Ce travail porte sur la précession du spin d’électrons chauds polarisés en spin. Celle-ci est induite par le champ d’échange d’une couche mince ferromagnétique dans une structure multicouche. La précession du spin électronique a déjà été mesurée dans des matériaux ferromagnétiques mais uniquement pour des électrons qui possèdent une énergie supérieure à 4eV au-dessus du niveau de Fermi. L’objectif premier de cette thèse est de mesurer la précession du spin de l’électron pour des faibles énergies, comprises entre 0.7eV et 2eV au-dessus du niveau de Fermi. Pour ce faire, un transistor tunnel magnétique comportant trois couches magnétiques avec les aimantations qui pointent dans les trois directions de l’espace doit être construit. Les électrons sont injectés à basse énergie grâce à une jonction tunnel. Une diode Schottky (interface entre du Cu et du Si) filtre en énergie les électrons incidents, permettant uniquement aux électrons balistiques de contribuer au courant mesuré dans le semi-conducteur. Le premier travail a consisté à obtenir une couche magnétique exhibant une anisotropie perpendiculaire. Ainsi, nous avons réussi à faire croître une multicouche de Co et Ni sur une diode Schottky qui possède une anisotropie perpendiculaire jusqu’à 5 répétitions. Le deuxième travail réalisé dans cette thèse était d’optimiser le magnéto-courant d’une la vanne de spin. En effet, le magnéto-courant détermine la sensibilité de notre transistor tunnel magnétique. Nous avons notamment démontré ici que le magnéto-courant augmente avec le nombre de répétitions de la multicouche [Co/Ni], pour atteindre quasiment le maximum de 100% théoriquement prédit dans une vanne de spin à aimantations croisées. Enfin, le troisième travail de cette thèse résidait dans l’étude de la précession du spin de l’électron dans différents matériaux ferromagnétiques. Cet effet a été mis en évidence ici pour des couches à aimantation planaire composée de Co, de CoFeB, ainsi que pour un alliage de CoAl et ceci en fonction de leur épaisseur / This work is about polarised hot electrons spin precession. This phenomenon is induced by the exchange field of a ferromagnetic thin film in a multilayer structure. The electronic spin precession has already been measured in ferromagnetic materials, but only for electrons whose energy is more than 4eV over the Fermi level. The initial aim of this PhD work is to measure the electron spin precession for weak energies, between 0.7 eV and 2eV over the Fermi level. In order to achieve that, a magnetic tunnel transistor composed of three magnetic layers with their magnetisations directions perpendicular to each other has to be elaborated. The electrons are injected at low energy by means of a tunnel junction. A Schottky diode (interface between Cu and Si) filters the incident electrons by their energies, which enables only ballistic electrons to contribute to the measured current in the semi-conductor. The first task consisted in obtaining a magnetic layer showing perpendicular magnetic anisotropy. We succeeded in growing cobalt nickel multilayers exhibiting a perpendicular magnetic anisotropy up to five repetitions. The second part of the job carried out during this PhD was to optimise the magneto-current of a spin valve. Indeed, it determines the magnetic tunnel transistor sensitivity. We have demonstrated that the magneto-current increases with the number of repetitions of the [Co/Ni] multilayer up to a maximum of nearly a hundred percent, which is the maximum theoretically predicted in a spin valve with crossed magnetisations. Eventually, the third task of this PhD was the study of the electron spin precession in various ferromagnetic materials. This effect has been evidenced here for thin layers with in-plane magnetisations composed of Co, CoFeB, and also for a CoAl alloy depending on the thickness of the layers

Orientador: Carlos Rettori / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-17T05:10:39Z (GMT). No. of bitstreams: 1 Garcia_FernandoAssis_D.pdf: 2669059 bytes, checksum: 122a24668135bef8b9e2ee683190a28a (MD5) Previous issue date: 2010 / Resumo: Esta tese aborda o problema geral dos efeitos de campos cristalino e rattling modes em compostos do tipo skutterudite. Emprega, predominantemente, a técnica de ressonância de spin eletrônico (ESR, ou EPR na literatura clássica) a fim de sondar a simetria local dos estados eletrônicos associados a íons de terras raras nestes compostos e investigar as inomogeneidades decorrentes da dinâmica (rattling modes) destes íons. Desenvolvemos o nosso estudo predominantemente sob o sistema Ce1-xRxFe4P12 (R = Gd, Dy, Er, Yb), usualmente