The Finite Difference Time Domain (FDTD) Method has been a powerful tool in numerical simulation of electromagnetic (EM) problems for decades. In recent years, it has also been applied to biomedical research to investigate the interaction between EM waves and biological tissues. In Wireless Body Area Networks (WBANs) studies, to better understand the localization problem within the body, an accurate source/receiver model must be investigated. However, the traditional source models in FDTD involve effective volume and may cause error in near field arbitrary direction. This thesis reviews the basic mathematical and numerical foundation of the Finite Difference Time Domain method and the material properties needed when modeling a human body in FDTD. Then Coincident Phase Centers (CPCs) point sources models have been introduced which provide nearly the same accuracy at the distances as small as 3 unit cells from the phase center. Simultaneously, this model outperforms the usual sources in the near field when an arbitrary direction of the electric or magnetic dipole moment is required.

Rotationally averaged polarizabilities and intrinsic electric dipole moments of sodium clusters are measured and reported. The experimental method is a molecular beam deflection. Our precision is the highest (<5%) and the range of the cluster sizes is the broadest to date (Na₁₀ ∼ Na₃₀₀). Compared to the earlier measurements, our data covers all sizes with no gaps up to the largest cluster. The fine structure in the polarizability curve is previously unobserved. We have carefully ruled out several possible explanations. And we find an earlier existing theory could explain the facts but will lead to magic numbers which were not seen in some previous experiments. A detailed theory is needed to understand the behaviors we see. Intrinsic electric dipole moments (EDM) of sodium clusters are probed to answer the intriguing question: Do metal clusters develop electric dipole moments like molecules? Some theories have predicted the existence of EDM in ground state sodium clusters and gave their magnitudes. We put upper bounds on the EDM of sodium clusters and find that they are orders of magnitude smaller than the predictions. This provokes an interesting question: how can one define metallicity in metal clusters? Our measurements are performed at cryogenic temperature 20 Kelvin. At this temperature the clusters are believed to be in their vibronic ground states.

Student Number : 9713144D - MSc Dissertation - School of Electrical Engineering - Faculty of Engineering and the Built Environment / The performance of the Log Periodic dipole array antenna has been characterized, in the form of parametric curves available in most antenna design handbooks and other relevant literature. These characteristic curves are often limiting in scope, as for instance they do not contain parametric curves giving the relationship between the boom-length 'L' and the number of dipole element 'N' for any given bandwidth, even when it is known that these two parameters are the main cost determinants of a LPDA Antenna. The concept of convergence is introduced to aid cost optimization of the LPDA Antenna in terms of number of dipole element 'N'. Although 'N' is used as the minimization criterion, the criteria for establishing convergence encompass all the main electrical characteristics of the LPDA Antenna, such as VSWR, gain and radiation patterns. Lastly, the effects of boomimpedance 'Zo' and length to diameter ration 'Ln/Dn', on the performance characteristics of the LPDA Antenna was investigated with the view to determining if neglecting the effects of these two parameters were responsible for the disparity in the directive gain values obtained by R. L Carrel compared to those obtained by later researchers. The investigation indicates that if an LPDA Antenna is converged, then the effects of Zo and Ln/Dn ratio though significant can not alone account for the fairly large disparity in the gain values. In other to perform these investigations, a modern scientific tool in the form of numerical modeling by method of moments based, Super Numerical electromagnetic code version2 was utilized. The numerical modeling tool was first validated by agreement between measured values and the values as predicted by the modeling tool. Next, simulation of the performance of LPDA antennas under variations of their number of elements was done. Thereafter, the means and standard deviations of the gain were extracted from the simulated numerical models. Trends in the pattern of variation of the means and standard deviations of the gain are used as the basis for deciding the value of number of element at which the antenna can yield acceptable performance (convergence criteria). These are presented as convergence curves, which gives for any given boom-length and operating bandwidth, the minimum number of elements required for the antenna to yield acceptable performance. Finally, the effect of length to diameter ratio and boom-impedance on the gain of optimized LPDA antennas are presented as parametric curves.

