In the Gamma-Alpha phase transition in Cerium, the magnetic properties change from a temperature-dependent Curie-Weiss susceptibility in the high temperature gamma phase to a temperature-independent paramagnetic susceptibility in the low temperature alpha phase. Taking into account the crystal-field split Hamiltonian, the energy eigenvalues and eigenfunctions are calculated for the doublet and the quartet ground states which are then used to determine the Kondo temperature for different crystal field splittings. The susceptibility and B-T phase diagrams are obtained for both the ground states and it is concluded that the doublet ground state is in better agreement with the experimental results. The P-T phase diagram is obtained using the Anderson model and the Non-Crossing Approximation. This numerical method determines the free energy for a given hybridization and subsequently the susceptibility, specific heat and the Fermi-liquid temperature. Using these, the P-V phase diagram is first obtained and then the P-T phase diagram derived from it using Maxwell's equal area construction. CePdSb is one of the few ternary compounds which exhibit Kondo lattice behavior along with a ferromagnetic ground state. The competitions between the RKKY interaction and the Kondo interactions are consistent with expectations based on the Doniach phase diagram. The susceptibility in the high temperature regime calculated using the linear response theory concurs well with the experimental results. At low temperatures, the exchange interaction dominates the magnetic behavior of the system. Taking into consideration the single-ion anisotropy and the Heisenberg interaction, the spin-wave dispersion spectrum is obtained using the Holstein-Primakoff transformation and renormalized Stevens coefficients. The spin-spin correlation function is obtained and found to agree very well with the inelastic neutron scattering data. / Physics

This research provides a proof of concept and background theory for the physics behind the state-of-the-art ultra-fast plasmonic spiking neurons (PSN), which can serve as a primary synaptic device for developing a platform for fast neural computing. Such a plasmonic-powered computing system allows localized AI with ultra-fast operation speed. The designed architecture for a plasmonic spiking neuron (PSN) presented in this thesis is a photonic integrated nanodevice consisting of two electro-optic and optoelectronic active components and works based on their coupling. The electro-optic active structure incorporated a periodic array of seeded quantum nanorods sandwiched between two electrodes and positioned at a near-field distance from the topmost metal layer of a sub-wavelength metal-oxide multilayer metamaterial. Three of the metal layers of the metamaterials form the active optoelectronic component. The device operates based on the coupling of the two active components through optical complex modes supported by the multilayer and switching between two of them. Both action and resting potentials occur through subsequent quantum and extraordinary photonics phenomena. These phenomena include the generation of plasmonic high-k complex modes, switching between the modes by enhanced quantum-confined stark effect, decay of the plasmonic excitations in each metal layer into hot-electrons, and collecting hot-electrons by the optoelectronic component. The underlying principles and functionality of the plasmonic spiking neuron are illustrated using computer simulation.

On étudie la cosmologie induite par un gaz parfait de supercordes à température finie. Les effets thermaux et quantiques du gaz de cordes génèrent un potentiel effectif au niveau d'une boucle. A certains points dans l'espace de modules où les masses des états génériquement massifs s'annulent, le potentiel effectif atteint des minima locaux. Les modules qui y sont attirés prennent des masses qui diminuent avec le temps, ce qui permet aux oscillations cohérentes des modules d'être diluées avant la nucléosynthèse. Ainsi le modèle ne rencontre pas le problème des modules cosmologiques. En particulier, on étudie la stabilisation des modules pour la cosmologie induite par i) un gaz de cordes hétérotiques maximalement supersymétriques, ii) un gaz de cordes de type II compactifiées sur un espace de Calabi-Yau de dimension complexe 3. Dans le 1er cas, les minima locaux du potentiel effectif apparaissent aux points de symétries de jauge étendues. Ceux-ci stabilisent tous les modules sauf le dilaton. Dans le 2ème cas, les minima locaux du potentiel effectif sont atteints où des 2- ou 3-sphères dans l'espace de Calabi-Yau s'évanouissent, produisant une transition conifold ou une extension du groupe non Abelien de jauge. Les états non massifs supplémentaires sont engendrés par des D-branes s'enroulant sur les sphères qui s'évanouissent et les modules stabilisés sont ceux qui contrôlent les volumes de ces sphères. Pour les deux cas, on étudie aussi respectivement les théories duales de type I maximalement supersymétrique et hétérotique compactifiée sur K3xT2. On trouve que les modules duaux sont stabilisés par des effets non perturbatifs impliqués par les dualités concernées.