descrito como um semicondutor com gap G ¿ 0.15 eV. Nossos resultados apontam que a correta determinação dos efeitos de campo cristalino requer uma nova abordagem no estudo de compostos com simetria Th. O grupo pontual Th é um dos cinco grupos pontuais cúbicos (O, Oh, T, Th, Td) e é obtido pelo produto T x sh, onde T descreve a simetria de um tetraedro regular. Desta maneira, Th não possui as operações 6C4 e 3C¿2 e precisamos utilizar um novo parâmetro de sexta ordem para descrever todos os invariantes da simetria do grupo pontual Th Este novo parâmetro causa uma mistura na simetria do estado fundamental de sistemas cúbicos que dá origem a fenômenos não triviais discutidos na tese. Resultado igualmente importante é que, a partir do estudo deste novo parâmetro, fomos capazes de mapear a inomogeneidade de campo cristalino decorrente da dinâmica dos íons de terra rara. Desta maneira, a técnica de ESR foi utilizada pela primeira vez no estudo da dinâmica de íons em sistemas do tipo gaiola. Os estudos em sistemas S (L = 0) mostraram ainda que os efeitos desta dinâmica nos espectros de ESR não depende da presença deste parâmetro, sendo algo mais geral. Esta tese examina estes dois importantes aspectos relacionados a estrutura cristalina da família das skutterudites e discute estes resultados no contexto das possíveis aplicações destes compostos na construção de dispositivos termoelétricos e também no estudo de fenômenos fortemente correlacionados / Abstract: This thesis addresses the general problem of crystall field effects and rattling modes in skut- terudite compounds. It employs, predominantly, the electron spin resonance technique (ESR or EPR, in the classical literature) aiming to probe the local point symmetry of the electronic states associated to the rare earth ions and the inohomogenities implied by their dynamics (rattling modes). Our study was mainly directed to the Ce 1-xRxFe4P12 (R = Gd, Dy, Er, Yb) compounds, usually described as a small gap semiconductor (G ¿ 0.15 eV). Our results pointed that the correct determination of the crystall field effects requires a new approach in the case of compounds with Th symmetry. The point group Th is one the five cubic point groups (O, 0h, T, Th, Td) being generated by the product T x sh, where T describes the full symmetry of a regular tetrahedron. In this sense, Th lacks the symmetry operations 6C4 e 3C¿2 and we need a new sixth order parameter, to describe all the invariants of point group T^ symmetry. This new parameter gives rise to a mix of the ground state symmetry of cubic systems, implying in non trivial phenomena discussed in this these. Interesting enough, following these results, we were able to map the crystall field ino-homogenities due to the dynamics of the rare earth ion within the skutterudite cage. In this sense, the ESR techinique was introduced as a probe to the guest ion dynamics in cage systems. Investigation of S systems (L = 0) showed that the effects of this dynamics in our experiment does not require the presence of this new sixth order parameter, being associated to a more general role of the rattling modes. We examine these two important aspects related to crystal structure of the skutterudite family and discuss the results in the context of possible applications of these compounds in the construction of thermoelectric devices and also in the study of strongly correlated phenomena / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Research on spintronics has galvanized the design of new devices that exploit the electronic spin in order to augment the performance of current microelectronic technologies. The sucessful implementation of these devices is largely contingent on a quantitative understanding of nonequilibrium magnetism in conducting ferromagnets. This thesis is largely devoted to expanding the microscopic theory of magnetization relaxation and current-induced spin torques in transition metals ferromagnets as well as in (III,Mn)V dilute magnetic semiconductors. We start with two theoretical studies of the Gilbert damping in electric equilibrium, which treat disorder exactly and include atomic-scale spatial inhomogeneities of the exchange field. These studies enable us to critically review the accuracy of the conventional expressions used to evaluate the Gilbert damping in transition metals. We follow by generalizing the calculation of the Gilbert damping to current-carrying steady states. We find that the magnetization relaxation changes in presence of an electric current. We connect this change with the non-adiabatic spin transfer torque parameter, which is an elusive yet