Cette thèse couvre l'auto-assemblage de particules dipolaires (magnétiques/électriques). Ces systèmes sont abondants en physique de la matière condensée (molécules et nanoparticules magnétiques, particules colloïdales magnétiques, bactérie magnétotactique, etc.). Sur un plan fondamental, ils représentent un défi important en raison de l'anisotropie et de la longue portée de l'interaction de paire. Le principal objectif de ce travail de recherche est de prédire les microstructures de ces systèmes en tenant compte de façon adéquate de l'interaction complexe dipôle-dipôle ainsi que des effets stériques et ceux dus à un éventuel confinement. Comprendre et revisiter les interactions de filaments dipolaires tels que des aiguilles et des chaînes faites de billes dipolaires est une première étape importante de cette thèse. En effet, les chaînes sont les constituants élémentaires de nombreux systèmes dipolaires, notamment sous l'effet d'un champ magnétique extérieur appliqué. Ensuite, l'agrégation colonnaire des chaînes dipolaires est examinée, ce qui conduit aussi naturellement à l'étude des cristaux dipolaires massifs où une nouvelle phase est découverte. Le cas plus générique des chaînes hélicoïdales est discuté en considérant les situations limites que sont les chaînes linéaires droites et en zigzag. L'association des chaînes dipolaires, dans le cas bidimensionnel, forme des rubans, puis une monocouche avec un réseau hexagonal. La réponse non triviale d'un tel réseau à un champ magnétique perpendiculaire imposé est aussi étudiée. Il est démontré qu'un réseau rhombique peut être induit de cette façon. Finalement, la sédimentation de particules paramagnétiques dans une monocouche inclinée en présence d'un champ magnétique est explorée via une étude mêlant expériences, théorie et simulations. L'ordre induit par gravité s'avère être une voie prometteuse pour l'élaboration contrôlée de réseaux bidimensionnels / This thesis covers the self-assembly of dipolar (magnetic/dielectric) particles. These systems are abundant in condensed matter physics (magnetic molecules and nanoparticles, magnetic colloidal particles, magnetotactic bacteria, etc). They also represent a fundamental challenge owing to the both long range and anisotropic nature of the pair interaction. The main objective of this research work is to predict the microstructures of these systems by properly handling the intricate dipole-dipole interaction combined with steric and possibly confinement effects. Understanding and revisiting the interaction of dipolar filaments such as needles or chains made up of dipolar beads is a first important achievement in this thesis. Indeed, the chains are the fundamental building blocks of many dipolar systems especially under applied external magnetic field. Then, the columnar aggregation of dipolar chains is investigated which naturally leads to the study of the bulk dipolar crystals. A new phase is discovered there. The more generic case of helical chains is discussed by considering limiting situations such as straight linear chains and zigzag chains. The association of dipolar chains in two-dimensions forms ribbons then a monolayer with triangular lattice symmetry. The interesting response of such a layer to an imposed perpendicular magnetic is addressed as well. It is demonstrated that rhombicity can be induced that way. Finally, sedimenting paramagnetic particles in a tilted monolayer in presence of a magnetic field are investigated by experiments, theory and simulations. The gravity-mediated ordering is found to be a promising route to elaborate tailored two-dimensional patterns

