Dans le modèle cosmologique actuel, l'Univers est majoritairement composé de matière noire dont la nature est inexpliquée par le Modèle Standard de la physique des particules. L'annihilation de particules issues de nouveaux cadres théoriques, peut induire un signal de rayons gamma de très hautes énergies, observable par des expériences d'astronomie gamma. Largement dominées par la matière noire, les galaxies naines sphéroïdes sont des cibles privilégiées pour conduire cette recherche. Le réseau de télescopes H.E.S.S. discuté dans la première partie est un parfait exemple d'expérience d'imagerie atmosphérique stéréoscopique permettant de conduire la recherche de matière noire. Une nouvelle méthode de discrimination des gerbes électromagnétiques et hadroniques permettant d'améliorer la recherche de sources faibles est présentée. Elle est appliquée aux données des observations de la galaxie naine du Sagittaire et la limite supérieure sur le flux de gamma en provenance de cet objet est calculée. En orbite à bord du satellite Fermi depuis Juin 2008, le télescope à conversion de paire LAT permet de rechercher la matière noire sur l'ensemble de la voûte céleste. La sensibilité théorique à détecter un signal de matière noire est déterminée pour deux galaxies naines spéciques. Au terme de la première année d'observations, les limites supérieures sur les ux de gamma sont dérivées pour un catalogue de galaxies naines sphéroïdes. Des modèles de physique au-delà du Modèle Standard sont confrontés avec les contraintes calculées sur les sections ecaces d'annihilation en fonction de la masse des particules obtenues à partir des observations effectuées avec ces deux expériences.

Dans le cadre d'une approche bottom-up, la fabrication de nanofils par une croissance catalysée ouvre la voie à nombres d'applications: nano--transistors verticaux à grille enrobantes, heterostructures cœur--coquilles... Avec ces nouveaux objets, de nouvelles interrogations apparaissent quant à l'influence du catalyseur et de la surface sur les propriétés électroniques des nanofils. Mon travail basé sur une étude spectroscopique via des expériences de photoluminescence a mis en évidence le rôle prépondérant de la surface sur les propriétés électroniques des nanofils. La passivation des états de surface a permis d'observer la recombinaison radiative des porteurs libres d'une phase dense : le liquide électron-trou, dans des nanofils catalysés par de l'or et du cuivre. Cette phase liquide a la particularité d'être stable thermodynamiquement et sa densité est constante. Cette propriété unique dans les semiconducteurs a conduit à l'étude quantitative de l'influence de la surface via la modification du ratio surface/volume. Une méthode originale de mesure de la vitesse de recombinaison de surface (VRS) a ainsi été développée et des VRS relativement faibles ont été mesurées indiquant une excellente passivation des états de surface. Les propriétés de volume de nanofils catalysés 'or' sont très similaires à celles d'un silicium massif utilisé en micro-électronique. Enfin, l'oxydation sacrificielle du silicium a permis d'obtenir des nanofils de diamètre inférieur à 10 nm. L'oxydation progressive des nanofils a permis d'observer un décalage de la raie vers le rouge attribué à la présence de contraintes, puis l'augmentation du gap est corrélée au confinement quantique des porteurs.

<p>When modelling the collapse of massive stars leading to supernova explosions and the cooling of neutron stars, understanding the microphysical processes, such as the interaction of neutrinos within a dense medium are of vital importance. The interaction of neutrinos with nucleons (neutrons and protons) is altered by the presence of the medium, compared to the same process with free nucleons. Neutrino scattering and production processes may be characterized in terms of the excitations that are created or destroyed in the nuclear medium. One way to analyse the effects of the medium is by using Landau's theory of normal Fermi liquids. This theory gives simple relationships between physical quantities such as the spin susceptibility or the response to a weak interaction probe in terms of Landau parameters, that are measures of the interaction between quasiparticles. One problem when using Landau Fermi liquid theory for nucleon matter is that the interaction has a tensor component. The tensor interaction does not conserve the total spin and, as a consequence, there are generally contributions to long-wavelength response functions from states that have more than one quasiparticle-quasihole pair in the intermediate state. Such contributions cannot be calculated in terms of Landau parameters alone, since in the usual formulation of Landau theory, only singlepair excitations are considered. In this thesis three problems are addressed. First, we obtain bounds on the contributions from more than one quasiparticle-quasihole pair by using sum-rule arguments. Second, we derive expressions for static response functions allowing for the tensor components of the interaction. We analyse which the most important effects are on the static response of nucleon matter, and find that the major contributions comes from renormalization of coupling constants and transitions to states with more than one quasiparticle-quasihole pair. Third, we show how contributions to the dynamical response coming from states containing two quasiparticle-quasihole pairs may be evaluated in terms of Landau theory if one allows for the effect of collisions in the Landau kinetic equation. We consider the case of asymmetric nuclear matter, and our work goes beyond earlier works in that they contain the effects of collisions in addition to those of the mean field.</p>