La méthode de factorisation est appliquée sur les données initiales d'un problème de mécanique quantique déja résolu. Les solutions (états propres et fonctions propres) sont presque tous retrouvés. / The factorization methode is applied to the initial data of an already solved quantum mechanics problem. The solutions (eigenfunctions and eigenvalues) are almost all rederived.

Un des aspects fascinants de la théorie des cordes, c'est qu'elle vit dans l'espace-temps de dix dimensions. Mais cela implique que, pour la relier à des observations phénoménologiques, elle devrait ȇtre compactifiées à quatre dimensions. Un cas particulièrement riche, mais toujours faisable correspond à la compactification sur une variété de Calabi-Yau qui donne à basse énergie une théorie effective avec la supersymétrie N=2. L'action de cette théorie est complètement déterminée par la métrique sur son espace des modules qui comporte deux composantes correspondant aux multiplets vectoriels et hypermultiplets. La première est classiquement exacte et bien comprise, alors que la dernière reçoit des corrections quantiques et est connue de porter une géométrie compliquée quaternion-Kählerrienne. Dans cette thèse, nous présentons nos résultats sur la description complète non-perturbative de l'espace des modules des hypermultiplets. Nous montrons comment toutes les corrections quantiques, qui comprennent des contributions perturbatives d'une boucle ainsi que celles non-perturbatives venant des D-branes et NS5-branes, sont incorporées dans le cadre de l'approche twisteurielle. Ce cadre, que nous élaborons ici en détail, fournit une description mathématique puissante des variétés hyperkähleriennes et quaternion-Kähleriennes et il est indispensable pour la formulation de la géométrie non-perturbative de l'espace des modules des hypermultiplets. Nous présentons également de nouveaux résultats sur la dualité-S, symétrie miroir quantique, les connexions à des modèles intégrables et aux cordes topologiques.

La production de paires de quarks top se désintégrant en deux leptons dont au moins un lepton tau est étudiée dans le cadre de l'expérience CDF auprès du collisionneur proton-antiproton, Tevatron, a FNAL aux USA. La sélection exige un électron ou un muon produit par désintégration du lepton tau ou par désintégration d'un W. L'analyse utilise toutes les données enregistrées, 9 fb-1, avec un déclenchement basé sur un électron ou muon à faible moment transverse et une trace chargée isolée. La section efficace de production de paires de top a cette énergie obtenue est de 8,2+-1.7(+1.2-1.1)+-0,5 pb, et le rapport de branchement en leptons tau est de 0,120+-0,027(+0,022-0,019)+-0,007 avec erreur statistique, systématique et sur la luminosité respectivement. Ce sont à jour les résultats les plus précis dans ce canal de désintégration du top, en bon accord avec les résultats obtenus au Tevatron avec tous les autres canaux de désintégration du top. Le rapport de branchement est aussi mesuré en séparant les événements tau plus lepton et avec deux leptons tau avec une méthode de maximum de vraisemblance. C'est la première fois que ces modes de désintégration sont identifiés séparément. Par une méthode de maximum de vraisemblance appliquée pour séparer ces deux canaux une mesure alternative du rapport de branchement du top en lepton tau de 0,098+-0,022(stat.)+-0,014(syst.) est obtenue, en bon accord avec les prédictions du Modèle Standard. Une limite supérieure de 0,159 pour ce rapport de branchement, avec 95% de niveau de confiance est extraite donnant un indice de Physique au delà du Modèle Standard en particulier un possible boson de Higgs chargé.

