Dissertationen zum Thema „Spin Polarized Molecular Systems“

Dans la thèse, nous avons utilisé des simulations de Monte Carlo combinées avec différentes techniques efficaces tels que les méthodes d'histogramme pour étudier les transitions de phase et transport des spins dans différents systèmes. La première partie est consacrée à l'étude des transition de phase dans les systèmes de spins frustrés: (i) le modèle J_1-J_2 avec des spins Ising dans le régime antiferromagnétique complet, (ii) le modèle HCP avec des spins Ising et des spins $XY$ dans le régime antiferromagnétique complet. Les résultats obtenus montrent en effet une transition du premier ordre que l'on trouve plus tôt dans d'autres systèmes frustrés. La deuxième partie montre les état fondamental et transitions de phase dans les cristaux moléculaires et dans les liquides de dimères. Pour faire face à ces systèmes, nous avons utilisé le modèle de Potts en tenant compte de l'interaction dipolaire pour expliquer structures périoques en couches observées expérimentalement. Les résultats montrent des effets étonnants de cette interaction à longue portée. L'effet de l'interaction d'échange de surface a été pris en compte dans ce travail. Finalement, nous avons calculé la résistivité des spins itinérants. Nous nous sommes concentrés en particulier sur les effets des fluctuations de spin dans la région de transition de phase. Des résultats intéressants ont été obtenus montrant une forte corrélation entre les fluctuations de spin et le comportement de la résistivité
In this thesis, we have used Monte Carlo simulations combined with different efficient techniques such as histogram methods to study the phase transitions and spin transport in various systems. The first part is devoted to the investigation of phase transition in frustrated spin systems: (i) the J_1-J_2 model with Ising spin in the full antiferromagnetic regime, (ii) the HCP lattice with both Ising and XY spin in the full antiferromagnetic regime. The results obtained show indeed a first-order transition as found earlier in other frustrated systems. The second part shows the ground state and phase transitions in molecular crystals and in dimer liquids. To deal with these systems, we have used the Potts model taking into the account the dipolar interaction to explain long-period layered structures experimentally observed. The results show amazing effects of this long-range interaction. The effect of surface exchange interaction has been considered in this work. Finally, we describe the resistivity of itinerant spins. We focused in particular on the effects of spin fluctuations in the phase transition region. Interesting results have been obtained showing a strong correlation between spin fluctuations and the behavior of the resistivity

L'effet Kondo observé dans des objets individuels constitue un système modèle pour l’étude de corrélations électroniques. Ces dernières jouent un rôle moteur dans le domaine émergent de l'électronique de spin (ou spintronique) où l’utilisation d’atomes issus des terres rares et des métaux de transition est incontournable. Dans ce contexte, l’étude de l'interaction d’une impureté Kondo avec des électrodes ferromagnétiques ou avec d’autres impuretés magnétiques peut donc s’avérer fondamental pour la spintronique. L’effet Kondo est sensible à son environnement magnétique car en présence d’interactions magnétiques la résonance ASK se dédouble. Dans une certaine mesure, la résonance ASK agit comme un niveau atomique discret doublement dégénérée qui subit un dédoublement Zeeman en présence d'un champ magnétique ou plus généralement d’un champ magnétique effectif. Inversement, la détection d'un dédoublement Zeeman indique l'existence d'un champ magnétique. Dans une boîte quantique, le couplage de la boîte avec les deux électrodes est faible en général et la largeur de la résonance ASK est donc de l'ordre de quelques meV. Beaucoup d’études de l’effet Kondo en présence d’interactions magnétiques ont été menées sur les boîtes quantiques, grâce notamment au contrôle qui peut être exercé sur la résonance ASK, mais aussi grâce au faible élargissement de la résonance qui peut alors être dédoublée avec un champ magnétique de l’ordre de 10 Tesla ou moins. A ces études, s’ajoutent de nombreux travaux similaires menés avec des dispositifs tels des jonctions cassées comprenant une molécule individuelle jouant le rôle de l’impureté magnétique. En revanche, peu d’études de ce type ont été consacrées aux atomes individuels. Cela est dû à l’hybridation plus marquée entre l'impureté atomique et la surface comparée aux boîtes quantiques, qui entraine une largeur typique de 10 meV ou plus pour la résonance ASK. Un champ magnétique d'environ 100 T ou plus est alors nécessaire afin de dédoubler la résonance et donc en pratique difficile