Ce travail porte sur la spectroscopie magnéto-optique de semi-conducteurs magnétiques dilués (DMS) : ZnO et GaN dopés manganese, fer et cobalt. Les deux semi-conducteurs hôtes, ZnO et GaN ont une grande bande interdite, une structure wurtzite, une faible interaction spin - orbite et une forte interaction d'échange excitonique entre trous et électrons. En présence de champs magnétiques, les ions magnétiques induisent un effet Zeeman géant dont l'interprétation est complexe : les excitons s'anti-croisent et leurs énergies de transition et leurs forces d'oscillateur sont fortement influencées par l'effet Zeeman géant.<br />On a mesuré expérimentalement le splitting Zeeman géant des excitons A et B avec des couches epitaxiées sur saphir (0001) et une propagation de la lumiere parallele a l'axe c du cristal et au champ magnétique (configuration Faraday). Le splitting Zeeman géant diminue avec la température et augmente non linéairement avec le champ magnétique en accord avec l'aimantation calculée des spins isolés. Une analyse quantitative nous a permis d'analyser les propriétés magnétiques et de mesurer les intégrales d'échange pour l'ensemble des matériaux étudiés. Pour des ions avec une configuration d5 (Mn2+ et Fe3+), l'aimantation suit une fonction de Brillouin, mais pour les configurations d7 et d4 (Co2+ ou Mn3+) l'interaction spin-orbite et le champ cristallin trigonal induisent une aimantation anisotrope, en accord avec l'analyse des transitions internes des ions mesurées en spectroscopie infrarouge. Pour (Ga,Mn)N, et (Ga,Fe)N, nous avons trouvé un signe positive pour l'intégrale d'échange entre trous et spins localisés (beta). En supposant une symétrie de la bande de valence dans ZnO correspondant a une interaction spin-orbite positive (Gamma_9, Gamma_7, Gamma_7), nous trouvons un signe négative de beta pour (Zn,Co)O, et beta est de pres de zéro pour (Zn,Mn)O. Toutefois, dans l'hypothese avec spin-orbite négative, nous trouvons un signe positif de beta. Les signes et les valeurs des intégrales d'échange déterminées a partir de nos mesures magnéto-optiques ne peuvent pas etre expliqués par des tendances matérielles et des modeles basés sur l'approximation de cristal virtuel. Ceci suggere que l'échange p-d en DMS a large bande interdite, soit dans le régime de couplage fort, et la nature de splitting Zeeman géant observé est différente qu'en semi-conducteurs magnétiques dilués classiques.

Ce travail se place dans le contexte des recherches sur les mémoires quantiques pour la lumière. L'information quantique est stockée dans un état de superposition atomique, dont la durée de vie détermine le temps maximum de stockage.On s'intéresse particulièrement aux matériaux capables de capturer la lumière par excitation résonnante d'une raie d'absorption, puis de conserver l'information quantique dans un état de superposition du fondamental électronique.Dans Tm3+:YAG, l'information est enregistrée dans un état de spin nucléaire. Cependant le champ magnétique qui lève la dégénérescence nucléaire entraîne les différents spins à des vitesses de précession différentes, ce qui tend à détruire l'aimantation initiale, porteuse de l'information.Une étude quantique du cristal est réalisée lors du premier chapitre de ce manuscrit. Les trois chapitres suivants traitent des différents mécanismes conduisant au déphasage des spins nucléaires. On y trouvera différente analyses théoriques qui seront confirmées par un ensemble de résultats expérimentaux, ainsi qu'une description détaillée du dispositif expérimental. Enfin le dernier chapitre, prospectif, exploite les outils développés au cours de la thèse pour préserver les cohérences optiques. Il présente quelques résultats expérimentaux prometteurs sur l'allongement du temps de vie de ces cohérences optiques.