potentially important quantity of nonequilibrium magnetism. This connection culminates in a concise analytical expression that will lead to the first ab initio estimates of the non-adiabatic spin transfer torque in real materials. Subsequently we predict that in gyrotropic ferromagnets the magnetic anisotropy can be altered by a dc current. In these systems spin-orbit coupling, broken inversion symmetry and chirality conspire to yield current-induced spin torques even for uniform magnetic textures. We thus demonstrate that a transport current can switch the magnetization of strained (Ga,Mn)As. This thesis concludes with the transfer of some fundamental ideas from nonequilibrium magnetism into the realm of superconductors, which may be viewed as easy-plane ferromagnets in the particle-hole space. We emphasize on the analogies between nonequilibrium magnetism and superconductivity, which have thus far been studied as completely separate disciplines. Our approach foreshadows potentially new effects in superconductors. / text

La réalisation d'un ordinateur quantique fonctionnel est l'un des objectifs technologiques les plus ambitieux pour les scientifiques d'aujourd'hui. Sa brique de base est composée d'un système quantique à deux niveaux, appelé bit quantique (ou qubit). Parmi les différents concepts existants, les dispositifs à base de spin sont très attractifs car ils bénéficient de la progression constante des techniques de nanofabrication et permettent la lecture électrique de l'état ​​du qubit. Dans ce contexte, les dispositifs à base de spins nucléaires offrent un temps de cohérence supérieur à celui des dispositifs à base de spin électronique en raison de leur meilleure isolation à l'environnement. Mais ce couplage faible a un prix: la détection et la manipulation des spins nucléaires individuels restent des tâches difficiles. De très bonnes conditions expérimentales étaient donc essentielles pour la réussite de ce projet. Outre des systèmes de filtrage des radiofréquences à très basses températures et des amplificateurs à très faible bruit, j'ai développé de nouveaux supports d'échantillons et des bobines de champ magnétique trois axes compacts avec l'appui des services techniques de l'Institut Néel. Chaque partie a été optimisée afin d'améliorer la qualité de l'installation et évaluée de manière quantitative. Le dispositif lui-même, un qubit réalisé grâce à un transistor de spin nucléaire, est composé d'un aimant à molécule unique couplé à des électrodes source, drain et grille. Il nous a permis de réaliser la lecture électrique de l'état d'un spin nucléaire unique, par un processus de mesure non destructif de son état quantique. Par conséquent, en sondant les états quantique de spin plus rapidement que le temps de relaxation caractéristique de celui-ci, nous avons réalisé la mesure de la trajectoire quantique d'un qubit nucléaire isolé. Cette expérience a mis en lumière le temps de relaxation T$ _1$ du spin nucléaire ainsi que son mécanisme de relaxation dominant. La manipulation cohérente du spin nucléaire a été réalisée en utilisant des champs électriques externes au lieu d'un champ magnétique. Cette idée originale a plusieurs avantages. Outre une réduction considérable du chauffage par effet Joule, les champs électriques permettent de contrôler et de manipuler le spin unique de façon très rapide. Cependant, pour coupler le spin à un champ électrique, un processus intermédiaire est nécessaire. Un tel procédé est l'interaction hyperfine, qui, si elle est modifiée par un champ électrique, est également désigné sous le nom d'effet Stark hyperfin. En utilisant cet effet, nous avons mis en évidence la manipulation cohérente d'un spin nucléaire unique et déterminé le temps de cohérence $ T^*_2 $. En outre, l'exploitation de l'effet Stark hyperfin statique nous avons permis de régler le qubit de spin nucléaire à et hors résonance par l'intermédiaire de la tension de grille. Cela pourrait être utilisé pour établir le contrôle de l'intrication entre les différents qubits nucléaires. En résumé, nous avons démontré pour la première fois la possibilité de réaliser et de manipuler un bit quantique basé sur un aimant à molécule unique, étendant ainsi le potentiel de la spintronique moléculaire au delà du stockage de données classique. De plus, la grande polyvalence des molécules aimants est très prometteuse pour une variété d'applications futures qui, peut-être un jour, parviendront à la réalisation d'un ordinateur quantique moléculaire. / The realization of a functional quantum computer is one of the most ambitious technologically goals of today's scientists. Its basic