Matériaux dans lesquels les électrons responsables des propriétés de basse énergie son soumis à fortes corrélations sont aujourd'hui très étudiés à la recherche de nouvelles phases émergentes aux propriétés surprenantes et/ou utiles.Les supraconducteurs à base de fer (IBSC) sont maintenant considérés dans cette classe de composés. En utilissant des techniques multi-corps nécessaires pour le traitement théorique de ces corrélations (théorie du champ moyen de spin esclave - SSMFT et théorie du champ moyen dynamique - DMFT - en conjonction avec la théorie du fonctionnelle de la densité, DFT), dans cette thèse, j'etudie plusieurs propriétés d'IBSC.D’abord, j'analyse les composés très dopés de la famille de IBSC, qui montrent expérimentalement certains comportements typiques des ``fermions lourds'', des composés typiquement des terres rares ou des actinides, où des électrons extrêmement corrélés coexistent avec des électrons moins corrélés. En particulier je me concentre sur la chaleur spécifique et le pouvoir thermoélectrique et je montre comment ces propriétés peuvent être comprises dans le paradigme récemment développé ``métaux de Hund''. En effet, l’échange intra-atomique (le ``couplage de Hund'') est responsable de ces matériaux à éléments métal de transition en montrant la physique des fermions lourds. Je montre aussi que les caractéristiques typiquement fermions-lourds du spectre d’excitation, connues car les singularités de Van Hove sont bien capturées par notre modélisation au sein de DFT+SSMFT. J'utilise ensuite DMFT dans un modèle afin d'étudier l'impact direct des singularités de Van Hove sur la force des corrélations.Dans une seconde partie, je montre comment FeSe, le IBSC actuellement le plus étudié, se trouve également dans une phase métal de Hund, mais il est amené à la frontière de cette phase par la pression. Cette frontière est liée à une augmentation de la compressibilité électronique qui est positivement corrélée à l’augmentation de la supraconductivité trouvée dans les expériences.Je réalise une étude analogue sur le détenteur du record pour la température supraconductrice critique la plus élevée, la monocouche FeSe où je trouve également une compressibilité augmentée. Cela appuie la récente proposition selon laquelle la frontière du métal de Hund favorise la supraconductivité à haute température.Enfin, j'étudie la nature du magnétisme dans une autre famille de IBSC, les germanides de fer. J'explore différents ordres magnétiques possibles avec des simulations DFT et leur concurrence (ce qui peut en principe favoriser la supraconductivité) dans plusieurs composés où différents substitutions sont appliquées au composé parent YFe2Ge2. J'étudie également l'effet de la pression chimique sur ce composé. / Materials where the electrons responsible for the low-energy properties experience strong correlations are today very investigated in search of emerging new phases with surprising and/or useful properties. Iron-based superconductors (IBSC) are now considered in this class of compounds. Using the many-body techniques necessary for the theoretical treatment of these correlations (slave-spin mean field theory - SSMFT- and dynamical mean field theory - DMFT- in conjunction with density functional theory, DFT), in this thesis I address several properties of IBSC.First I analyze the very hole-doped compounds in the IBSC family, that show experimentally some behaviors typical of the so-called “heavy fermions”, compounds typically of rare earth or actinides, where extremely correlated electrons coexist with others less correlated. In particular I focus on the specific heat and the thermoelectric power and show how these properties can be understood in the recently developed paradigm of “Hund’s metals”. Indeed the intra-atomic exchange (the “Hund’s coupling”) is responsible for these materials of transition metal elements showing heavy-fermionic physics. I show also that typical heavy-fermionic features of the excitation spectrum, known as Van Hove singularities are well captured by our modelization within DFT+SSMFT. I then use DMFT in a model in order to study the direct impact of the Van Hove singularities on the strength of correlations.In a second part I show how FeSe, the presently most studied IBSC, is also in a Hund’s metal phase, but it is brought to the frontier of this phase by pressure. This frontier is connected to an enhancement of the electronic compressibility which correlates positively then with the enhancement of superconductivity found in experiments. I perform an analogous study on the record holder for the highest critical superconducting temperature, the monolayer FeSe where I also find an enhanced compressibility. This supports the recent proposal that the frontier of a Hund's metal favors high-temperature superconductivity.Finally I study the nature of magnetism in another family of IBSC, the iron-germanides. I explore different possible magnetic orders with DFT simulations and study their competition (which can in principle favor superconductivity) in several compounds where different chemical substitutions are applied to the parent compound YFe2Ge2. I also study the effect of chemical pressure on this compound.