Magnetic nanorings are the object of increasing scientific interest because they possess the vortex (stray field free) state which ensures lower magnetostatic interactions between adjacent ring elements in high packing density memory devices. In addition, they have other potential applications such as single magnetic nanoparticle sensors, microwave-frequency oscillators and data processing. The stabilization of magnetization state, types of domains and domain wall structures depends on the competing energies such as magnetostatic, exchange and anisotropy. The nucleation/ pinning of domain walls depends on the local inhomogeneity in shape such as roughness, notches etc, which play an important role in stabilizing domain configurations that can be controlled by magnetic field/spin polarized current etc. The information gained by the study of magnetization reversal in the nanoring devices could help in understanding the possible stable magnetization states, which can be incorporated into the development of magnetic logic and recording devices in a NR-based architecture. The magnetization reversal and the stable states in the symmetric cobalt nanorings (NRs) attached with nanowires (NWs) (at diametrically opposite points), is studied through magnetoresistance (MR) measurements by application of in-plane magnetic field (H). Here, a strong in-plane shape anisotropy is introduced in cobalt thin films by patterning them into NR and NWs. The presence or absence of a DW in the device is detected utilizing the AMR property of the material, where the presence of DW leads to a decrease in the resistance of the probed section of the device. It is demonstrated that the magnetization reversal of the device with smaller width, proceeds through four distinct magnetization states, one of these is the stabilized vortex state that persists over a field range of 0.730 kOe. The effect of width (from 70 nm to 1 µm) and diameter (from 2 µm to 6 µm) on the switching behavior is demonstrated. The magnetization states observed in the MR measurements are well supported by micromagnetic simulations. A statistical analysis of switching fields in these devices was demonstrated by histogram plot (of switching counts) to understand the repeatability and reproducibility of switching characteristics. In addition, the magnetization reversal of permalloy NR is also studied by MR experiment when two NWs are attached to it in two different configurations. It has been demonstrated that a vortex state can be stabilized if the NWs are attached in a way that they are at an obtuse angle with respect to each other (type-II device) which is not the case if the NWs are attached at diametrically opposite points (type-I device). This occurs because the NWs reverse at different fields as they are asymmetric with respect to applied magnetic field at every angle. The angular dependence study of the magnetization states indicates that the vortex state could be always stabilized in the type-II device irrespective of the direction of in-plane applied magnetic field while it is not the case in type-I device. The experimental observations are in good agreement with micromagnetic simulations performed on similar device structures. Further, in the last part of the thesis, the magnetization reversal of geometrically engineered cobalt NR (of width 80 nm) devices are studied by application of H. Two types of cobalt nanoring devices were fabricated. In type-1 devices the NR is attached with two nanowires (NWs) at diametrically opposite positions. In type-2 devices the NR is attached with one NW, whose other end is attached to a 5 µm x 5 µm square pad. In type-2 device, the pad reverses first, thus causing the generation of a DW at the junction of the nucleation pad and the NW. The device type-2 possesses five distinct magnetization states, one of these is the vortex state. Easy nucleation of domain walls (DWs) results in a decrease of switching field corresponding to the reversal of the nanowire. This leads to an increase in the range of fields, where the vortex state exists. In addition, angular dependence of the switching behavior indicates that the vortex state can be stabilized at all in-plane orientations of H. This occurs because of the fact that symmetry was broken due to the presence of single domain wall pinning center which was the junction of the NR and NW. The results of our micromagnetic simulations are in a good agreement with the experimental results. These results are important to understand the role of NWs which allows the formation of vortex state at every angle of the in-plane H. In type-1 device, the simulation shows that when the field is applied at any angle away from the axis of the NW, the vortex state cannot be stabilized. The width dependent study of switching fields indicates, that the switching fields decrease with increasing the width of NR devices due to a reduction of the demagnetization field.

Made available in DSpace on 2014-06-12T18:01:22Z (GMT). No. of bitstreams: 2 arquivo2356_1.pdf: 8441876 bytes, checksum: 31a6f5c664996110b64b2134094cdd42 (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2009 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Nesta tese, realizamos experiências relacionadas com o armazenamento de informação óptica em um ensemble de átomos frios de césio. A configuração experimental utilizada foi semelhante à de mistura de quatro ondas degenerada resolvida no tempo, usando a geometria conhecida como conjugação de fase óptica. Investigamos, tanto teórica quanto experimentalmente, diversos efeitos coerentes e mostramos explicitamente o armazenamento de uma grade de coerência entre os subníveis Zeeman pertencentes a um nível hiperfino do estado fundamental do césio. Analisamos o comportamento da energia dos pulsos gerados em função das intensidades dos campos envolvidos e observamos diferentes comportamentos de saturação. Em outro experimento, estudamos as correlações de energia entre diferentes modos temporais e espaciais gerados coerentemente pelo meio, o qual permitiu observar que os efeitos de emissão espontânea são predominantes quando cada transição do sistema é acessada com mesma intensidade de leitura. As larguras temporais dos modos envolvidos no processo mostraram ser invariantes quando a intensidade total de leitura se mantém constante. Realizamos também, usando um campo magnético externo, uma investigação sobre a evolução coerente do estado armazenado. Medimos a evolução temporal do sinal difratado em duas condições experimentais distintas: com o campo magnético externo aplicado paralelo e perpendicularmente à direção de propagação dos campos ópticos. Em ambos os casos, observamos uma série de collapses and revivals que estão intimamente relacionados com a precessão de Larmor da grade Zeeman armazenada em torno do campo magnético aplicado externamente. Finalmente, realizamos experiências envolvendo modos do campo eletromagnético que possuem momentum angular orbital (MAO). Demonstramos o armazenamento de estados de MAO e de suas superposições coerentes, bem como sua manipulação com campos magnéticos externos. Mostraremos que o MAO óptico pode ser armazenado, manipulado e recuperado coerentemente a partir do sistema atômico.