à mettre en oeuvre. Cette thèse est consacrée précisément à l’étude de l'interaction entre une impureté Kondo individuel et son environnement magnétique à l’aide d’un STM. Une nouvelle stratégie est adoptée ici par rapport aux études antérieures de ce genre. Tout d'abord, nous éliminons la barrière tunnel en établissons un contact pointe-atome. Nous formons ainsi un point de contact quantique comprenant une seule impureté Kondo. Deuxièmement, nous utilisons des pointes ferromagnétiques. Le contact pointe-atome permet de sonder l'influence du ferromagnétisme sur l'impureté Kondo vial’observation de la résonance ASK. La géométrie de contact permet tout particulièrement de produire une densité de courant polarisé en spin suffisamment élevée pour qu’elle entraîne un dédoublement de la résonance ASK. Ce dédoublement constitue la première observation à l’échelle atomique d’un phénomène connu sous le nom d’accumulation de spin, laquelle se trouve être une propriété fondamentale de la spintronique
The Kondo effect of these single objects represents a model system to study electron correlations, which are nowadays of importance in relation to the emerging field of spin electronics, also known as spintronics, where chemical elements with partially filled d or f shells play a central role. Also of particular interest to spintronics is the interaction of single Kondo impurities with ferromagnetic leads or with other magnetic impurities. A Kondo impurity is in fact sensitive to its magnetic environment as the ASK resonance is usually split into two resonances in the presence of magnetic interactions. To some extent, the ASK resonance acts as a two-fold degenerate energy level of an atom which undergoes a Zeeman splitting in the presence of an effective magnetic field. Conversely, the detection of a Zeeman splitting indicates the existence of a magnetic field. In a QD, the coupling of the QD to the two leads is very weak in general, and the Kondo resonance is in the range of a few meV. Many studies focusing on magnetic interaction have been carried out on QDs, due to the high control that can be extended to the ASK resonance and its low energy range, allowing to split the resonance with a magnetic field of 10 T. Similar work has also been carried out in single-molecule or lithographically-defined devices. Although STM is an ideal tool to study the Kondo effect of single atoms, there is still a strong lack of experimental studies concerning atoms in the presence of magnetic interactions. This is partly due to the stronger impurity-metal hybridization compared to QDs, which places the ASK width in the range of 10 meV. An effective magnetic field of 100 T would be needed to split the resonance. The present Thesis is devoted precisely at studying the interaction between a single Kondo impurity with its magnetic environment through STM. A new strategy is adopted herecompared to former studies of this kind. Firstly, we contact a single-magnetic atom on a surface with a STM tip thereby eliminating the vacuum barrier. Secondly, we use ferromagnetic tips. The contact with a single atom allows probing the influence of ferromagnetism on the Kondo impurity i. e. its ASK resonance. But most importantly, the contact geometry produces sufficiently high current densities compared to the tunneling regime, so that the ASK resonance becomes sensitive to the presence of a spin-polarized current. This constitutes the first atomic scale detection of a spin-polarized current with a single Kondo impurity

L'objectif de cette thèse est de contribuer à la compréhension des phénomènes de mouvement de l'électron induits par le spin. Ces phénomènes aparaissent lorsqu'un électron se déplace à travers un environnement (partiellement) magnétique, de telle sorte que son moment magnétique (spin) peut interagir avec l'environnement. La nature quantique pure du spin nécessite des modèles de transport qui traitent des effets comme la cohérence quantique, l'intrication (corrélation) et la dissipation quantique. Sur le niveau méso- et macroscopique, il n'est pas encore clair dans quelles circonstances ces effets quantiques du spin peut transparaitre. Le but de ce travail est, d'une part, de dériver des nouveaux modèles de transport de spin à partir des principes de base et, d'autre part, de développer des algorithmes numériques qui permettent de trouver une solution de ces modèles. Cette thèse se compose de quatre parties. La première partie introductive contient un aperçu des concepts fondamentaux liés au transport polarisé en spin, tels que la magnéto-résistance géante (GMR), le couple de transfert de spin dans les multi-couches magnétiques et le caractère matriciel des équations de transport qui prennent en compte la cohérence de spin. L'accent est mis sur la modélisation du couple de transfert