Ce travail de thèse a été consacré à l'étude des noyaux avec quelques trous dans la fermeture de couche N = 82, qui présente une très riche variété d'excitations à hauts spins. La particularité de ces noyaux est la présence de coexistence de forme (sphérique et/ou triaxiale) à des spins très élevés. Durant ma thèse, j’ai analysé une expérience effectuée avec le multidétecteur Euroball à l’IPHC (Strasbourg) pour l'étude des états de haut spin dans le noyau ¹⁴¹Nd, peuplé en utilisant la réaction de fusion-évaporation : ⁹⁶Zr (⁴⁸Ca, 3n). Nous avons identifié plusieurs bandes à spin élevé et développé le schéma de niveaux jusqu'à une énergie d'excitation et spin de l’ordre de 19 MeV et 81/2⁻ respectivement, qui sont bien plus élevés par rapport à celui publié précédemment (9.4 MeV et 49/2h). Trois nouvelles bandes dipôlaires et trois nouvelles bandes rotationnelles quadrupolaires ont été identifiées. La séquence principale yrast a aussi été étendue jusqu’au spin 61/2⁻.Nous avons réalisé des calculs théoriques détaillés des bandes observées, en intégrant, pour la première fois, les résultats qu’on a obtenus par les modèles Cranked Nilsson-Strutinsky (CNS) et Tilted Axis Cranking (TAC). Une interprétation cohérente de la plupart des bandes observées a été réalisée, qui semblent être basées sur des minimas d’énergies presque sphérique pour les séquences principales, légèrement déformée pour les bandes dipoalires et triaxiale déformée pour les bandes quadrupolaires. La structure de niveau observée dans le noyau ¹⁴¹Nd révèle la capacité des noyaux avec quelques trous dans la fermeture de couche N = 82 à acquérir des formes différentes de tourner autour d'un axe principal ou d'un axe incliné par rapport au système de référence intrinsèque, comme dans le cas des noyaux ¹³⁸Nd et ¹⁴⁰Nd récemment étudiés par notre groupe. Tous ces résultats représentent donc un fort soutien à l'existence de la forme nucléaire triaxiale stable à hauts spins dans cette région de masse. / This PhD work was devoted to the study of exotic nuclear rotation and stable triaxiality at very high spin. In, nuclei with a few holes in the N = 82 shell closure, which exhibit a large variety of excitations at medium and high spins. The peculiar feature of these nuclei is the existence of coexisting shapes, spherical and triaxial, up to very high spins.During my PhD I analyzed an experiment performed with the Euroball multidetector at IPHC (Strasbourg) for the study of high-spin states in ¹⁴¹Nd populated using the ⁹⁶Zr (⁴⁸Ca, 3n) fusion-evaporation reaction. We have identified several high-spin bands and developed the level scheme up to an excitation energy and spin of 19 MeV and 81/2⁻ , respectively, which are much higher than previously published (9.4 MeV and 49⁄2 ℏ). Three new dipole bands and three new ΔI = 2 rotational bands have been identified. The main yrast sequence was extended up to spin 61/2⁻.A detailed interpretation of the observed bands was performed, for the first time, using theoretical calculations with Cranked Nilsson-Strutinsky (CNS) and the Tilted Axis Cranking (TAC) models. A consistent interpretation of most of the observed bands was realized. They appear to be based on nearly spherical for the main sequence, slightly deformed for the dipole bands and triaxially deformed minima for the ΔI = 2 bands. The observed level structure of ¹⁴¹Nd reveals the capability of the nuclei with a few holes in the N = 82 shell closure to acquire different shapes and to rotate around a principal or a tilted axis relative to the intrinsic reference system, as in the case of the ¹³⁸Nd and ¹⁴⁰Nd nuclei recently studied by our group.