We study the expansion velocity and ion temperature evolution of ultracold neutral plasmas (UNPs) of calcium atoms under the influence of a uniform magnetic field that ranges up to 200 G. In the experiments, we use a magneto-optical trap (MOT) to capture the neutral atoms and laser-induced fluorescence (LIF) to take images of the plasma. We vary the magnetic field strengths and the initial electron temperatures and observe the plasma evolution in time. We compare the ion temperature evolution to the theory introduced in the paper by Pohl et. al. [Phys. Rev. A 70, 033416 (2004)]. The evolution of the gradient of expansion velocity suggests the presence of ion acoustic waves (IAWs). We speculate that our measurements showing that the ion temperature remains relatively high throughout the evolution is a biproduct of the IAW.

No description available.

A l'aide d'un spectre de fluorescence de résonance à bande étroite obtenu avec une nano-cellule d'épaisseur L= [lambda]/2 et des résonances VSOP formées pour une épaisseur L =[lambda] ([lambda] est la longueur d'onde de la radiation résonnante), cette thèse présente pour la première fois une étude expérimentale du comportement en fréquence et en intensité (probabilités de transition) des transitions atomiques de la structure hyperfine entre sous-niveaux Zeeman des raies D1 and D2 pour le 85Rb et le 87Rb en présence de champs magnétiques extérieurs compris entre 5 et 7000 G. Le comportement d'une dizaine de transitions atomiques inétudiées à ce jour a été analysé et nous avons démontré que l'intensité de ces raies peut montrer alternativement de grandes variations : jusqu'à un facteur 10 de taux de croissance ou de décroissance. Pour la première fois, nous avons parfaitement enregistré des résonances VSOP dans le cas où un gaz tampon (néon de pression partielle 6 torr) est introduit dans la nano-cellule d'épaisseur L =[lambda], alors que l'addition d'un gaz tampon (néon) même à une pression partielle de 0,1 torr, dans une cellule d'épaisseur centimétrique conduit à une complète disparition de ces résonances VSOP obtenues par la méthode usuelle d'absorption saturée. Enfin, nous avons montré pour la première fois que la largeur spectrale d'un spectre de fluorescence de résonance d'une nano-cellule de rubidium d'épaisseur L= [lambda]/2, quelques soient les pressions du gaz tampon (néon), est beaucoup plus étroite (6 à 8 fois) que celle obtenue avec une cellule centimétrique de rubidium pour les mêmes valeurs de pressions / Using a narrow-band resonant fluorescence spectra from a nano-cell with a thickness of L= [lambda]/2, and VSOP resonances formed at a thickness L =[lambda] ([lambda] is the wavelength of the resonant radiation), for the first time it was experimentally investigated the behaviour of the frequency and intensity (transition probabilities) of the atomic hyperfine structure transitions between the 85Rb, 87Rb, D1 and D2 lines Zeeman sublevels in external magnetic fields in range 5 - 7000G. The behaviour of tens of previously unstudied atomic transitions was analyzed and it is demonstrated that the intensities of these lines can both greatly increase, and decrease (tenfold). For the first time it is demonstrated that, in the case of partial pressure of neon buffer gas up to 6~torr into the nano-cell of thickness L = [lambda] filled with Rb, VSOP resonances are recorded confidently, while the addition of 0.1~torr neon buffer gas in a cell of a centimeter thickness leads to the complete disappearance of VSOP resonances formed with the help of the widely used technique of saturated absorption. It is demonstrated for the first time that the spectral width of the resonant fluorescence spectra of the rubidium nano-cell with thickness L= [lambda]/2, for all values of the neon buffer gas pressures is much narrower (6-8 times) compared with the resonant fluorescence spectra of an ordinary centimeter cell containing rubidium with the same pressures of neon

Ce travail montre l'intérêt, par deux aspects, des semiconducteurs semimagnétiques CdMnTe pour l'étude des propriétés structurales, magnétooptiques et électroniques des puits quantiques II-VI. La première partie est consacrée à la caractérisation des interfaces CdTe/CdMnTe dans des puits quantiques asymétriques. Le large effet Zeeman dû à l'interaction d'échange porteurs-Manganèse de la transition excitonique fondamentale, déterminé par des mesures de magnétoréflectivité et d'effet Kerr, permet une analyse précise du profil de concentration du Manganèse à proximité de l'interface. Nous mettons ainsi en évidence le mécanisme de ségrégation qui résulte l'échange d'atomes entre deux monocouches adjacentes en cours de croissance. Dans la seconde partie, l'effet Zeeman géant de la bande de conduction est utilisé pour polariser totalement le spin des électrons d'un gaz bidimensionnel de densité intermédiaire présent dans un puits quantique semimagnétique. Cette propriété permet de découpler les phénomènes dépendant et non-dépendant du spin des électrons et d'étudier ainsi les effets à N corps au sein d'un gaz d'électrons bidimensionnel. L'analyse des mesures de magnéto-transmission inter-niveaux de Landau met en évidence la création de paires électron-trou liés par l'interaction coulombienne. L'énergie de liaison de ce complexe est toutefois fortement réduite par deux mécanismes dus au gaz d'électrons : l'écrantage et le remplissage de l'espace des phases. Un autre effet à N corps est manifeste : l'interaction d'échange électron-électron qui permet d'expliquer, conjointement avec l'interaction d'échange porteurs-Manganèse, la polarisation complète du gaz à un faible champ magnétique. Un modèle au premier ordre, incluant ces trois effets, reproduit le comportement des énergies des transitions en champ.