Lorsque l’on place des atomes suffisamment proches les uns des autres, l’interaction dipôle-dipôle résonante entre les atomes modifie leurs propriétés. Les atomes se comportent alors de manière collective. Ces effets collectifs se produisent lorsque les distances interparticulaires sont de l’ordre de l/(2Pi), où l est la longueur d’onde de la transition atomique. La densité atomique est alors de l’ordre de 10^14 at/cm^3. Afin de créer des échantillons d’atomes froids présentant des densités aussi élevées, nous avons mis en place plusieurs méthodes de chargement de nos pinces optiques de taille micrométrique. L’une d’elles utilise un processus d’évaporation forcée qui amène les atomes proches de la dégénérescence quantique. En utilisant des nuages denses contenant quelques centaines d’atomes à des densités spatiales élevées, et en étudiant les modifications de la diffusion de la lumière qui en résultent, nous avons pu mettre en évidence des effets collectifs entre les atomes. Nous avons par ailleurs mesuré le retard de Wigner associé à la diffusion élastique de la lumière par un atome unique de rubidium. Nous avons mesuré un retard proche de la valeur théorique, c’est-à-dire deux fois la durée de vie de la transition atomique (52 ns). / When several atoms are placed close to each other, the resonant dipole-dipole interactionbetween atoms modifies the atomic properties and atoms behave collectively. These collective effects occur for interatomic distances on the order of l/(2Pi) where l is the wavelength of the atomic transition. The atomic density is then on the order of 10^14 at/cm^3. To create such cold atomic samples, we load optical tweezers with a microscopic size according to several loading schemes. One of them uses forced evaporative cooling and brings the atoms close to quantum degeneracy. We have used dense clouds containing a few hundred atoms with a high spatial density to demonstrate collective effects between the atoms. In particular, we have studied how these effects modify the scattering of light by the cloud. Besides, we have also measured for the first time the time-delay associated to the elastic scattering of light by a single rubidium atom, the so-called Wigner delay. We have shown that this delay is close to the theoretical prediction of twice the lifetime of the atomic transition (52 ns).

Cette thèse présente différentes études sur la formation et la détection de molécules froides. Différents états moléculaires de grandes élongations, pour la molécule Cs2, sont étudié par spectroscopie de photoassociation et d'ionisation. Ces différentes études ont permis d'affiner notre compréhension des mécanismes de photoassociation d'atomes froids formant des molécules dans l'état fondamental triplet (a 3Σu+).<br />Une détection non sélective a été développée, pour la recherche de mécanismes de formation de molécules froides dans l'état fondamental singulet avec peu de vibration. Avec cette nouvelle détection, un nouveau mécanisme de formation de molécules par photoassociation d'atomes froids de césium a été trouvé. Celui-ci permet de former efficacement des molécules dans une distribution de niveaux avec très peu de vibration dans l'état fondamental (X 1Σg+).<br />En utilisant un laser femtoseconde (large spectralement) façonné, un refroidissement vibrationnel des molécules a été démontré, permettant la formation de molécules froides sans vibrations. Le laser femtoseconde, permet d'exciter les nombreux niveaux vibrationnels, créés par photoassociation, il réalise ainsi un pompage optique des molécules. Le laser est façonné de manière à rendre l'état de vibration zéro, noir pour ce laser, et ainsi accumuler toutes les molécules vers ce seul état. <br />Ce résultat est également simulé par un model théorique simple. Cette simulation permet de généraliser l'idée au refroidissement de la rotation des molécules. <br /><br />Une partie (résumée) présente, en s'appuyant sur les différents articles publiés, les études sur les interactions dipôle-dipôle, à grandes portées, entre atomes de Rydberg.

We present an efficient method to compute the electrostatic fields, torques and forces in dipolar systems, which is based on the fast Fourier transform for nonequispaced data (NFFT). We consider 3d-periodic, 2d-periodic, 1d-periodic as well as 0d-periodic (open) boundary conditions. The method is based on the corresponding Ewald formulas, which immediately lead to an efficient algorithm only in the 3d-periodic case. In the other cases we apply the NFFT based fast summation in order to approximate the contributions of the nonperiodic dimensions in Fourier space. This is done by regularizing or periodizing the involved functions, which depend on the distances of the particles regarding the nonperiodic dimensions. The final algorithm enables a unified treatment of all types of periodic boundary conditions, for which only the precomputation step has to be adjusted.

Thesis (Ph.D.); Submitted to the University of California at Berkeley, Berkeley, CA (US); 1 Sep 2003. / Published through the Information Bridge: DOE Scientific and Technical Information. "LBNL--55023" Li, Liang-shi. USDOE Director. Office of Science. Office of Basic Energy Sciences (US) 09/01/2003. Report is also available in paper and microfiche from NTIS.