Ressonância magnética nuclear é particularmente útil na caracterização de síntese molecular. Quase todos os compostos moleculares de interesse contêm átomos de nitrogênio e oxigênio em sua estrutura. Como esses elementos químicos costumam desempenhar funções estratégicas em reações e interações intra e intermoleculares, seus espectros NMR têm particular interesse. Com foco na blindagem magnética nuclear e no acoplamento entre spins nucleares, investigamos a dependência dessas constantes magnéticas com relação ao meio. A polarização eletrônica e relaxação geométrica do soluto, devido à presença do solvente, foram sistematicamente consideradas utilizando um tratamento sequencial QM/MM. Nossas observações gerais mostram que efeitos de relaxação de geometria mediados pelo meio têm pouca influência sobre o mecanismo da blindagem magnética nuclear, pelo menos quanto aos sistemas aqui investigados. Em contrapartida, os efeitos da polarização eletrônica são cruciais sobre essa propriedade molecular. Para o nitrogênio em piridina, amônia e formamida, por exemplo, as contribuições isoladas provenientes da relaxação geométrica são da ordem de 1,2 ppm. Note-se que este efeito é muito pequeno se considerarmos que com frequência são observados desvios gás-líquido da ordem de -26 ppm para o nitrogênio. Sobre o oxigênio, esses efeitos podem chegar até 100 ppm. Assim, é crucial entender as origens desses efeitos se desejamos utilizar corretamente espectroscopia RMN em caracterização molecular. Relevantes contribuições para a blindagem magnética têm origens em interações específicas entre soluto e solvente, como ligações de hidrogênio, as quais apenas podem ser isoladas e quantificadas através de modelagem molecular. Sistematicamente constatamos que a blindagem magnética é drasticamente afetada pelo caráter doador ou aceitador do sítio atômico envolvido em ligações de hidrogênio. Com respeito ao nitrogênio, nossos resultados indicam que o comportamento aceitador de hidrogênio de um elemento é responsável por um desvio gás-líquido positivo, enquanto que o caráter doador causa um desvio negativo. Nossas investigações também mostram que a blindagem magnética nuclear é independente com relação à hibridização do nitrogênio e oxigênio, indicando que as contribuições principais têm origens em interações específicas soluto-solvente, as quais devem ser corretamente modeladas. Investigamos sistematicamente os efeitos do solvente sobre o acoplamento indireto entre spins nucleares em amônia líquida. Embora a polarização do soluto seja realmente importante, para essa propriedade há outras contribuições fundamentais. Sem considerar efeitos de geometria, calculamos o acoplamento ANTPOT. 1 J(N-H) como -67,8 Hz. Após incluir esses efeitos, conseguimos uma descrição teórica mais apurada, obtendo um acoplamento de -63,9 Hz. Esses resultados mostram que efeitos de relaxação geométrica mediado pelo meio têm grande influência sobre o acoplamento indireto entre spins nucleares. / Nuclear magnetic resonance is helpful on molecular characterization. Most organic molecular compounds of interest contain nitrogen or oxygen atoms which play strategic functions in chemical reactions and molecular interactions. The NMR technique provides local atomic scale information on molecular properties, thus the study of nuclear magnetic properties of these elements in molecules is of particular interest. Focusing on nuclear magnetic shielding and spin-spin coupling constants we investigated the dependence of these magnetic constants with respect to the medium. Electronic polarization and geometry relaxation effects due to solvent were systematically studied. Our general findings indicate that geometry contributions are negligible in understanding the variation of shielding constants of the investigated systems, but polarization effects are crucial for this molecular property. On nitrogen in pyridine, ammonia and formamide, for instance, isolated contributions from geometry relaxation to shielding constants are around 1,2 ppm. Nitrogen shielding constants are very sensitive to the medium, where solvent effects around -26 ppm are often observed. On oxygen-17 magnetic shielding, the solvent effects can easily reach 100 ppm. It is crucial to understand the origins of these effects if one desires to correctly use NMR spectroscopy for molecular characterization. Our investigations also show that magnetic shielding constants are totally independent of the nitrogen or oxygen hybridization, which indicated that the main contributions arise from solute-solvent interactions. Relevant contributions to shielding constants come from specific solute-solvent interactions like hydrogen-bonds, which can only be quantified by explicit molecular modeling, and we observed that this property has a very strong dependence on the donor or acceptor character of the atomic site involved in hydrogen-bonding interactions. On nitrogen, the acceptor behavior is responsible by the positive gas-liquid shift in shielding constants, while the donor character causes positive shift. We systematically investigated the solvent effects on indirect spin-spin coupling constants of liquid ammonia. Solute polarization is very important, but there are other fundamental contributions to this property. Without including geometry effects we calculated the 1J (N-H) coupling to be -67,8 Hz. After accounting for the solute geometry relaxation we improved the theoretical prediction obtaining the coupling constant value of -63,9 Hz. These results show that geometry relaxation has drastic influence on indirect spin-spin coupling constants.