Using a narrow-band resonant fluorescence spectra from a nano-cell with a thickness of L= [lambda]/2, and VSOP resonances formed at a thickness L =[lambda] ([lambda] is the wavelength of the resonant radiation), for the first time it was experimentally investigated the behaviour of the frequency and intensity (transition probabilities) of the atomic hyperfine structure transitions between the 85Rb, 87Rb, D1 and D2 lines Zeeman sublevels in external magnetic fields in range 5 - 7000G. The behaviour of tens of previously unstudied atomic transitions was analyzed and it is demonstrated that the intensities of these lines can both greatly increase, and decrease (tenfold). For the first time it is demonstrated that, in the case of partial pressure of neon buffer gas up to 6~torr into the nano-cell of thickness L = [lambda] filled with Rb, VSOP resonances are recorded confidently, while the addition of 0.1~torr neon buffer gas in a cell of a centimeter thickness leads to the complete disappearance of VSOP resonances formed with the help of the widely used technique of saturated absorption. It is demonstrated for the first time that the spectral width of the resonant fluorescence spectra of the rubidium nano-cell with thickness L= [lambda]/2, for all values of the neon buffer gas pressures is much narrower (6-8 times) compared with the resonant fluorescence spectra of an ordinary centimeter cell containing rubidium with the same pressures of neon

Recent efforts to classify off-shell representations of supersymmetry without a central charge have focused upon directed, supermultiplet graphs of hypercubic topology known as Adinkras. These encodings of Super Poincare algebras, depict every generator of a chosen supersymmetry as a node-pair transformtion between fermionic bosonic component fields. This research thesis is a culmination of investigating novel diagrammatic sums of gauge-quotients by supersymmetric images of other Adinkras, and the correlated building of field theoretic worldline Lagrangians to accommodate both classical and quantum venues. We find Ref [40], that such gauge quotients do not yield other stand alone or "proper" Adinkras as afore sighted, nor can they be decomposed into supermultiplet sums, but are rather a connected "Adinkraic network". Their iteration, analogous to Weyl's construction for producing all finite-dimensional unitary representations in Lie algebras, sets off chains of algebraic paradigms in discrete-graph and continuous-field variables, the links of which feature distinct, supersymmetric Lagrangian templates. Collectively, these Adiankraic series air new symbolic genera for equation to phase moments in Feynman path integrals. Guided in this light, we proceed by constructing Lagrangians actions for the N = 3 supermultiplet YI /(iDI X) for I = 1, 2, 3, where YI and X are standard, Salam-Strathdee superfields: YI fermionic and X bosonic. The system, bilinear in the component fields exhibits a total of thirteen free parameters, seven of which specify Zeeman-like coupling to external background (magnetic) fluxes. All but special subsets of this parameter space describe aperiodic oscillatory responses, some of which are found to be surprisingly controlled by the golden ratio, [phi] = 1.61803, Ref [52]. It is further determined that these Lagrangians allow an N = 3 - > 4 supersymmetric extension to the Chiral-Chiral and Chiral-twistedChiral multiplet, while a subset admits two inequivalent such extensions. In a natural proiii gression, a continuum of observably and usefully inequivalent, finite-dimensional off-shell representations of worldline N = 4 extended supersymmetry are explored, that are variate from one another but in the value of a tuning parameter, Ref [53]. Their dynamics turns out to be nontrivial already when restricting to just bilinear Lagrangians. In particular, we find a 34-parameter family of bilinear Lagrangians that couple two differently tuned supermultiplets to each other and to external magnetic fluxes, where the explicit parameter dependence is unremovable by any field redefinition and is therefore observable. This offers the evaluation of X-phase sensitive, off-shell path integrals with promising correlations to group product decompositions and to deriving source emergences of higher-order background flux-forms on 2-dimensional manifolds, the stacks of which comprise space-time volumes. Application to nonlinear sigma models would naturally follow, having potential use in M- and F- string theories.