de spin, qui représente l'intersection de ces concepts. En particulier, nous considérons pour sa description le modèle diffusif de Zhang-Levy-Fert (ZLF) qui se compose de l'équation de Landau-Lifshitz et d'une équation de diffusion matricielle pour le spin. Un schéma de différences finies est développé pour résoudre numériquement ce système non-linéaire dans des structures multi-couches. Le modèle est testé par comparaison des résultats obtenus aux données expérimentales récentes. Les parties deux et trois forment le noyau thématique de cette thèse. Dans la deuxième partie nous proposons une équation de Boltzmann matricielle qui permet la description de la cohérence de spin sur le niveau cinétique. La nouveauté est un opérateur de collision dans lequel les taux de transition de la quantité de mouvement sont modélisés par une matrice 2x2 hermitienne; par conséquent, les libre parcours moyens des électrons spin-up et spin-down sont représentés par les valeurs propres de cette matrice de scattering. Après une dérivation formelle de l'équation de Vlasov matricielle à partir de l'équation de Wigner, l'équation cinétique qui suit est étudiée en ce qui concerne l'existence, l'unicité et la positivé d'une solution. En outre, le nouveau opérateur de collision est étudié rigoureusement et la limite de diffusion tc -> 0, correspondant à l'annulation de la moyenne de temps de scattering, est effectué. Les équations de drift-diffusion matricielle qui sont obtenues représentent une amélioration par rapport au modèle traité dans la première partie. Ce dernier est obtenu dans la limite ou la différence entre les deux valeurs propres de la matrice de scattering va disparaître. La troisième partie est consacrée à l'obtention de l'opérateur de collision matricielle introduit auparavant, à partir des principes quantiques. Pour cela, nous augmentons l'équation de von Neumann d'un système composite par un terme dissipatif qui fait tendre l'opérateur de densité totale vers l'approximation de Born. En vertu de la prémisse que la relaxation est le processus dominant, on obtient une hiérarchie d'équations non-Markoviennes. Celles-ci découlent d'une expansion de l'opérateur de densité en termes de tr, le temps de relaxation. Dans la limite de Born-Markov, tr -> 0, l'équation de Lindblad est récupérée. Elle a la même structure que l'opérateur de collision proposé dans la deuxième partie. Cependant, l'équation de Lindblad est encore une équation microscopique; donc la prochaine étape serait de procéder à la limite semi-classique du résultat obtenu. Dans la quatrième partie nous procédons à une étude numérique d'un modèle quantique-diffusif de spin qui décrit le transport dans un gaz d'électrons bidimensionnel avec un couplage spin-orbite de Rashba. Ce modèle suppose que les électrons sont dans un état d'équilibre quantique sous la forme d'un opérateur de Maxwell. Nous présentons deux discrétisations espace-temps du modèle couplé par l'équation de Poisson. Dans une première étape on applique une discrétisation en temps et on montre que les systèmes sont bien définis. Ceux-ci sont basés sur un formalisme fonctionnel pour traiter les relations non-locales entre les densités de spin. Nous utilisons ensuite des discrétisations espace-temps pour simuler la dynamique dans une géométrie typique d'un transistor. Les approximations différences finies sont du premier ordre en temps et du second ordre en espace. Les fonctionnelles discrètes sont minimisée à l'aide d'un algorithme du gradient conjugué et la méthode de Newton est appliquée afin de trouver les minima dans la direction désirée
The aim of this thesis is to contribute to the understanding of spin-induced phenomena in electron motion. These phenomena arise when electrons move through a (partially) magnetic environment, in such a way that its magnetic moment (spin) may interact with the surroundings. The pure quantum nature of the spin requires transport models that deal with effects like quantum coherence, entanglement (correlation) and quantum dissipation. On the meso- and macroscopic level it is not yet clear under which circumstances these quantum effects may transpire. The purpose of this work is, on the one hand, to derive novel spin transport models from basic principles and, on the other hand, to develop numerical algorithms that allow for a solution of these new and other existing model equations. The thesis consists of four parts. The first part has introductory character; it comprises an overview of fundamental spin-related concepts in electronic transport such as the giant-magneto-resistance (GMR) effect, the spin-transfer torque in metallic magnetic multilayers and the matrix-character of transport equations that take spin-coherent electron states into account. Special emphasis is placed on the modeling of the spin-transfer