Ressonância magnética nuclear é particularmente útil na caracterização de síntese molecular. Quase todos os compostos moleculares de interesse contêm átomos de nitrogênio e oxigênio em sua estrutura. Como esses elementos químicos costumam desempenhar funções estratégicas em reações e interações intra e intermoleculares, seus espectros NMR têm particular interesse. Com foco na blindagem magnética nuclear e no acoplamento entre spins nucleares, investigamos a dependência dessas constantes magnéticas com relação ao meio. A polarização eletrônica e relaxação geométrica do soluto, devido à presença do solvente, foram sistematicamente consideradas utilizando um tratamento sequencial QM/MM. Nossas observações gerais mostram que efeitos de relaxação de geometria mediados pelo meio têm pouca influência sobre o mecanismo da blindagem magnética nuclear, pelo menos quanto aos sistemas aqui investigados. Em contrapartida, os efeitos da polarização eletrônica são cruciais sobre essa propriedade molecular. Para o nitrogênio em piridina, amônia e formamida, por exemplo, as contribuições isoladas provenientes da relaxação geométrica são da ordem de 1,2 ppm. Note-se que este efeito é muito pequeno se considerarmos que com frequência são observados desvios gás-líquido da ordem de -26 ppm para o nitrogênio. Sobre o oxigênio, esses efeitos podem chegar até 100 ppm. Assim, é crucial entender as origens desses efeitos se desejamos utilizar corretamente espectroscopia RMN em caracterização molecular. Relevantes contribuições para a blindagem magnética têm origens em interações específicas entre soluto e solvente, como ligações de hidrogênio, as quais apenas podem ser isoladas e quantificadas através de modelagem molecular. Sistematicamente constatamos que a blindagem magnética é drasticamente afetada pelo caráter doador ou aceitador do sítio atômico envolvido em ligações de hidrogênio. Com respeito ao nitrogênio, nossos resultados indicam que o comportamento aceitador de hidrogênio de um elemento é responsável por um desvio gás-líquido positivo, enquanto que o caráter doador causa um desvio negativo. Nossas investigações também mostram que a blindagem magnética nuclear é independente com relação à hibridização do nitrogênio e oxigênio, indicando que as contribuições principais têm origens em interações específicas soluto-solvente, as quais devem ser corretamente modeladas. Investigamos sistematicamente os efeitos do solvente sobre o acoplamento indireto entre spins nucleares em amônia líquida. Embora a polarização do soluto seja realmente importante, para essa propriedade há outras contribuições fundamentais. Sem considerar efeitos de geometria, calculamos o acoplamento ANTPOT. 1 J(N-H) como -67,8 Hz. Após incluir esses efeitos, conseguimos uma descrição teórica mais apurada, obtendo um acoplamento de -63,9 Hz. Esses resultados mostram que efeitos de relaxação geométrica mediado pelo meio têm grande influência sobre o acoplamento indireto entre spins nucleares. / Nuclear magnetic resonance is helpful on molecular characterization. Most organic molecular compounds of interest contain nitrogen or oxygen atoms which play strategic functions in chemical reactions and molecular interactions. The NMR technique provides local atomic scale information on molecular properties, thus the study of nuclear magnetic properties of these elements in molecules is of particular interest. Focusing on nuclear magnetic shielding and spin-spin coupling constants we investigated the dependence of these magnetic constants with respect to the medium. Electronic polarization and geometry relaxation effects due to solvent were systematically studied. Our general findings indicate that geometry contributions are negligible in understanding the variation of shielding constants of the investigated systems, but polarization effects are crucial for this molecular property. On nitrogen in pyridine, ammonia and formamide, for instance, isolated contributions from geometry relaxation to shielding constants are around 1,2 ppm. Nitrogen shielding constants are very sensitive to the medium, where solvent effects around -26 ppm are often observed. On oxygen-17 magnetic shielding, the solvent effects can easily reach 100 ppm. It is crucial