building block is composed of a two-level quantum system, namely a quantum bit (or qubit). Among the other existing concepts, spin based devices are very attractive since they benefit from the steady progress in nanofabrication and allow for the electrical read-out of the qubit state. In this context, nuclear spin based devices exhibit an additional gain of coherence time with respect to electron spin bases devices due to their better isolation from the environment. But weak coupling comes at a price: the detection and manipulation of individual nuclear spins remain challenging tasks. Very good experimental conditions were important for the success of this project. Besides innovative radio frequency filter systems and very low noise amplifiers, I developed new chip carriers and compact vector magnets with the support of the engineering departments at the institute. Each part was optimized in order to improve the overall performance of the setup and evaluated in a quantitative manner. The device itself, a nuclear spin qubit transistor, consisted of a TbPc$_2$ single-molecule magnet coupled to source, drain, and gate electrodes and enabled us to read-out electrically the state of a single nuclear spin. Moreover, the process of measuring the spin did not alter nor demolish its quantum state. Therefore, by sampling the spin states faster than the characteristic relaxation time, we could record the quantum trajectory of an isolated nuclear qubit. This experiment shed light on the relaxation time T$_1$ of the nuclear spin and its dominating relaxation mechanism. The coherent manipulation of the nuclear spin was performed by means of external electric fields instead of a magnetic field. This original idea has several advantages. Besides a tremendous reduction of Joule heating, electric fields allow for fast switching and spatially confined spin control. However, to couple the spin to an electric field, an intermediate quantum mechanical process is required. Such a process is the hyperfine interaction, which, if modified by an electric field, is also referred to as the hyperfine Stark effect. Using the effect we performed coherent rotations of the nuclear spin and determined the dephasing time $T^*_2$. Moreover, exploiting the static hyperfine Stark effect we were able to tune the nuclear qubit in and out of resonance by means of the gate voltage. This could be used to establish the control of entanglement between different nuclear qubits. In summary, we demonstrated the first single-molecule magnet based quantum bit and thus extended the potential of molecular spintronics beyond classical data storage. The great versatility of magnetic molecules holds a lot of promises for a variety of future applications and, maybe one day, culminates in a molecular quantum computer.

Na primeira parte deste trabalho, são discutidos alguns fatores que determinam a forma correta de operação de um espectro convencional de RPE. Um sistema específico, possuindo o circuito usual de \"ponte de microonda\", é analisado e as condições ideais para a realização da medida das duas componentes da susceptibilidade magnética complexa são determinadas. A seguir, mostra-mos os resultados encontrados para a relaxação eletrônica do hidrogênio atômico intersticial no CaF2. As medidas foram efetuadas utilizando técnicas de microonda pulsada, a 3 KGauss, para temperatura entre 1.6 e 4.2 0K. Os resultados experimentais foram interpretados, através dos dois processos seguintes: - difusão espectral entre spins pertencentes a uma mesma linha de absorção, e relaxação cruzada entre as diferentes componentes hiperfinas do espectro de absorção. A estes dois processos relacionamos tempos característicos da ordem de 10 ms a 60ms, respectivamente. / In the first part of this work some factors are discussed wich determine the correct way of operating of a convenctional EPR spectrometer. A specific system, having the usual \"microwave bridge\" circuit, is analysed and the complex conditions for the measurement of both components of the complex magnetic susceptibility are determined. We show results for the electronics relaxation of an interstitial atomic hydrogen in CaF2. Measurement were performed using the pulsed microwave technique at 3 kilogauss for temperatures between 1.6 and 4.2 0K.The experimental results were interpreted by means of two processes: - Spectral diffusion between spin states belonging to the same absorption line, and cross relaxation between different hyperfine components of the absorption spectrum. The characteristic times assigned to these processes are of the order or 10 ms and 60 ms respectively.