torque which represents the intersection of these concepts. In particular, we consider the diffusive Zhang-Levy-Fert (ZLF) model, an exchange-torque model that consists of the Landau-Lifshitz equation and a heuristic matrix spin-diffusion equation. A finite difference scheme based on Strang operator splitting is developed that enables a numerical, self-consistent solution of this non-linear system within multilayer structures. Finally, the model is tested by comparison of numerical results to recent experimental data. Parts two and three are the thematic core of this thesis. In part two we propose a matrix-Boltzmann equation that allows for the description of spin-coherent electron transport on a kinetic level. The novelty here is a linear collision operator in which the transition rates from momentum k to momentum k' are modeled by a 2x2 Hermitian matrix; hence the mean-free paths of spin-up and spin-down electrons are represented by the eigenvalues of this scattering matrix. After a formal derivation of the matrix-Vlasov equation as the semi-classical limit of the one-electron Wigner equation, the ensuing kinetic equation is studied with regard to existence, uniqueness and positive semi-definiteness of a solution. Furthermore, the new collision operator is investigated rigorously and the diffusion limit tc -> 0 of the mean scattering time is performed. The obtained matrix drift-diffusion equations are an improvement over the heuristic spin-diffusive model treated in part one. The latter is obtained in the limit of identical eigenvalues of the scattering matrix. Part three is dedicated to a first step towards the derivation of the matrix collision operator, introduced in part two, from first principles. For this, we augment the von Neumann equation of a composite quantum system by a dissipative term that relaxes the total state operator towards the Born approximation. Under the premise that the relaxation is the dominant process we obtain a hierarchy of non-Markovian master equations. The latter arises from an expansion of the total state operator in powers of the relaxation time tr. In the Born-Markov limit tr -> 0 the Lindblad master equation is recovered. It has the same structure as the collision operator proposed in part two heuristically. However, the Lindblad equation is still a microscopic equation; thus the next step would be to carry out the semi-classical limit of the result obtained. In part four we perform a numerical study of a quantum-diffusive, two-component spin model of the transport in a two-dimensional electron gas with Rashba spin-orbit coupling. This model assumes the electrons to be in a quantum equilibrium state in the form of a Maxwellian operator. We present two space-time discretizations of the model which also comprise the Poisson equation. In a first step pure time discretization is applied in order to prove the well-posedness of the two schemes, both of which are based on a functional formalism to treat the non-local relations between spin densities via the chemical potentials. We then use fully space-time discrete schemes to simulate the dynamics in a typical transistor geometry. Finite difference approximations applied in these schemes are first order in time and second order in space. The discrete functionals introduced are minimized with the help of a conjugate gradient-based algorithm in which the Newton method is applied to find the desired line minima

Laser-cooled atoms offer an excellent platform for testing new ideas of quantum control and measurement. I will discuss experiments where we use light and magnetic fields to drive and monitor non-trivial quantum dynamics of a large spin-angular momentum associated with an atomic hyperfine ground state. We can design Hamiltonians to generate arbitrary spin states and perform a full quantum state reconstruction of the results. We have implemented and verified time optimal controls to generate a broad variety of spin states, including spin-squeezed states useful for metrology. Yields achieved are of the range 0.8-0.9.We present a first experimental demonstration of the quantum kicked top, a popular paradigm for quantum and classical chaos. We make `movies' of the evolving quantum state which provides a direct observation of phase space dynamics of this system. The spin dynamics seen in the experiment includes dynamical tunneling between regular islands, rapid spreading of states throughout the chaotic sea, and surprisingly robust signatures of classical phase space structures. Our data show differences between regular and chaotic dynamics in the sensitivity to perturbations of the quantum kicked top Hamiltonian and in the average electron-nuclear spin entanglement during the first 40 kicks. The difference, while clear, is modest due to the small size of the spin.