to understand the origins of these effects if one desires to correctly use NMR spectroscopy for molecular characterization. Our investigations also show that magnetic shielding constants are totally independent of the nitrogen or oxygen hybridization, which indicated that the main contributions arise from solute-solvent interactions. Relevant contributions to shielding constants come from specific solute-solvent interactions like hydrogen-bonds, which can only be quantified by explicit molecular modeling, and we observed that this property has a very strong dependence on the donor or acceptor character of the atomic site involved in hydrogen-bonding interactions. On nitrogen, the acceptor behavior is responsible by the positive gas-liquid shift in shielding constants, while the donor character causes positive shift. We systematically investigated the solvent effects on indirect spin-spin coupling constants of liquid ammonia. Solute polarization is very important, but there are other fundamental contributions to this property. Without including geometry effects we calculated the 1J (N-H) coupling to be -67,8 Hz. After accounting for the solute geometry relaxation we improved the theoretical prediction obtaining the coupling constant value of -63,9 Hz. These results show that geometry relaxation has drastic influence on indirect spin-spin coupling constants.

Cette thèse porte sur le développement et l'application en physique statistique d'un nouveau paradigme pour les méthodes sans rejet de Monte-Carlo par chaînes de Markov irréversibles, grâce à la mise en œuvre du filtre factorisé de Metropolis et du concept de lifting. Les deux premiers chapitres présentent la méthode de Monte-Carlo et ses différentes applications à des problèmes de physique statistique. Une des principales limites de ces méthodes se rencontre dans le voisinage des transitions de phase, où des phénomènes de ralentissement dynamique entravent fortement la thermalisation des systèmes. Le troisième chapitre présente la nouvelle classe des algorithmes de Metropolis factorisés et irréversibles. Se fondant sur le concept de lifting des chaînes de Markov, le filtre factorisé de Metropolis permet de décomposer un potentiel multidimensionnel en plusieurs autres unidimensionnels. De là, il est possible de définir un algorithme sans rejet de Monte-Carlo par chaînes de Markov irréversibles. Le quatrième chapitre examine les performances de ce nouvel algorithme dans une grande variété de systèmes. Des accélérations du temps de thermalisation sont observées dans des systèmes bidimensionnels de particules molles, des systèmes bidimensionnels de spins XY ferromagnétiques et des systèmes tridimensionnels de verres de spins XY. Finalement, une réduction importante du ralentissement critique est exposée pour un système tridimensionnel de spins Heisenberg ferromagnétiques. / This thesis deals with the development and application in statistical physics of a general framework for irreversible and rejection-free Markov-chain Monte Carlo methods, through the implementation of the factorized Metropolis filter and the lifting concept. The first two chapters present the Markov-chain Monte Carlo method and its different implementations in statistical physics. One of the main limitations of Markov-chain Monte Carlo methods arises around phase transitions, where phenomena of dynamical slowing down greatly impede the thermalization of the system. The third chapter introduces the new class of irreversible factorized Metropolis algorithms. Building on the concept of lifting of Markov chains, the factorized Metropolis filter allows to decompose a multidimensional potential into several unidimensional ones. From there, it is possible to define a rejection-free and completely irreversible Markov-chain Monte Carlo algorithm. The fourth chapter reviews the performance of the irreversible factorized algorithm in a wide variety of systems. Clear accelerations of the thermalization time are observed in bidimensional soft-particle systems, bidimensional ferromagnetic XY spin systems and three-dimensional XY spin glasses. Finally, an important reduction of the critical slowing down is exhibited in three-dimensional ferromagnetic Heisenberg spin systems.