L'exploration du monde quantique au moyen d'objets macroscopiques constitue l'un des défis centraux de ces dernières décennies pour la recherche en physique. Parmi les systèmes proposés pour atteindre cet objectif, les systèmes hybrides, qui couplent un résonateur nanomécanique à un qubit unique, font figure de paradigme.L'excitation cohérente d'un oscillateur mécanique macroscopique par un unique spin électronique ouvrirait en particulier de nouvelles perspectives pour la création d'états quantiques arbitraires du mouvement.Dans ce manuscrit, nous considérons un système hybride constitué d'un oscillateur nanomécanique et du spin électronique d'un unique centre NV, couplés entre eux par une interaction magnétique. Nous nous concentrons sur le cas d'une interaction paramétrique où la vibration mécanique module l'énergie du qubit, et plus précisément sur le cas où le qubit ainsi forcé et l'oscillateur mécanique évoluent sur des échelles de temps comparables.Dans cette situation, nos observations montrent une synchronisation de la dynamique du qubit sur l'oscillation mécanique. Le phénomène est dans un premier temps abordé par une expérience-test qui remplace le mouvement mécanique par un champ radiofréquence en couplage paramétrique avec le spin. Cette première implémentation permet de dégager les propriétés essentielles de l'effet paramétrique, qui est dans un second temps observé sur l'expérience principale.Dans cette seconde expérience, un centre NV est attaché à l'extrémité d'un nanofil de carbure de silicium en vibration placé dans un fort gradient de champ magnétique. Le caractère bidimensionnel des déformations du nanofil octroie alors à la synchronisation des signatures vectorielles encore inédites, qui peuvent aussi être interprétées comme la manifestation d'un triplet de Mollow phononique, ainsi qu'il a été observé dans les premières expériences d'électrodynamique quantique.Finalement, nous explorons la robustesse de la synchronisation vis-à-vis du mouvement Brownien du résonateur, et démontrons la possibilité de protéger le qubit de cette source de décohérence additionnelle grâce à une excitation mécanique de faible amplitude. / Probing the quantum world with macroscopic objects has been a core challenge for research in physics during the past decades. Proposed systems to reach this goal include hybrid devices that couple a nanomechanical resonator to a single spin qubit. In particular, the coherent actuation of a macroscopic mechanical oscillator by a single electronic spin would open perspectives in the creation of arbitrary quantum states of motion.In this manuscript, we investigate a hybrid system coupling a nanomechanical oscillator and a single electronic spin of a NV defect in magnetic interaction. We focus on the parametric interaction case, when the mechanical motion modulates the qubit energy, and in particular when the driven qubit and mechanical oscillators evolves on similar timescales. In that situation a synchronization of the qubit dynamics onto the mechanical motion is observed. The phenomenon is first explored on a test experiment where mechanical motion is replaced by a parametrically coupled RF field. It allows to establish the main properties of the phenomenon, which is subsequently investigated on the core experiment. It consists of a NV defect attached at the vibrating extremity of a silicon carbide nanowire, immersed in a strong magnetic field gradient. The bidimensional character of the nanowire deformations is responsible for novel vectorial signatures in the synchronization, which can also be viewed as a phononic Mollow triplet as observed in early quantum electrodynamics experiments. We finally explore the robustness of the synchronization against the Brownian motion of the resonator and demonstrate the possibility to protect the qubit against this additional decoherence source by applying a small coherent mechanical drive.