One of the important issues of Quantum Chromodynamics (QCD) - the fundamental theory of strong interaction, is the understanding of the role of the quark-gluon interactions in the processes involving nuclear targets. One direction in such studies is to explore the onset of the quark gluon degrees of freedom in nuclear dynamics. The other direction is using the nuclear targets as a “micro-labs” in studies of the QCD processes involving protons and neutrons bound in the nucleus. In the proposed research, we work in both directions considering high energy photo- and electro-production reactions involving deuteron and 3 He nuclei. In the first half of the research, we study the high energy break-up of the 3 He nucleus, caused by a incoming photon, into a proton-deuteron pair at the large center of mass scattering angle. The main motivation of the research is the theoretical interpretation of recent experimental data which revealed the unprecedentedly large exponent s −17 , for the energy dependence of the differential cross section. In the present research, we extend the theoretical formalism of the hard QCD rescattering model to calculate energy and angular dependences of the absolute cross section of the γ 3 He → pd reaction in high momentum transfer limit. The second half of the research explores the deep-inelastic scattering of a polarized electron off the polarized deuteron and 3 He nuclei, to explore the quark-gluon structure of polarized neutron. The main reason of using deuteron is that it is the most simple and best understood nucleus. While the reason of using polarized 3 He as an effective polarized neutron target is that because of the Pauli-principle, the two protons in the target are in the opposite spin states and thus the neutron has all the polarization of the 3 He nucleus. However this approximation is exact only for the S-state and becomes less accurate with the increase of the internal momentum of the bound nucleons in the nucleus. There are several planned experiments which will be performed during next few years at the kinematics in which the internal momenta of the probed neutron cannot be neglected. Therefore, for the reliable interpretation of the data, all the nuclear effects, especially the effects related to the relativistic treatment of high momentum component of the nuclear wave function, should be taken into account. In this work, we developed a comprehensive theoretical framework for calculation of the all relevant nuclear effects that will allow the accurate extraction of the neutron data from deepinelastic scattering involving deuteron and 3 He targets.