Des informations sur la nouvelle physique peuvent être extraites de plusieurs secteurs indépendants et en particulier : les recherches directes du Higgs et de nouvelles particules aux collisionneurs, qui sont entrées dans une nouvelle ère avec le démarrage du LHC, les informations indirectes des données de physique des saveurs, en utilisant les résultats obtenus aux usines à B et récemment aussi au LHC, et enfin les informations indirectes sur la densité relique de mati ère noire et les recherches directes de mati ère noire, en particulier au vu des résultats récents des expériences XENON, CoGENT, CRESST, ... Combiner les informations des différents secteurs est en effet riche d'implications et permet de réduire l'espace des paramètres des scénarios de nouvelle physique. Nous avons démontré l'existence de telles synergies dans le contexte de la supersymétrie pour différents scénarios contraints ainsi que pour un scénario plus g én éral du MSSM (pMSSM).

<p>The physics of complex systems has grown considerably as a field in recent decades, largely due to improved computational technology and increased availability of systems level data. One area in which physics is of growing relevance is molecular biology. A new field, systems biology, investigates features of biological systems as a whole, a strategy of particular importance for understanding emergent properties that result from a complex network of interactions. Due to the complicated nature of the systems under study, the physics of complex systems has a significant role to play in elucidating the collective behavior.</p><p>In this dissertation, we explore three problems in the physics of complex systems, motivated in part by systems biology. The first of these concerns the applicability of Boolean models as an approximation of continuous systems. Studies of gene regulatory networks have employed both continuous and Boolean models to analyze the system dynamics, and the two have been found produce similar results in the cases analyzed. We ask whether or not Boolean models can generically reproduce the qualitative attractor dynamics of networks of continuously valued elements. Using a combination of analytical techniques and numerical simulations, we find that continuous networks exhibit two effects -- an asymmetry between on and off states, and a decaying memory of events in each element's inputs -- that are absent from synchronously updated Boolean models. We show that in simple loops these effects produce exactly the attractors that one would predict with an analysis of the stability of Boolean attractors, but in slightly more complicated topologies, they can destabilize solutions that are stable in the Boolean approximation, and can stabilize new attractors.</p><p>Second, we investigate ensembles of large, random networks. Of particular interest is the transition between ordered and disordered dynamics, which is well characterized in Boolean systems. Networks at the transition point, called critical, exhibit many of the features of regulatory networks, and recent studies suggest that some specific regulatory networks are indeed near-critical. We ask whether certain statistical measures of the ensemble behavior of large continuous networks are reproduced by Boolean models. We find that, in spite of the lack of correspondence between attractors observed in smaller systems, the statistical characterization given by the continuous and Boolean models show close agreement, and the transition between order and disorder known in Boolean systems can occur in continuous systems as well. One effect that is not present in Boolean systems, the failure of information to propagate down chains of elements of arbitrary length, is present in a class of continuous networks. In these systems, a modified Boolean theory that takes into account the collective effect of propagation failure on chains throughout the network gives a good description of the observed behavior. We find that propagation failure pushes the system toward greater order, resulting in a partial or complete suppression of the disordered phase.</p><p>Finally, we explore a dynamical process of direct biological relevance: asymmetric cell division in <italic>A. thaliana</italic>. The long term goal is to develop a model for the process that accurately accounts for both wild type and mutant behavior. To contribute to this endeavor, we use confocal microscopy to image roots in a SHORTROOT inducible mutant. We compute correlation functions between the locations of asymmetrically divided cells, and we construct stochastic models based on a few simple assumptions that accurately predict the non-zero correlations. Our result shows that intracellular processes alone cannot be responsible for the observed divisions, and that an intercell signaling mechanism could account for the measured correlations.</p> / Dissertation