Na primeira parte deste trabalho, são discutidos alguns fatores que determinam a forma correta de operação de um espectro convencional de RPE. Um sistema específico, possuindo o circuito usual de \"ponte de microonda\", é analisado e as condições ideais para a realização da medida das duas componentes da susceptibilidade magnética complexa são determinadas. A seguir, mostra-mos os resultados encontrados para a relaxação eletrônica do hidrogênio atômico intersticial no CaF2. As medidas foram efetuadas utilizando técnicas de microonda pulsada, a 3 KGauss, para temperatura entre 1.6 e 4.2 0K. Os resultados experimentais foram interpretados, através dos dois processos seguintes: - difusão espectral entre spins pertencentes a uma mesma linha de absorção, e relaxação cruzada entre as diferentes componentes hiperfinas do espectro de absorção. A estes dois processos relacionamos tempos característicos da ordem de 10 ms a 60ms, respectivamente. / In the first part of this work some factors are discussed wich determine the correct way of operating of a convenctional EPR spectrometer. A specific system, having the usual \"microwave bridge\" circuit, is analysed and the complex conditions for the measurement of both components of the complex magnetic susceptibility are determined. We show results for the electronics relaxation of an interstitial atomic hydrogen in CaF2. Measurement were performed using the pulsed microwave technique at 3 kilogauss for temperatures between 1.6 and 4.2 0K.The experimental results were interpreted by means of two processes: - Spectral diffusion between spin states belonging to the same absorption line, and cross relaxation between different hyperfine components of the absorption spectrum. The characteristic times assigned to these processes are of the order or 10 ms and 60 ms respectively.

Orientador: Gilberto Medeiros Ribeiro / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-09T10:48:50Z (GMT). No. of bitstreams: 1 GonzalezHernandez_FelixGuillermo_D.pdf: 12837677 bytes, checksum: 70e82c96ea88ab1de4fa785d908c9af6 (MD5) Previous issue date: 2007 / Resumo: Medidas experimentais foram realizadas para determinar as escalas de tempo de relaxação e decoerência do spin eletrônico como bit quântico. A estrutura dos estados de exciton foi investigada com o objetivo de servir como estados intermediários na manipulação do spin. O sistema utilizado para o estudo de decoerência é um ensemble de pontos quânticos auto-formados semicondutores. Dois temas servem como eixos centrais dos três experimentos desenvolvidos nesta tese: a polarização de spin e o fator g de Landé. No primeiro experimento, ao incluir o efeito do reservatório térmico, foi obtido o grau de polarização do spin (populações dos níveis up e down) para as camadas s e p. O desdobramento dos níveis orbitais em subníveis de spin permitiu obter a magnitude do fator g para estes estados. Mudando a orientação do campo magnético, foram observadas as anisotropias do tensor g e a sua relação com os detalhes do potencial de confinamento. Estas características permitiram inferir o tempo de relaxação T1. A medida da polarização resolvida no tempo foi realizada através de es-pectroscopia óptica de bombeio-prova. Os pulsos de luz e o campo magnético transverso permitem que uma polarização líquida seja inicializada. A rotação de Kerr permitiu observar oscilações desta polarização em torno do campo magnético com freqüência determinada pelo fator g. A perda da coerência de fase do spin resulta no decaimento destas oscilações numa escala de tempo T2. Medidas realizadas num ensemble de spins implicam em que o tempo de decoerência encontra-se limitado pela escala de defasagem T¤2< T2. Uma técnica semelhante à refocalização por spin-eco em experimentos de ressonância magnética nuclear, foi aplicada utilizando pulsos de laser para reverter a defasagem do ensemble. Tanto a possibilidade de medir o sinal de eco como o tempo de decoerência foram medidos como função da temperatura. A estrutura de níveis de exciton e a sua distribuição no ensemble foi estudada também com espectroscopia de bombeio-prova. Foram observados batimentos quânticos entre os níveis de estrutura fina do exciton para sis-temas 0D e 2D limitados pelo tempo de recombinação / Abstract: Experimental measurements were carried out to determine the scales of the relaxation and decoherence time for the electronic spin as quantum bit. The structure of the exciton states was investigated with the objective to serve as intermediate states in the spin manipulation. The system studied for the implementation of the quantum computation is an ensemble of self-assembled semiconductor quantum dots. Two subjects serve as central axes of the three experiments developed in this thesis: the spin polarization and the Landé g-factor. In the first experiment, when including the effect of the thermal reservoir, the degree of spin polarization (populations for the up and down levels) was measured for layers s and p. The splitting of the orbital levels in spin sublevels allowed to get the magnitude of factor g for these states. Changing the orientation of the magnetic field, the g-tensor anisotropies and its relation with the details of the confinement potential had been observed. These characteristics had allowed to infer the relaxation time T1. The time resolved polarization measurement was carried out by optical pump-probe spectroscopy. The pulses of light and the transverse magnetic field allow the initialization of a net polarization. The Kerr rotation allowed to observe oscillations of this polarization around the magnetic field with frequency determined for factor g. The loss of the spin phase coherence results in the decay of these oscillations in a time scale T2. Measurements carried out in an ensemble of spins imply that the decoherence time is limited by the ensemble dephasing time T¤2 < T2. A technique similar to the spin-echo refocalization in nuclear magnetic resonance experiments using laser pulses was applied to reverse the ensemble dephasing. The possibility to measure the echo signal and the decoherence time was measured as a function of the temperature. The structure of exciton levels and its distribution in ensemble were also studied with pump-probe spectroscopy. Quantum beats were observed be-tween the fine structure exciton levels for 0D and 2D systems, yet limited by the recombination time / Doutorado / Física da Matéria Condensada / Doutor em Ciências

A novel experimental setup has been realized to measure weak magnetic moments which can be modulated at radio frequencies (~1–5 MHz). Using an optimized radio-frequency (RF) pickup coil and lock-in amplifier, an experimental sensitivity of 10 -15 Am2 corresponding to 10 -18 emu has been demonstrated with a one second time constant. The detection limit at room temperature is 9.3 10 -16 Am2/√Hz limited by Johnson noise of the coil. In order to demonstrate the sensitivity of this technique it was used to electrically detect the polarized spins in semiconductors in zero applied magnetic fields. For example in GaAs, the magnetic moment due to a small number (~ 7 x 108) of spin polarized electrons generated by polarization modulated optical radiation was detected. Spin polarization was generated by optical injection using circularly polarized light which is modulated rapidly using an electro-optic cell. The modulated spin polarization generates a weak time-varying magnetic field which is detected by the sensitive radio-frequency coil. Using a radio-frequency lock-in amplifier, clear signals were obtained for bulk GaAs and Ge samples from which an optical spin orientation efficiency of ~ 10–20% could be determined for Ge at 1342 nm excitation wavelength at 127 K. In the presence of a small external magnetic field, the signal decayed according to the Hanle Effect, from which a spin lifetime of 4.6 ± 1.0 ns for electrons in bulk Ge at 127 K was extracted. The spin dynamics in n-Ge was further explored and the temperature dependence of the spin lifetime was plotted for a temperature range of about 90 K to 180 K. The temperature dependence of the optical pumping efficiency was also measured though no quantitative conclusions could be derived. The signals observed for semi-insulating GaAs, n-GaAs, GaSb and CdTe which are direct gap semiconductors are much larger than expected (almost two orders of magnitude). An attempt was made to explain this unexpected behavior of these direct gap semiconductors using the spin hall effect.