Macroscopic and microscopic properties of molecular and solid-state systems are intimately related to the their electronic structure. The electron position and spin densities, which represent the probability distributions to find all or unpaired electrons in the space, contain information concerning several chemical-relevant properties, such as the chemical bonding and the magnetic behaviour. Understanding the fine atomic-level mechanism behind these properties is a key step to design chemical modifications to properly tune and develop materials or molecules with specific features. Topological descriptors can be used to extract information from these electron distributions. In this work, novel applications of the source function descriptor have been developed to gain further insights on the electron and spin density-related properties. These developments, together with other topological descriptors, were used to get further insights on relevant chemical systems. Firstly, the source function reconstruction was enlarged to a multi-dimensional grid of points with a particular focus on the two-dimensional maps. This analysis allows to see the ability of chosen subsets of atoms to reconstruct the density in the selected area within a cause-effect relationship and to rationalise the chemical or magnetic behaviours. The source function partial reconstructed maps depict if in a molecular region the atomic contributions are important, modest or negligible. Besides, they may also be useful for a proper selection of the reference points and for a full understanding of the source function percentages analysis. In fact, the choice of the reference point where to reconstruct the studied density is neither easy nor objective for non-standard situations, such as for the spin density. This novel application was applied to the study of the spin density on a couple of azido Cu complexes. The source function partial reconstructed maps allow to unravel the different role played by the paramagnetic centre Cu and the ligand atoms and to explain the spin transmission mechanism at a molecular level. Moreover, they enable to highlight the nature of the spin density differences between the two complexes and among adopted computational approaches. DFT functionals tend to over-delocalise the spin density towards the ligand atoms introducing a biased spin-polarization mechanism between the Cu and the ligand atoms. The same descriptor was then applied to the study of the hydrogen bonds in the DNA base pairs. The source function reveals the delocalised nature of these interactions, highlighting that distant groups and rings have non-negligible effects on the reconstruction of the electron density in the intermolecular region. Besides, the analysis demonstrates that the purine and pyrimidine bases equally contribute to the reconstruction of the electron density at the hydrogen bond critical points. The source function also reveals that subtle variations of the atomic source contributions occur when the pairs are ionized, revealing that sources and sinks effects redistribution plays an important role in the stabilization of the DNA base pairs. The source function was also used to develop a method to extract full population matrices purely based on the electron density distribution and then amenable to experimental determination. The peculiar features of this descriptor, in particular the cause-effect relationship, assign a profound chemical meaning to the matrix elements in contrast with other population analyses such as the Mulliken's one, where the matrix elements are associated to orbital overlaps. The latest breakthroughs on the development of this method are shown together with some numerical examples on very simple compounds. The full population matrices obtained using the source function descriptor are able to retrieve the major chemical features. A detailed analysis on the intermolecular interactions involved in the in vivo molecular recognition of the antimalarial drug chloroquine with the heme moiety has been carried out using a combined topological-energetic analysis. This work reveals that charged-assisted hydrogen bonds set up between the lateral chains of the chloroquine and the propionate group of the heme are the most important interactions in the drug:substrate recognition process.

Thesis advisor: Ilija Zeljkovic
We used spectroscopic-imaging scanning tunneling microscopy (SI-STM) and spin-polarized STM (SP-STM) to unveil new electronic phenomena in several different quantum systems. We explored: (1) a potential topological superconductor heterostructure Bi₂Te₃/Fe(Te, Se), (2) high-Tc superconductors − Bi₂Sr₂CaCu₂O₈₊ₓ and Fe(Te, Se), and (3) doped spin-orbit Mott insulators Sr₂IrO₄ and Sr₃Ir₂O₇. In Bi₂Te₃/Fe(Te, Se), we observed superconductivity (SC) on the surface of Bi₂Te₃ thin film, induced by the iron-based superconductor substrate. By annealing the optimally-doped cuprate superconductor Bi₂Sr₂CaCu₂O₈₊ₓ, we drastically lowered the surface hole doping concentration to detect a unidirectional charge stripe order, the first reported charge order on an insulating (defined by the spectral gap with zero conductance spanning the Fermi level) cuprates surface. In the high-Tc SC Fe(Te, Se) single crystal, we found local regions of electronic nematicity, characterized by C₂ quasiparticle interference (QPI) induced by Fermi surface anisotropy and inequivalent spectral weight of dyz and dxz orbitals near Fermi level. Interestingly, the nematic order is locally strongly anti-correlated with superconductivity. Finally, utilizing SP-STM, we observed a short-range antiferromagnetic (AF) order near the insulator-metal transition (IMT) in spin-orbital Mott insulators Sr₂IrO₄ and Sr₃Ir₂O₇. The AF order inhomogeneity is found not to be strongly correlated with the charge gap. Interestingly, the AF order in the bi-layered Sr₃Ir₂O₇ shows residual memory behavior with temperature cycling. Overall, our work revealed new phenomena in a range of today’s most intriguing materials and set the stage for using SP-STM in other complex oxides