Le travail de cette thèse porte sur l'étude des lieux conjugué et de coupure de métriques riemanniennes ou pseudo-riemanniennes en dimension 2. On se place du point de vue du contrôle optimal pour appliquer le principe du maximum de Pontryagin afin de caractériser les extrémales des problèmes considérés.On va utiliser des méthodes géométriques, numériques et d'intégrabilité pour étudier des métriques de Clairaut-Liouville ou de Liouville sur la sphère. Dans le cas dégénéré de révolution, l'étude de l'ellipsoïde utilise des méthodes géométriques pour déterminer le lieu de coupure et la nature du lieu conjugué dans les cas oblat et prolat. Dans le cas général, les extrémales auront deux types de comportements distincts qui se rapportent à ceux observés dans le cas de révolution, et sont séparés par celles passant par des points ombilicaux. Les méthodes numériques sont utilisées pour retrouver rapidement la dernière conjecture géométrique de Jacobi : le lieu de coupure est un segment et le lieu conjugué contient quatre points de rebroussement.L'étude d'une métrique pseudo-riemannienne vient d'un problème de contrôle quantique où le but est de transférer en temps minimal l'état d'un spin à travers une chaîne de trois spins couplés par des interactions de type Ising. Après réduction, la métrique obtenue possède une intégrale première supplémentaire et on peut donc la mettre sous forme de Liouville, ce qui nous donne les équations des géodésiques. En dehors du cas particulier de Grushin, dont la caustique est décrite, on utilise les méthodes numériques pour étudier le lieu conjugué et le lieu de coupure dans le cas général. / The work of this thesis is about the study of the conjugate and cut loci of 2D riemannian or almost-riemannian metrics. We take the point of view of optimal control to apply the Pontryagin Maximum Principle in the purpose of characterize the extremals of the problem considered.We use geometric, numerical and integrability methods to study some Liouville and Clairaut-Liouville metrics on the sphere. In the degenerate case of revolution, the study of the ellipsoid uses geometric methods to fix the cut locus and the nature of the conjugate locus in the oblate and prolate cases. In the general case, extremals will have two distinct type of comportment which correspond to those observed in the revolution case, and are separated by those which pass by umbilical points. The numerical methods are used to find quickly the Jacobi's Last Geometric Statement : the cut locus is a segment and the conjugate locus has exactly four cusps.The study of an almost-riemannian metric comes from a quantum control problem in which the aim is to transfer in a minimal time the state of one spin through an Ising chain of three spins. After reduction, we obtain a metric with a second first integral so it can be written in the Liouville normal form, which leads us to the equations of geodesics. Outside the particular case of Grushin, of which the caustic is described, we use numerical methods to study the conjugate locus and the cut locus in the general case.

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.