In this thesis, non-collinear magnetism has been studied by using density functional theory and the augmented plane wave method with local orbitals (APW+lo). Two conditions for non-collinear instabilities have been identified in this thesis. First, the Fermi energy should cut through both spin up and down states. Secondly, strong nesting between the spin up and spin down Fermi surfaces is needed. The two criteria described here can be fulfilled by tuning the exchange-splitting and/or by modifying the volume. Calculations on several elements; bcc V, bcc and fcc Mn, bcc Fe, bcc and fcc Co, and bcc and fcc Ni show that a non-collinear state can be stabilized provided that the criteria discussed above are met. More complex materials have also been analyzed in terms of these two criteria. The substitutional alloys TlCo2Se2-xSx are found in experiments to possess spin spiral structures for x = {0-1.5} and at a concentration x = 1.75 the alloys become ferromagnetic. As S takes the place of Se in the crystal structure the distance between the Co layers is reduced and the turn angle of the spin spiral becomes smaller until it totally vanishes at x = 1.75. This thesis show that the evolution of the magnetic structure in these alloys is the consequence of a modification of the distance between Co layers, which induces a change in the interlayer exchange coupling. Fermi surfaces have been analyzed in TbNi5 in order to determine nesting features which would be responsible for the magnetic spin spiral observed in this material. The electronic structure of CeRhIn5 is also reported in this thesis. Furthermore, the 3-k magnetic structure of UO2 was investigated and the crystal field levels were calculated. Transition metal systems such as Fe in the superconducting high-pressure hcp phase and in the fcc crystal structure were also studied. The results obtained for fcc Fe are in accordance with previous reports. However the paramagnetic state in hcp Fe is found to be more stable than the antiferromagnetic configurations discussed earlier in the literature as being favored in the volume range where the hcp phase is stable and superconductivity appears (~ 15 GPa). The complex non-collinear magnetic structure in Mn3IrSi was calculated and the results are found to be in good agreement with experiments.

When modelling the collapse of massive stars leading to supernova explosions and the cooling of neutron stars, understanding the microphysical processes, such as the interaction of neutrinos within a dense medium are of vital importance. The interaction of neutrinos with nucleons (neutrons and protons) is altered by the presence of the medium, compared to the same process with free nucleons. Neutrino scattering and production processes may be characterized in terms of the excitations that are created or destroyed in the nuclear medium. One way to analyse the effects of the medium is by using Landau's theory of normal Fermi liquids. This theory gives simple relationships between physical quantities such as the spin susceptibility or the response to a weak interaction probe in terms of Landau parameters, that are measures of the interaction between quasiparticles. One problem when using Landau Fermi liquid theory for nucleon matter is that the interaction has a tensor component. The tensor interaction does not conserve the total spin and, as a consequence, there are generally contributions to long-wavelength response functions from states that have more than one quasiparticle-quasihole pair in the intermediate state. Such contributions cannot be calculated in terms of Landau parameters alone, since in the usual formulation of Landau theory, only singlepair excitations are considered. In this thesis three problems are addressed. First, we obtain bounds on the contributions from more than one quasiparticle-quasihole pair by using sum-rule arguments. Second, we derive expressions for static response functions allowing for the tensor components of the interaction. We analyse which the most important effects are on the static response of nucleon matter, and find that the major contributions comes from renormalization of coupling constants and transitions to states with more than one quasiparticle-quasihole pair. Third, we show how contributions to the dynamical response coming from states containing two quasiparticle-quasihole pairs may be evaluated in terms of Landau theory if one allows for the effect of collisions in the Landau kinetic equation. We consider the case of asymmetric nuclear matter, and our work goes beyond earlier works in that they contain the effects of collisions in addition to those of the mean field.

Dark matter (DM) constitutes one of the most intriguing but so far unresolved issues in physics. In many extensions of the Standard Model of particle physics, the existence of a stable Weakly Interacting Massive Particle (WIMP) is predicted. The WIMP is an excellent DM particle candidate. One of the most interesting scenarios is the creation of monochromatic gamma-rays from the annihilation or decay of these particles. This type of signal would represent a “smoking gun” for DM, since no other known astrophysical process should be able to produce it. In this thesis, the search for spectral lines with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi) is presented. The satellite was successfully launched from Cape Canaveral in Florida, USA, on 11 June, 2008. The energy resolution and performance of the detector are both key factors in the search and are investigated here using beam test data, taken at CERN in 2006 with a scaled-down version of the Fermi-LAT instrument. A variety of statistical methods, based on both hypothesis tests and confidence interval calculations, are then reviewed and tested in terms of their statistical power and coverage. A selection of the statistical methods are further developed into peak finding algorithms and applied to a simulated data set called obssim2, which corresponds to one year of observations with the Fermi-LAT instrument, and to almost one year of Fermi-LAT data in the energy range 20–300 GeV. The analysis on Fermi-LAT data yielded no detection of spectral lines, so limits are placed on the velocity-averaged cross-section, <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%3C%5Csigma%20v%3E_%7B%5Cgamma%20X%7D" />, and the decay lifetime, <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Ctau_%7B%5Cgamma%20X%7D" />, and theoretical implications are discussed. / QC20100525 / GLAST

In this thesis work, we have mainly worked on two topics of GPU performance analysis. First, we have developed an analytical method and a timing estimation tool (TEG) to predict CUDA application's performance for GT200 generation GPUs. TEG can predict GPU applications' performance in cycle-approximate level. Second, we have developed an approach to estimate GPU applications' performance upper bound based on application analysis and assembly code level benchmarking. With the performance upper bound of an application, we know how much optimization space is left and can decide the optimization effort. Also with the analysis we can understand which parameters are critical to the performance.

This thesis presents the experimental study on the two-spin component, strongly interacting 6 Li Fermi gases in 3D and 1D traps. The interaction strength is tuned from the molecular BEC regime to the BCS regime using a Feshbach resonance. The trap dimension can be tuned from 3D to 1D with the implementation of optical lattice. The evaporation of imbalanced Fermi gases in 3D trap is studied. The anisotropic and fast evaporation is the cause of the deformation observed in the 2006 Rice experiment. In a balanced Fermi system, the fraction of correlated states is measured as a function of interaction and temperature. At unitarity, the fraction of correlated states is ∼85% and exists above T c . The one-body-like photoexcitation rate can be related to the contact quantity. Lastly, the spin-imbalance in a one-dimensional Fermi gas is studied. The 1D phase diagram is mapped out. The result agrees well with the 1D theory, in which the partially polarized regime is predicted to be a FFLO phase, an exotic superfluid with pairs carrying finite center-of-mass momentum proposed almost 50 years ago.

This thesis presents theoretical investigations of triplet superfluidity (triplet superconductivity) in quasi-one-dimensional organic conductors and ultra-cold Fermi gases. Triplet superfluidity is different from its s-wave singlet counterpart since the order parameter is a complex vector and the interaction between fermions is in general anisotropic. Because of these distinctions, triplet superfluids have different physical properties in comparison to the s-wave case. The author discusses in this thesis the interplay between triplet superconductivity and spin density waves in quasi-one-dimensional organic conductors, and proposes a coexistence region of the two orders. Within the coexistence region, the interaction between the two order parameters acquires a vector structure, and induces an anomalous magnetic field effect. Furthermore, the author analyzes the matter-wave interference between two p-wave Fermi condensates, and proposes a polarization effect. For a single harmonically trapped p-wave Fermi condensate, the author also shows that the expansion upon release from the trap can be anisotropic, which reflects the anisotropy of the p-wave interaction.

This dissertation discusses the Landau-Zener (LZ) theory and its application in noisy environments and in many-body systems. The first project considers the effect of fast quantum noise on LZ transitions. There are two important time intervals separated by the characteristic LZ time. For each interval we derive and solve the evolution equation, and match the solutions at the boundaries to get a complete solution. Outside the LZ time interval, we derive the master equation, which differs from the classical equation by a quantum commutation term. Inside the LZ time interval, the mixed longitudinal-transverse noise correlation renormalizes the LZ gap and the system evolves according to the renormalized LZ gap. In the extreme quantum regime at zero temperature our theory gives a beautiful result which coincides with that of other authors. Our initial attempts to solve two experimental puzzles - an isotope effect and the quantized hysteresis curve of a single molecular magnet - are also discussed. The second project considers an ultracold dilute Fermi gas in a magnetic field sweeping across the broad Feshbach resonance. The broad resonance condition allows us to use the single mode approximation and to neglect the energy dispersion of the fermions. We then propose the Global Spin Model Hamiltonian, whose ground state we solve exactly, which yields the static limit properties of the BEC-BCS crossover. We also study the dynamics of the Global Spin Model by converting it to a LZ problem. The resulting molecular production from the initial fermions is described by a LZ-like formula with a strongly renormalized LZ gap that is independent of the initial fermion density. We predict that molecular production during a field-sweep strongly depends on the initial value of magnetic field. We predict that in the inverse process of molecular dissociation, immediately after the sweeping stops there appear Cooper pairs with parallel electronic spins and opposite momenta.