Thesis (PhD) — Boston College, 2020
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics

This PhD thesis reports original research results concerning the development of theoretical models and computational protocols for the quantification and analysis of two of the most important quantum observables of open-shell systems: the electron spin density and the phosphorescence energy gap. In the first part, a comprehensive theory of the electron spin density topology is proposed for the first time [1]. Several new notions (spin density critical points, molecular spin graphs, spin density basins, spin maxima and spin minima joining paths etc.) and descriptors (local and integral spin polarization indeces, basin average spin density etc.) are introduced. This analysis reveals that the spin density topology, based on precise mathematical notions, can unveil precious information on the physical structure of spin-polarized systems. In particular, it enables to describe and quantify spin polarization and delocalization mechanisms and, at the same time, to evaluate the dependence of spin density distributions on the adopted level of theory. In the second part instead, the performance of the domain-based local pair natural orbital (DLPNO) variant of the “gold standard” CCSD(T) method for the prediction of phosphorescence energies of aromatic chromophores is investigated for the first time [2]. An extensive analysis of both accuracy and computational cost of the main parameters of the method (basis set, triples correction approximation, dimension of PNOs space) is conducted. Two procedures, the Gold DLPNO-CCSD(T) aimed at maximizing the accuracy and the Silver DLPNO-CCSD(T) aimed at minimizing the computational cost, which result in an excellent agreement with experimental data, are proposed. 1. G. Bruno, G. Macetti, L. Lo Presti and C. Gatti, “Spin Density Topology,” Molecules, 25, 3537, 2020. 2. G. Bruno, B. de Souza, F. Neese, and G. Bistoni, “Can domain-based local pair natural orbitals approaches accurately predict phosphorescence energies?,” Phys. Chem. Chem. Phys., 24, 14228–14241, 2022.

In this thesis Moore and Yeo's proposed mapping of the structural glass to the Ising spin glass in a random field is presented. In contrast to Random First Order Theory and Mode Coupling Theory, this mapping predicts that there should be no glass transition at finite temperature. However, a growing correlation length is predicted from the size of rearranging regions in the supercooled liquid, and from this a growing structural relaxation time is predicted. Also presented is a study of the propensity of binary fluids (i.e. fluids containing particles of two sizes) to phase separate into regions dominated by one type of particle only. Binary fluids like this are commonly used as model glass formers and the study shows that this phase separation behaviour is something that must be taken into account.The mapping relies on the use of replica theory and is therefore very opaque. Here a model is presented that may be mapped directly to a system of spins, and also prevents the process of phase separation from occurring in binary fluids. The system of spins produced in the mapping is then analysed through the use of an effective Hamiltonian, which is in the universality class of the Ising spin glass in a random field. The behaviour of the correlation length depends on the spin-spin coupling J and the strength of the random field h. The variation of these with packing fraction and temperature T is studied for a simple model, and the results extended to the full system. Finally a prediction is made for the critical exponents governing the correlation length and structural relaxation time.

Spintronique moléculaire : de la vanne de spin à la détection d'un spin unique. Parmi les thématiques qui ont émergé ces dix dernières années, la spintronique moléculaire est intéressante de par son caractère hybride, à la croisée entre l'électronique de spin, l'électronique moléculaire et le magnétisme moléculaire. Dans ce nouveau domaine, on cherche à exploiter les propriétés magnétiques et quantiques des aimants moléculaires pour créer des dispositifs originaux, utiles en spintronique ou en information quantique. Mon projet de thèse s'inscrit dans cette perspective en voulant combiner un transistor à nanotube de carbone avec des aimants à molécule unique, en les couplant par des interactions supramoléculaires. L'objectif est d'observer le renversement magnétique d'une seule molécule par des mesures de transport électronique à travers le nanotube. En effet, le diamètre de ce dernier étant comparable aux dimensions d'un aimant moléculaire, le couplage devrait être suffisamment fort pour en permettre la détection. La réalisation d'un tel dispositif, un défi technique, et la question de savoir s'il était réellement possible de détecter et de caractériser le moment d'une seule molécule ont constitué les deux enjeux majeurs de cette thèse. