Le phénomène de Magnéto Résistance Géante (GMR) est une manifestation macroscopique de l'intrication entre les variables magnétique et électronique dans les systèmes ferromagnétiques. Depuis sa découverte expérimentale en 1988, de nombreuses théories semi-classiques ont permis de bien décrire ce phénomène dans le régime magnétique statique. Nous nous sommes intéressés au problème du comportement de l'effet GMR dans un capteur micrométrique de type vanne de spins soumis à un champ magnétique et/ou d'une tension hyperfréquences. Pour cela, nous avons étudié la réponse électrique du capteur en utilisant le principe de démodulation des signaux afin de nous affranchir des phénomènes parasites intervenant dans les mesures hyperfréquences. En comparant les mesures expérimentales avec un modèle semi-classique, nous montrerons comment l'étude de l'effet GMR permet de remonter à la dynamique de chacune des couches composant le système puis nous présenterons des résultats préliminaires montrant l'influence de la fréquence de la tension appliquée au capteur sur l'amplitude de l'effet GMR.<br /><br />Cette thèse porte sur le comportement de l'effet de Magnéto Résistance Géante (GMR) dans un capteur micrométrique de type vanne de spin soumis à un champ magnétique hyperfréquence et à une tension hyperfréquence. Nous avons pour cela développé une technique originale basée sur l'utilisation du capteur en tant que démodulateur in situ. L'étude de la dynamique de l'aimantation des vannes de spin grâce à des micro antennes nous permis de déterminer les lois régissant la résonance ferromagnétique de la couche douce du capteur et de mettre en avant le rôle des couplages dipolaires et des diffusions électroniques sur les phénomènes de relaxation. Ces résultats nous ont permis de modéliser la réponse de l'effet GMR en présence d'un champ hyperfréquence montrant par là même la très bonne sensibilité des capteurs à ce type d'excitation. De plus, la dynamique de la couche dure a pu être détectée grâce au profil fréquentiel de l'effet GMR. Enfin, nous avons obtenu un résultat préliminaire correspondant à une décroissance apparente de l'effet GMR lorsque la fréquence de la tension appliquée est supérieure à la fréquence de la résonance ferromagnétique de la couche douce.

Dans cette thèse, nous utilisons différentes inégalités fonctionnelles<br />(Poincaré, Sobolev logarithmique, etc.) pour étudier deux questions.<br />Nous appliquons d'abord des inégalités affaiblies à l'étude d'une<br />diffusion inhomogène, analogue continu de l'algorithme de recuit<br />simulé, dans la lignée d'un travail de L. Miclo. Nous montrons un<br />résultat de convergence de la diffusion, sous des hypothèses plus<br />faibles que celles posées précédemment : le potentiel dans lequel la<br />diffusion évolue peut croître très lentement à l'infini.<br />Dans le cadre d'un modèle de mécanique statistique à spins non-bornés,<br />en nous basant sur des résultats de T. Bodineau et B. Helffer, N. Yoshida<br />et G. Royer, nous éclaircissons ensuite les liens entre différentes<br />inégalités fonctionnelles, des propriétés de mélange et l'unicité de<br />la mesure de Gibbs en volume infini. Nous montrons en particulier<br />l'unicité si les mesures en volume fini et pour une seule condition<br />aux bords vérifient uniformément une inégalité de Beckner.

Figure skating is a competitive sport that requires intensive training which can be taught in a variety of settings. There are various methods to teaching figure skaters new skills such as positive and corrective feedback, modeling and coaching procedures, and physical guidance. These different approaches may lead to a lack of consistency among coaches. Over the years, these established coaching strategies have not changed substantially as training methods are passed down from coach to student. Also, research in the area of what constitutes effective coaching methods is lacking. Skaters may progress more quickly in skill development if coaches are implementing empirically based successful coaching methods. These teaching approaches may also be enhanced by incorporating the latest technology available. This study evaluated the effectiveness of a video feedback coaching procedure using the Dartfish application. A multiple baseline design was utilized to document the impact of this video feedback coaching procedure on the demonstration of six established figure skating moves, three moves for one skater and three different moves for two other skaters. Results showed utilizing video feedback improved figure skater's performance levels on the targeted moves to an acquisition of 80% accuracy or higher.

xiv, 114 p. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / Electron spin coherence can arise through a coherent superposition of two spin states in the conduction band of a semiconductor and can persist over remarkably long time and length scales. The robust nature of electron spin coherence makes it an excellent model system for exploring coherent quantum phenomena in semiconductors. This dissertation presents both spectral- and time-domain nonlinear optical studies of electron spin coherence through &Lambda;-type three-level systems in two- and zero-dimensional semiconductor systems. The spectral domain study focuses on the experimental realization of electromagnetically induced transparency (EIT), a phenomenon that exploits destructive interference induced by the spin coherence. Coherent Zeeman Resonance (CZR), a precursor to EIT, is demonstrated in two 2D systems, a GaAs mixed-type quantum well (MTQW) and a modulation doped CdTe quantum well (QW). For these studies, &Lambda;-type three-level systems are formed via dipole coupling of a trion to two electron spin states. The CZR response can be described qualitatively by effective density matrix equations. In addition, effects of manybody Coulomb interactions on CZR are investigated by varying the electron density in the MTQW via optical carrier injection. Time-domain studies based on transient differential transmission (DT) are carried out to explore the excitation, manipulation, and detection of electron spin coherence and to better understand how manybody interactions affect coherent nonlinear optical processes in semiconductors. While electron spin coherence can be formed and detected via resonant excitation of excitons or trions, a surprising observation is that injecting excitons into the 2D electron gas in a modulation doped CdTe QW can significantly alter the oscillatory nonlinear response of the electron spin coherence, while the response remains qualitatively unchanged when trions are injected. These behaviors are attributed to an interplay between manybody effects and carrier heating generated by trion formation from excitons. Finally, donor-bound electrons in GaAs are used as a model of localized electron spins. Spin decoherence of order 10 ns, limited by nuclear hyperfine interactions, is observed. Electron spin rotation induced by a nearly resonant laser pulse is also observed, opening the door for further work on mitigating electron spin decoherence time through optical spin echoes. / Adviser: Hailin Wang

xvii, 110 p. : ill. (some col.) / Electron spin states in semiconductors feature long coherence lifetimes, which have stimulated intense interest in the use of these spins for applications in spin based electronics and quantum information processing (QIP). A principal requirement for these spins to be viable candidates in QIP is the ability to coherently control the spins on timescales much faster than the decoherence times. The ability to optically control the spin state can meet this requirement. The spin states of electrons exhibit strong radiative coupling to negatively charged exciton (trion) states, and this radiative coupling makes coherent optical control of spin states possible. This dissertation presents experimental demonstration of coherent control of an electron spin ensemble in a two-dimensional electron gas in a CdTe quantum well. We present two complimentary techniques to optically manipulate these electron spins using a Raman transition. The first demonstration is with a single off-resonant ultrafast optical pulse. This ultrafast pulse acts like an effective magnetic field in the propagation direction of the optical pulse. The second experiment utilizes phase-locked Raman resonant pulse pairs to coherently rotate the quantum state, where the relative phase of the pulse pair sets the axis of rotation. The Raman pulse pair acts like a microwave field driving the spin states. This research demonstrates two significant contributions to the field of coherent optical interactions with semiconductors. First, we have advanced the potential use of electron spin ensembles in semiconductors for optics based quantum information processing hardware through our demonstration of coherent spin flips and complete coherent control. Second, we have experimentally realized full coherent control through the use of phase-locked Raman pulse pairs that overcomes inherent limitations of the single-pulse optical rotation technique, which is the current standard technique used in coherent control. This dissertation includes previously published and unpublished co-authored material. / Committee in charge: Dr. Miriam Deutsch, Chairperson; Dr. Hailin Wang, Advisor; Dr. Steven van Enk, Member; Dr. Raghuveer Parthasarathy, Member; Dr. Catherine Page, Outside Member

Made available in DSpace on 2016-06-02T20:15:24Z (GMT). No. of bitstreams: 1 4500.pdf: 3879542 bytes, checksum: b5f64bb4a570b611b18559c59dc97dd1 (MD5) Previous issue date: 2009-06-25 / Financiadora de Estudos e Projetos / In this work we used the technique of scattering matrix to calculate the transmissivity of polarized spins in a semiconductor symmetrical double-barrier heterostruture. The dynamics of carriers is described by effective mass approximation applied to models of Dresselhaus and Bychkov-Rashba. In the former case, the hamiltonian was studied in its full form, free of approximations. Both models describe interactions of type Spin-orbit, which is basically a coupling between the orbital angular moment of the electron with its magnetic moment of spin. This type of coupling as a consequence creates a splitting of some levels of energy, initially