L'étude menée dans cette thèse concerne la dynamique de systèmes quantiques hors de l'équilibre, et plus particulièrement leurs propriétés d'intrication. En effet, l'intrication est devenue un concept fondamental dans la physique moderne, grâce notamment au développement de l'information quantique. Nous avons dans un premier temps étudié la dynamique d'un modèle de bosons sur réseau après la trempe de leur potentiel de confinement. Dans la limite de coeur dur, nous avons développé une théorie hydrodynamique qui reproduit parfaitement les différents comportements observés. Nous nous sommes ensuite intéressés à la dynamique de deux spins défauts couplés à une chaîne d'Ising. Dans un premier temps, ces défauts ont été préparés dans un état séparable. Nous avons dans ce cas établi une formule donnant l'évolution temporelle de la matrice de densité réduite, qui nous a permis d'avoir accès à l'intrication créée par l'intermédiaire du couplage à la chaîne. Puis, nous avons considéré le cas de deux spins défauts initialement intriqués, et nous avons étudié l'influence d'un environnement hors de l'équilibre sur leurs propriétés de désintrication. Finalement, la dernière partie de cette thèse est consacrée à l'étude d'un système couplé à un environnement décrit par le processus d'interactions répétées. Nous avons étudié la relaxation du système dans deux régimes temporels différents. Pour des temps courts, l'état est bien décrit par un état stationnaire hors équilibre, dans lequel nous avons mis en évidence les propriétés d’échelle de certaines observables. Enfin, pour des temps longs, le système atteint un état stationnaire d'équilibre composé d'un produit d'états de Bell / The study carried in this thesis concerns the dynamics of out-Of-Equilibrium quantum systems, and more particularly their entanglement properties. Indeed, entanglement became a fundamental concept in modern physics, especially with the development of quantum information. We have in a first part studied the dynamics of a model of bosons on a lattice after the quench of their trapping potential. In the hard-Core limit, we developed an hydrodynamical theory which perfectly reproduced the observed behavior. Then, we have looked at the dynamics of two defect spins coupled to an Ising chain. When these defects have been prepared into a separable state, we have established a formula giving the evolution of the reduced density matrix, allowing us to have access to the entanglement create through the coupling to the chain. We considered then the case of two initially entangled defect spins, and we studied the influence of a non-Equilibrium environment on the disentanglement properties. Finally, the last part of this thesis is devoted to the study of a system coupled to an environment by means of the repeated interactions process. We studied the relaxation of the system in two different time regimes. For short times, the state is well described by a non-Equilibrium-Steady-State, in which we highlighted the scaling properties of some observables. For long times, the system reaches an equilibrium steady state made of a product of Bell states

Submitted by Haroudo Xavier Filho (haroudo.xavierfo@ufpe.br) on 2016-04-14T16:39:36Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) tese_final_biblioteca.pdf: 10701235 bytes, checksum: 1af9d6bb745e78ea9fe6b261b7baa59c (MD5) / Made available in DSpace on 2016-04-14T16:39:36Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) tese_final_biblioteca.pdf: 10701235 bytes, checksum: 1af9d6bb745e78ea9fe6b261b7baa59c (MD5) Previous issue date: 2015-05-08 / Nesta tese tratamos teoricamente algumas propriedades magnéticas e excita c~oes de spins em materiais ferromagn eticos e antiferromagn eticos com o objetivo de interpretar as medidas e observa c~oes experimentais. Usamos a formula c~ao de ondas de spins de Holstein-Primako para investigar propriedades da din^amica de propaga c~ao de ondas de spins no material ferrimagn etico YIG e tamb em no sistema antiferromagn etico FeF2. Para cada um deles, desenvolvemos modelos te oricos que permitiram ajustar as rela c~oes de dispers~ao de ondas de spins medidas experimentalmente nesses. Al em disso, investigamos a renormaliza c~ao da energia de m agnons usando o formalismo das fun c~oes de Green de dois tempos dependentes da temperatura de Bogoliubov-Tyablikov. A aplica c~ao para o YIG, permitiu calcular as varia c~oes da rela c~ao de dispers~ao para m agnons com a temperatura. Tamb em se investigou a renormaliza c~ao da energia de m agnons no sistema FeF2, mas esta vez considerando uma aproxima c~ao do tipo RPA1. Na renormaliza c~ao da energia foram consideradas duas contribui c~oes: exchange isotr opica tipo Heisenberg e anisotropia uniaxial. Em nossas considera c~oes, foi inclu da a depend^encia