nouvelles contraintes sur les propriétés physiques des nébuleuses de pulsar ainsi que sur les mécanismes d’émission à l’aide de modèles et d’études sur la population totale détectée par le LAT. / The Fermi Gamma-ray Space Telescope was launched on 2008 June 11, carrying the Large Area Telescope (LAT), sensitive to gamma-rays from 20 MeV to more than 300 GeV. Its constantly improving sensitivity and performance offer a new opportunity to understand the sources of the gamma-ray sky including Pulsar Wind Nebulae (PWNe). PWNe are powered by the constant injection of a relativistic wind of electrons and positrons from their central pulsars. These charged particles are accelerated at the shock front forming the PWN and emit photons which can be observed along the entire electromagnetic spectrum, including the highenergy gamma-ray domain via inverse Compton scattering.This thesis present the detailed analysis of two sources of gamma-ray emission potentially asso4021ciated to PWNe : MSH 11-62 and HESS J1857+026. The combination of the spatial and spectral analyses provide new elements to confirm these associations.In a second step, we describe a search for counterparts to sources detected by Cerenkov telescopes.This search led to the detection of six new LAT sources potentially associated with PWNe. These studies bring new insights and constraints on the physical properties of the sources as well as on emitting processes by constraining the models and allowing population studies.

Dans ce travail, nous nous sommes intéressés à la brisure de la symétrie d'isospin dans les noyaux N≈Z et à son effet sur l'élément de matrice de transitions β de Fermi super-permises 0+ ➝ 0+ dans le cas de la décroissance β+ du Mn50.Dans le cadre de l'approche microscopique Highly Truncated Diagonalization Approach (HTDA), dédiée à la description des corrélations au-delà du champ moyen et conservant explicitement de nombre de particules, nous avons étudié (en particulier) le rôle joué par les corrélations d'appariement dans les mécanismes de brisure de la symétrie d'isospin dans l'état fondamental de noyaux N≈Z. Une étude de sensibilité du degré de cette brisure, en fonction de l'intensité de l'interaction résiduelle décrivant l'appariement dans HTDA, a été menée et une interprétation des mécanismes recherchés a été proposée à l'aide d'une approximation développée dans ce travail. Cette étude a mis évidence toute la complexité d'un bon traitement de la symétrie d'isospin, tant au niveau de la description de la source de brisure qu'au niveau de la limitation des biais du modèle. Nous avons également montré la nécessité d'une description très fine des fonctions d'onde corrélées dans un tel problème. Plus précisément, nous avons obtenu une valeur de la correction δc de mélange d'isospin à l'élément de matrice de transition de Fermi de (0.2±0.1)%. Cette valeur a été confrontée à celles obtenues dans d'autres approches. Compte tenu des effets négligés dans notre travail, notre valeur de δc est supposée représenter une borne minimum. / In this work, we are interested in the breaking of the isospin symmetry in the N≈Z nuclei and in its effect on the matrix element of super-allowed 0+ ➝ 0+ Fermi β transitions in the case of the β+ decay of the Mn50. Within the framework of the Highly Truncated Diagonalization microscopic Approach, dedicated to the descritption of correlations beyond the mean field and conserving explicitely the particle number, we have studied (in particular) the role played by pairing correlations in the breaking mechanisms of this symmetry in the ground state of N≈Z nuclei. A sensitivity study of the isospin mixing, as a function of the strength of the residual interaction decribing the pairing correlations in HTDA, has been carried out and an interpretation of the mechanims at work has been proposed in terms of an approximation developped in this work. This study has pointed out the complexity of a good treatment of the isospin symmetry, in the description of the breaking sources as well as in the reduction of model biases. We have also paid attention to the necessity of a very fine description of the correlated wave functions in such a problem. More precisely, we have obtained a value for the isospin mixing correction δc to the Fermi transition matrix element of (0.2±0.1)%. This value has been compared to those obtained in other approaches. Taking account of the neglected effects in our work, our value of δc is expected to be a lower bound.