Dans ce manuscrit, nous présentons une étude expérimentale d’un gaz de Bose de spin-1 avec des interactions antiferromagnétiques, réalisée pour des atomes de sodium ultra-froids dans l’état hyperfin F=1. Gr au refroidissement évaporatif, nous obtenons un condensat de Bose-Einstein (CBE) spineur, soit dans un piège très confinant (« piège 0D »), soit sous la forme d’un quasi-condensat quasi-unidimensionnel dans un piège très allongé. Les deux systèmes présentent un ordre magnétique a très basse température, qui résulte de la compétition entre les interactions d’échange et l’énergie Zeeman quadratique q dans un champ magnétique externe. Nous étudions dans un premier temps l’ordre magnétique se forme dans le piège 0D. À très bassetempérature deux phases magnétiques sont possible : une phase dite « antiferromagnétique » pour q < Us, ou une phase dite « à aimantation transverse » dans le cas inverse. Dans ce travail, nous nous plaçons près de la température critique. Nous mesurons plusieurs scénarios de condensation séquentielles en changeant la magnétisation et le champ magnétique externe, ou une composante Zeeman condense toujours en premier et ou l’ordre magnétique n’apparait qu’à une seconde température de condensation. Les résultats expérimentaux pour les températures critiques sont bien décrits par une théorie d’Hartree-Fock simplifiée dans les cas ou une seule composante Zeeman est condensée. Dans un second temps, nous étudions l’ordre magnétique du système quasi-unidimensionnel a basse température. On observe la formation de domaines de spin ou les composantes Zeeman se sépare spontanément en domaines disjoints en l’absence de force extérieure (par exemple, un gradient de champ magnétique). On étudie l’état d’équilibre du système en fonction de la magnétisation et du champ magnétique. On observe une transition de phase entre une phase miscible et une phase immiscible ou la composante Zeeman mF = 0 forme un domaine séparé de mF = ±1 dans le centre du piège. L’équation d’état d’un nuage polarisé (atomes dans l’état mF = +1) est utilisée pourmesurer la température du système. Enfin, nous mesurons la réponse mécanique a une force magnétique appliquée pour un système binaire mF = 0, +1. Nous mesures une exaltation de la réponse par rapport a l’attente na basée sur l’effet Zeeman habituel, d’un facteur qui peut varier de plusieurs dizaines a environ cent. La configuration spatiale des domaines est ainsi sensible a de très faibles gradients de champ magnétique inférieurs au mG/cm. / In this thesis we present the experimental study of a spin-1 Bose gas of ultra-cold Na atoms with antiferromagnetic interactions in the F=1 manifold. Thanks to evaporative cooling in optical traps we obtain, depending on the trap geometry, quasi-pure spinor Bose-Einstein condensates (BEC) in 0D traps and quasi-condensates in quasi-1D traps. The quantum-statistical Bose enhancement, typical of BEC, allows inter-component interactions (between the different Zeeman components) to order the system just below the Bose-Einstein condensation temperature. The magnetic ordering of the system is set: by contact interactions, that do not change the Zeeman populations, by spin-exchange interactions (U_s spin-exchange energy), that do, and by the quadratic Zeeman energy q. In particular, for q < U_s the system is in the antiferromagnetic phase while, for q > U_s, is in the transverse magnetised phase. We study first in which order the magnetic ordering appears, in the 0D trap, near to the critical temperature for BEC. We experimentally study different condensations scenarii varying q and magnetisation. The condensation of the different components is sequential and strongly influenced by interactions. We find a good agreement between the experimental data and a simplified Hartree-Fock model.Then we study the magnetic ordering, at T=0, in a quasi-1D trap. The system presents the formation of spin domains. We study the ground state of the system varying magnetisation and q. We observe a transition from the miscible to the immiscible phase, associated with the transition from the antiferromagnetic to the transverse magnetised phase. This is due to the relative strengths of inter-species contact interaction. To measure the temperature of the system, we measure the equation of state for a polarised cloud (all atoms in m_F=+1). Finally, we prepare the system in the immiscible phase m_F=0,+1 and we measure the spin-dipole polarisability of the system.