com a temperatura da anisotropia uniaxial que permitiu estimar o campo critico de transi c~ao de spin- op para o limite de estabilidade da fase antiferromagn etica. Tamb em, foi feito um estudo te orico e experimental da anisotropia magneto cristalina do material antiferromagn etico RbMnF3, baseado nas intera c~oes de campo cristalino e intera c~ao de spin- orbita, com o objetivo de calcular a varia c~ao com a temperatura do campo de spin- op. As ondas de spins em YIG foram estudadas em mais detalhes calculando as taxas de relaxa c~ao por processos de espalhamento de 3-m agnons e 4-m agnons. Igualmente foram estimadas empregando campos magn eticos intensos da ordem de 102 kOe. Finalmente, estudamos propriedades t ermicas de transporte de m agnons em YIG submetido a campos magn eticos intensos. Calculamos o calor 1Random Phase Approximation especi co de m agnons e a condutividade t ermica em baixas temperaturas e campos magn eticos externos intensos para serem comparados com as medidas experimentais encontrando um bom acordo entre teoria e experimento [145] / In this thesis we theoretically treat some magnetic properties and excitations of spins in ferromagnetic and antiferromagnetic materials in order to interpret measurements and experimental observations. We use the formulation of quantization of spin waves of Holstein-Primako to investigate properties of the spin wave propagation dynamics in the ferrimagnetic material YIG and also in the antiferromagnetic system FeF2. For each of them, we have developed theoretical models to adjust the dispersion relations of spin waves measured experimentally in these systems. In addition, we investigated the magnon energy renormalization using the formalism of two-time temperature dependent Green's functions of Bogoliubov-Tyablikov. The application for the YIG allowed calculate the variation of the dispersion relation for magnons with temperature. We also investigate the magnon energy renormalization in the system FeF2, but this time considering an approximation of the RPA type. In the energy renormalization were considered two contributions: exchange isotropic Heisenberg and uniaxial anisotropy type. In our calculations the temperature dependence of the uniaxial anisotropy eld allows estimating the critical spin- op transition for the antiferromagnetic phase stability limit. Also, we have undertaken a theoretical and experimental study of the magneto crystalline anisotropy of the antiferromagnetic material RbMnF3, based on the crystal eld interactions and spin-orbit interaction, in order to calculate the variation with temperature of the eld spin- op. The spin waves in YIG were studied in more detail calculating the relaxation rate due to 3-magnons and 4-magnons scattering processes considering strong magnetic elds of hundreds of kOe. Finally, we have studied thermal transport properties of magnons in YIG subject to intense magnetic elds. We have, calculate of the speci c heat and thermal conductivity of magnons at low temperatures and high elds to compare with the experimental measurements nding a good agreement between theory and experiment [145].

Dans les systèmes de spins quantiques, la frustration et la basse dimensionnalité génèrent des fluctuations quantiques et donnent lieu à des phases exotiques. Cette thèse étudie un modèle d'échelle de spins avec des couplages frustrants le long des montants, motivé par les expériences sur le cuprate BiCu$_2$PO$_6$. Dans un premier temps, on présente une méthode variationnelle originale pour décrire les excitations de basse énergie d'une seule chaîne frustrée. Le diagramme de phase de deux chaînes couplées est ensuite établi à l'aide de méthodes numériques. Le modèle exhibe une transition de phase quantique entre une phase dimérisée est une phase à liens de valence résonnants (RVB). La physique de la phase RVB et en particulier l'apparition de l'incommensurabilité sont étudiées numériquement et par un traitement en champ moyen. On étudie ensuite les effets d'impuretés non-magnétiques sur la courbe d'aimantation et la loi de Curie à basse température. Ces propriétés magnétiques sont tout d'abord discutées à température nulle à partir d'arguments probabilistes. Puis un modèle effectif de basse énergie est dérivé dans la théorie de la réponse linéaire et permet de rendre compte des propriétés magnétiques à température finie. Enfin, on étudie l'effet d'un désordre dans les liens, sur une seule chaîne frustrée. La méthode variationnelle, introduite dans le cas non-désordonné, donne une image à faible désordre de l'instabilité de la phase dimérisée, qui consiste en la formation de domaines d'Imry-Ma délimités par des spinons localisés. Ce résultat est finalement discuté à la lumière de la renormalisation dans l'espace réel à fort désordre.