degenerate, resulting in a spectrum of energy more complex than that obtained without taking it into account such interactions. The transmissivity is calculated as a function of some parameters, for the type InAs=GaSb=InAs=GaSb=InAs. All results are compared with literature, and provide new information about the systems. The effects of spin-orbit interactions of Rashba and Dresselhaus show is very favorable to engineering in the manufacture of .lter spin and spintronic devices, as well as the injection of spin in quantum wells and semiconductor detectors based on non-magnetic asymmetric double barrier. / Neste trabalho usamos a técnica da matriz de espalhamento para calcular a trans- missividade de spins polarizados através de heteroestruturas semicondutoras de dupla barreira simétrica. A dinâmica dos portadores é descrita pela aproximação de massa efetiva aplicada aos modelos de Dresselhaus e Bychkov-Rashba. Sendo o primeiro hamil- toniano estudado em sua forma completa, isenta de aproximações. Ambos os modelos descrevem interações do tipo spin órbita, que é, basicamente, um acoplamento entre o momento angular orbital do elétron com seu momento magnético de spin. Esse tipo de acoplamento gera como consequência um desdobramento de alguns níveis de ener- gia, inicialmente degenerados, acarretando em um espectro de energia mais complexo do que àquele obtido sem levar-se em conta tais interações. A transmissividade é cal- culada como função de alguns parâmetros que controlam a e.ciência de polarização, para uma heteroestrutura do tipo InAs=GaSb=InAs=GaSb=InAs. Os efeitos das interações spin órbita de Dresselhaus e Rashba mostram-se bastante favoráveis à fabricação de dispositivos spintrônicos, bem como a injeção de spin em poços quânticos e detectores baseados em semicondutores não magnéticos em dupla barreira assimétrica.

La frustration est un concept physique qui apparaît lorsque toutes les interactions d'un système ne peuvent être satisfaites en même temps. La glace de spins artificielle est un exemple de système magnétique frustré par la compétition entre interactions dipolaires. Jusqu'ici, les propriétés de ce métamatériau ont été étudiées à l'aide de techniques microscopiques. Ces dernières ne permettent qu'un accès limité à la dynamique du système. Cette dernière pourrait être étudiée par les techniques de diffusion qui ont une meilleure résolution temporelle.La diffusion magnétique résonante des rayons-x mous (SXRMS) est une technique qui a une résolution spatiale comparable avec les tailles standards de la glace de spins artificielle. Le but de cette thèse est de démontrer que cette technique peut apporter des informations intéressantes sur l'organisation magnétique. Elle est organisée en deux parties principales. Tout d'abord, la fabrication d'échantillon est décrite étape par étape. Ensuite, les études de la glace de spins carrée et kagome sont présentées. Des signaux d'origine magnétique ont été enregistré qui apportent une vision de l'organisation magnétique. L'interprétation de ces signaux est complexe à cause de l'espace réciproque. Le dernier chapitre présente une série de mesures utilisant des techniques non-microscopiques sur des systèmes avec des fluctuations magnétiques.Les différentes études présentées montrent que non seulement la glace de spin artificielle peut être étudiée par la diffusion et les autres techniques apportent des informations sur le système. Cette thèse ouvre la voie à des techniques plus avancées comme, dans le domaine des rayons X, l'illumination de l'échantillon avec un faisceau cohérent. Le travail présenté est par conséquent une première étape vers l'étude de la dynamique dans la glace de spins artificielle. / Frustration is a concept in physics which appears when all interactions in a system cannot be satisfied at the same time. Artificial spin ice is an example of a magnetic system that is frustrated by the competition between the dipolar interactions. So far, the properties of this metamaterial has been studied by means of microscopic techniques with the disadvantage of a limited dynamical range. At a time when dimensions of the nanomagnets allow the access to fast dynamics using magnetic fluctuations, the time-resolution of scattering techniques could make them relevant for such studies.Soft x-ray resonant magnetic scattering (SXRMS) is a magnetic-sensitive technique with a resolution compatible with the dimensions of artificial spin ice. The scope of this thesis is to demonstrate that SXRMS can bring relevant information about the magnetic organisation in artificial spin ice. It is organized in two parts. The fabrication of scattering-compatible artificial spin ice samples is described step by step. Then, the studies of both artificial square and kagome spin ice are presented. Signals of magnetic origin have been recorded, bringing insights in the magnetic organization of the system. While most of the studies have been performed on static systems, a small chapter is presenting a serie of studies using non-microscopic techniques which include systems with fluctuating magnetic moments.The different studies performed within this thesis demonstrate that artificial spin ice can not only be studied by microscopy techniques, but that scattering and non-microscopic methods are also relevant. In particular, it opens the way to more advanced experiments such as, in the x-ray domain, the illumination of the system with a coherent beam in order to resolve the nature of the very short magnetic correlations using x-ray photon correlation spectroscopy technique. The presented work is therefore a first step toward the study of dynamics in artificial spin ice.

Submitted by Ronildo Prado (bco.producao.intelectual@gmail.com) on 2017-12-21T17:12:13Z No. of bitstreams: 1 AZEVEDO_José_2017.pdf: 2048175 bytes, checksum: 8f9c77b624ba3e310f344bcb798ee84d (MD5) / Approved for entry into archive by Ronildo Prado (bco.producao.intelectual@gmail.com) on 2017-12-21T17:15:40Z (GMT) No. of bitstreams: 1 AZEVEDO_José_2017.pdf: 2048175 bytes, checksum: 8f9c77b624ba3e310f344bcb798ee84d (MD5) / Approved for entry into archive by Ronildo Prado (bco.producao.intelectual@gmail.com) on 2017-12-21T17:15:51Z (GMT) No. of bitstreams: 1 AZEVEDO_José_2017.pdf: 2048175 bytes, checksum: 8f9c77b624ba3e310f344bcb798ee84d (MD5) / Made available in DSpace on 2017-12-21T17:16:03Z (GMT). No. of bitstreams: 1 AZEVEDO_José_2017.pdf: 2048175 bytes, checksum: 8f9c77b624ba3e310f344bcb798ee84d (MD5) Previous issue date: 2007-03-15 / Fundação de Amparo à Pesquisa do Estado do Amazonas (FAPEAM) / In this thesis, we will study the quantum phase transition of frustrated quantum spin models: (i) van Hemmen model ( S = 1) with transverse and anisotropic biaxial field (ii) Heisenberg model (S = 1/2 ) with competitive interaction first and second nearest neighbours (J1-J2 model) (iii) Ising model with transverse field and first magnetic model is studied to simulate the spin glass properties in real systems like the magnetic susceptibility cusp. We use the bimodal and gaussian probability distribution for random interactions. Applying the first-order approximation to decouple the products of exponential of operators, we calculate free energy and order parameter. Both, the transverse field and anisotropic transverse field destroy the spin glass order. In the second model, we use the effective field theory with differential operator technique and effective field renormalization group (EFRG) formalism. The phase diagrams are determined where are observe ferromagnetic (F), antiferromagnetic (AF) and superantiferromagnetic (SAF) states. In case of Heisenberg model in a square lattice at T=0, we have a quantum paramagnetic state that has been considered as a spin-liquid (SL) state in literature. For a simple cubic lattice, this spin-liquid state has not been observed. Which shows that the dimension of the system has influences on the quantum fluctuation at T=0. In the phase diagrams are the presence of first and second order phase transitions. Finally, are consider the critical behavior of the frustrated quantum Ising model and at T=0 we have the states with energy gap proportional to the transverse field intensity. Depending in the frustration parameter the system also shows first and second order transitions. / Nesta tese estudaremos a transição de fase quântica dos modelos de spins quânticos frustrados: i) Modelo de van Hemmen de spin S=1 com campo transverso e anisotropia biaxial; ii) modelo de Heisenberg de spin ½ anisotrópico com interações competitivas entre primeiros e segundos vizinhos (modelo J1-J2); iii) modelo de Ising com campo transverso e com interações de primeiros e segundos vizinhos. O primeiro modelo magnético é estudado para simular as propriedades de vidro de spin em sistemas reais, como, por exemplo, a cúspide da susceptibilidade magnética. Usamos as densidades de probabilidades bimodal e gaussiana nas ligações aleatórias. Aplicando a aproximação de primeira ordem para desacoplar o produto de exponenciais de operadores, calculamos a energia livre e parâmetro de ordem. Tanto o campo transverso quanto as anisotropias transversais, individualmente, atuam como agentes de destruição da ordem vidro de spin. Neste modelo são estudadas transições de fases quânticas de primeira e segunda ordem. No segundo modelo usamos o formalismo da teoria de campo efetivo via técnica do operador diferencial e grupo de renormalização na aproximação de campo efetivo (EFRG). São determinados diagramas de fases, onde observamos os estados ferromagnético (F), antiferromagnético (AF), superantiferromagnético (SAF) (denominado de colinear para uma rede quadrada e laminar para uma rede cúbica simples). No caso Heisenberg numa rede quadrada , em T=0 temos um estado paramagnético quântico que é sugerido a ser o estado spin-líquido (SL) discutido na literatura. Para a rede cúbica simples esse estado spin-líquido não foi observado, mostrando que o efeito da dimensionalidade no sistema influencia nas flutuações quânticas em T=0. Nos diagramas de fases temos as presenças de transições de primeira e segunda ordem. Finalmente, tratamos da criticalidade do modelo de Ising quântico frustrado, e em T=0 temos estados com gap de energia proporcional a intensidade do campo transverso, que dependendo do parâmetro de frustração o sistema presencia também transições de primeira e segunda ordem.

Made available in DSpace on 2014-12-17T15:14:54Z (GMT). No. of bitstreams: 1 SelmaRV_TESE.pdf: 3758039 bytes, checksum: 6ed2e51c943660db7972e9c51961c80a (MD5) Previous issue date: 1999-06-18 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Ising and m-vector spin-glass models are studied, in the limit of infinite-range in-teractions, through the replica method. First, the m-vector spin glass, in the presence of an external uniform magnetic field, as well as of uniaxial anisotropy fields, is consi-dered. The effects of the anisotropics on the phase diagrams, and in particular, on the Gabay-Toulouse line, which signals the transverse spin-glass ordering, are investigated. The changes in the Gabay-Toulouse line, due to the presence of anisotropy fields which favor spin orientations along the Cartesian axes (m = 2: planar anisotropy; m = 3: cubic anisotropy), are also studied. The antiferromagnetic Ising spin glass, in the presence of uniform and Gaussian random magnetic fields, is investigated through a two-sublattice generalization of the Sherrington-Kirpaktrick model. The effects of the magnetic-field randomness on the phase diagrams of the model are analysed. Some confrontations of the present results with experimental observations available in the literature are discussed / Modelos de vidros de spins, dos tipos Ising e m-vetorial, com intera??es de al-cance infinito, s?o estudados atrav?s do m?todo das r?plicas. Inicialmente, o vidro de spins TO-vetorial, na presen?a de um campo magn?tico externo uniforme e de campos de anisotropias uniaxiais, ? considerado. Os efeitos das anisotropias nos diagramas de fases e, em particular, na linha de Gabay-Toulouse, que sinaliza o ordenamento dos graus de liberdade de vidro de spins transversos, s?o investigados. As modifica??es na linha de Gabay-Toulouse, devido ? presen?a de campos de anisotropia que privilegiam orienta??es segundo os eixos cartesianos (m = 2: anisotropia planar; m 3: anisotropia c?bica), tamb?m s?o estudadas. O vidro de spins de Ising com favorecimento antiferromagn?tico, na presen?a de campos magn?ticos uniforme e aleat?rio obedecendo uma distribui??o de probabilidades gaussiana, ? investigado atrav?s de uma generaliza??o do modelo Sherrington-Kirpaktrick em duas subredes. Os efeitos da aleatoriedade no campo magn?tico sobre os diagramas de fases do modelo s?o analisados. Algumas confronta??es entre resultados deste trabalho e medidas experimentais existentes na literatura s?o discutidas

Made available in DSpace on 2014-12-17T15:14:55Z (GMT). No. of bitstreams: 1 JoaoMA_TESE.pdf: 3210677 bytes, checksum: b439c16c54cbcf828e9759b7638b198c (MD5) Previous issue date: 2000-01-14 / In this work we have studied the effects of random biquadratic and random fields in spin-glass models using the replica method. The effect of a random biquadratic coupling was studied in two spin-1 spin-glass models: in one case the interactions occur between pairs of spins, whereas in the second one the interactions occur between p spins and the limit p > oo is considered. Both couplings (spin glass and biquadratic) have zero-mean Gaussian probability distributions. In the first model, the replica-symmetric assumption reveals that the system presents two pha?ses, namely, paramagnetic and spin-glass, separated by a continuous transition line. The stability analysis of the replica-symmetric solution yields, besides the usual instability associated with the spin-glass ordering, a new phase due to the random biquadratic cou?plings between the spins. For the case p oo, the replica-symmetric assumption yields again only two phases, namely, paramagnetic and quadrupolar. In both these phases the spin-glass parameter is zero. Besides, it is shown that they are stable under the Almeida-Thouless stability analysis. One of them presents negative entropy at low temperatures. We developed one step of replica simmetry breaking and noticed that a new phase, the biquadratic glass phase, emerge. In this way we have obtained the correct phase diagram, with.three first-order transition lines. These lines merges in a common triple point. The effects of random fields were studied in the Sherrington-Kirkpatrick model consi?dered in the presence of an external random magnetic field following a trimodal distribu?tion {P{hi) = p+S(hi - h0) +Po${hi) +pS(hi + h0))- It is shown that the border of the ferromagnetic phase may present, for conveniently chosen values of p0 and hQ, first-order phase transitions, as well as tricritical points at finite temperatures. It is verified that the first-order phase transitions are directly related to the dilution in the fields: the extensions of these transitions are reduced for increasing values of po- In fact, the threshold value pg, above which all phase transitions are continuous, is calculated analytically. The stability analysis of the replica-symmetric solution is performed and the regions of validity of such a solution are identified / Nesse trabalho estudamos os efeitos de acoplamentos biquadr?ticos e campos aleat?rios em vidros de spins utilizando o m?todo das r?plicas. O efeito do acoplamento biquadr?tico aleat?rio ? estudado em um vidro de spins com spin 1 em duas situa??es diferentes: no caso onde as intera??es s?o entre pares de spins e no caso onde a intera??o ocorre entre p spins no limite p > oo. Ambos os acoplamen?tos (vidro de spins e biquadr?tico) t?m distribui??o de probabilidades gaussianas com m?dia zero. Para intera??es entre pares de spins a solu??o com simetria entre r?plicas fornece as fases paramagn?tica e vidro de spins separadas por uma linha de transi??es cont?nuas. A an?lise de estabilidade da solu??o com simetria entre r?plicas fornece, al?m da instabilidade usual associada com o ordenamento vidro de spins, uma nova fase devido ao acoplamento biquadr?tico entre os spins. Para o caso p = oo, a solu??o com simetria entre r?plicas fornece apenas duas fases, marginalmente est?veis com rela??o ? an?lise de estabilidade de de Almeida e Thouless. Entretanto, uma dessas fases apresenta entropia negativa a baixas temperaturas. Implementamos tamb?m um passo de quebra de sime?tria nos par?metros vidros de spins e vidro quadrupolar e obtivemos o diagrama de fases correto, incluindo a fase vidro de spins. Nesse caso, todas as transi??es s?o de primeira ordem e as tr?s fases coexistem em um ponto triplo. Os efeitos de campos aleat?rios s?o estudados no modelo de Sherrington-Kirkpatrick, onde consideramos um campo aleat?rio com distribui??o de probabilidades trimodal {P(hi) = p+?(hi - h0) +po?(h0) +p-6(hi + ho))- Mostramos que para valores convenientes de po e h0, a fronteira ferromagn?tica pode apresentar transi??es de primeira ordem e pontos tricr?ticos a temperaturas finitas. Verificamos que as transi??es de primeira ordem est?o diretamente relacionadas com a dilui??o do campo: as extens?es dessas transi??es s?o reduzidas quando aumentamos o valor de po- De fato, o valor pl acima do qual todas as transi??es s?o cont?nuas ? calculado analiticamente. Fizemos tamb?m a an?lise de es?tabilidade da solu??o com simetria entre r?plicas e determinamos as regi?es de validade desta solu??o