<p>Ultrarelativistic heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) are thought to have produced a state of matter called the Quark-Gluon-Plasma (QGP). The QGP forms when nuclear matter governed by Quantum Chromodynamics (QCD) reaches a temperature and baryochemical potential necessary to achieve the transition of hadrons (bound states of quarks and gluons) to {it deconfined} quarks and gluons. Such conditions have been achieved at RHIC, and the resulting QGP created exhibits properties of a near perfect fluid. In particular, strong evidence shows that the QGP exhibits a very small shear viscosity to entropy density ratio &eta/s, near the lower bound predicted for that quantity by Anti-deSitter space/Conformal Field Theory (AdS/CFT) methods of &eta/s = $hbar$/ 4 &pi $k_B$ where $hbar$ is Planck's constant and $k_B$ is Boltzmann's constant. As the produced matter expands and cools, it evolves through a phase described by a hadron gas with rapidly increasing $eta/s$.</p><p>This thesis presents robust calculations of $eta/s$ for hadronic and partonic media as a function of temperature using the Green-Kubo formalism. An analysis is performed for the behavior of $eta/s$ to mimic situations of the hadronic media at RHIC evolving out of chemical equilibrium, and systematic uncertainties are assessed for our method. In addition, preliminary results are presented for the bulk viscosity to entropy density ratio $zeta/s$, whose behavior is not well-known in a relativistic heavy ion collisions. The diffusion coefficient for baryon number is investigated, and an algorithm is presented to improve upon the previous work of investigation of heavy quark diffusion in a thermal QGP. </p><p>By combining the results of my investigations for $eta/s$ from our microscopic transport models with what is currently known from the experimental results on elliptic flow from RHIC, I find that the trajectory of $eta/s$ in a heavy ion collision has a rich structure, especially near the deconfinement transition temperature $T_c$. I have helped quantify the viscous hadronic effects to enable investigators to constrain the value of $eta/s$ for the QGP created at RHIC.</p> / Dissertation

Phonons and electrons are two types of excitations which are responsible for many properties of condensed matter materials. The interaction between them plays an important role in condensed matter physics. In this thesis we present some theoretical investigations of the effects due to the interactions between phonons and electrons interactions. We show evidence that a structural martensitic transition is related to significant changes in the electronic structure, as revealed in thermodynamic measurements made in high magnetic fields. The effect of the magnetic field is considered unusual, as many influential investigations of martensitic transitions have emphasized that the structural transitions are primarily lattice dynamical and are driven by the entropy due to the phonons. We provide a theoretical frame-work which can be used to describe the effect of a magnetic field on the lattice dynamics in which the field dependence originates from the dielectric constant. The temperature-dependence of the phonon spectrum of alpha-uranium has recently been measured by Manley et al. using inelastic neutron scattering and x-ray scattering techniques. Although there is scant evidence of anharmonic interactions, the phonons were reported to show some softening of the optic modes at the zone boundary. The same group of authors later reported that an extra vibrational mode was observed to form at a temperature above 450 K. The existence of the proposed new mode is inconsistent with the usual theory of harmonic phonons, as applied to a structure composed of a monoclinic Bravais lattice with a two-atom basis. We investigate the effect that the f electron-phonon interaction has on the phonon spectrum and its role on the possible formation of a breathing mode of mixed electronic and phonon character. We examine the model by using Green’s function techniques to obtain the phonon spectral density. Some materials undergo phase transitions from a high temperature state with periodic translational invariance to a state in which the electronic charge density is modulated periodically. The wave vector of the modulation may be either commensurate or incommensurate with the reciprocal lattice vectors of the high temperature structure. In the case of an incommensurate charge density wave, the system supports phason excitation. For an incommensurate state, the new ground state has a lower symmetry than the high temperature state since the charge density does not have long-ranged periodic translational order. If the metal is ideal (with no impurities), a charge density wave should be able to slide throughout the crystal without resistance, resulting in current flow similar to that of a superconductor. The phason is an excitation of the charge density wave which is related to the collective motion of electrons. We estimate the phason density of states, and the phason contribution to the specific heat. Angle-resolved photoemission experiments have been performed on USb2, and very narrow quasiparticle peaks have been observed in a band which local spin-density approximation (LSDA) predicts to osculate the Fermi energy. The observed band is found to be depressed by 17 meV below the Fermi energy. The experimentally observed quasiparticle dispersion relation for this band exhibits a kink at an energy of about 23 meV below the Fermi energy. The kink is not found in LSDA calculations and, therefore, is attributable to a change in the quasiparticle mass renormalization by a factor of approximately 2. The existence of a kink in the quasiparticle dispersion relation of a band which does not cross the Fermi energy is unprecedented. The kink in the quasiparticle dispersion relation is attributed to the effect of the interband self-energy involving transitions from the osculating band into a band that does cross the Fermi energy. / Physics