SENA FILHO, Roberto Ferreira. Teoria microscópica de ondas de spin em nanofios magnéticos. 2007. 74 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2007. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-05-22T19:45:31Z No. of bitstreams: 1 2007_dis_rfsenafilho.pdf: 3612063 bytes, checksum: 5cbe75be9b4bf3c77cd9b1136fad2a2a (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-05-22T19:46:04Z (GMT) No. of bitstreams: 1 2007_dis_rfsenafilho.pdf: 3612063 bytes, checksum: 5cbe75be9b4bf3c77cd9b1136fad2a2a (MD5) / Made available in DSpace on 2015-05-22T19:46:04Z (GMT). No. of bitstreams: 1 2007_dis_rfsenafilho.pdf: 3612063 bytes, checksum: 5cbe75be9b4bf3c77cd9b1136fad2a2a (MD5) Previous issue date: 2007 / The dynamical behavior of spins in magnetic materials is affected by its geometry and dimensionality. One can find several new results in the literature exploiting the magnetic properties of low dimension systems with different geometries, since the development of new devices such as: nanosensors, high density magnetic storage, etc., is closely related to new geometries. In this piece of work, we study the propagation of spin waves on cylindrical magnetic nanowires described by a microscopic theory through the Heisenberg Hamiltonian, where we consider the spins fixed at the sites lattice and the transversal section of the wire is hexagonal. Our model takes into account the exchange interaction between the spins that can be ferromagnetic or antifferomagnetic, the interaction of an external field with the spins (Zeeman interaction), anisotropic interactions due to a preferred direction of magnetization, and finally dipole-dipole interactions. The spins are described by boson operators through Holstein-Primakoff representation. The equations of motion for the spins are written in terms of these operators and translational symmetry in a preferential direction allows us to calculate several excitations spectra. / O comportamento dinâmico de spins em materiais magnéticos é influenciado pela geometria que eles apresentam. Além disso, outro aspecto relevante é a dimensionalidade do sistema. Trabalhos recentes comprovam o interesse do estudo das propriedades magnéticas em sistemas de baixa dimensionalidade, que é devido em grande parte as aplicações tecnológicas, tais como: nanosensores, gravadores magnéticos de alta densidade, dispositivos magneto-eletrônicos, etc. Neste trabalho estudamos a propagação de ondas de spin em nanofios magnéticos cilíndricos, onde a abordagem é feita utilizando teoria microscópica, através do Hamiltoniano de Heisenberg, em que os spins são considerados fixos nos sítios da rede e cuja geometria da seção transversal dos cilindros é hexagonal. Entre as interações magnéticas estudadas consideramos: a interação de troca que pode ser ferromagnética se os primeiros vizinhos dos spins estão numa configuração paralela, ou antiferromagnética se estiverem antiparelelos; a interação Zeeman que é devido ao campo magnético externo aplicado ao sistema; a interação de Anisotropia, esta sendo responsável pela direção de magnetização preferida que diversos sistemas magnéticos reais apresentam e a interação dipolar de natureza magnetostática, presente em todos os materiais. O formalismo leva em consideração a dependência espacial dos spins no sistema, onde os operadores de spin do hamiltoniano são escritos em termos de operadores bosônicos de criação e aniquilação através da Representação de Holstein-Primakoff. Em seguida, aproveitando-se da simetria translacional em uma direção devido a periodicidade da rede, realizamos a transformada de Fourier para estes operadores fornecendo um sistema de equações matriciais no espaço dos vetores de onda. A partir desse sistema de equações obtemos vários espectros de excitação como: a relação de dispersão para as ondas de spin, que é o gráfico onde mostra como a frequência de ondas de spin varia em função do vetor de onda e a variação da energia do sistema com o campo aplicado.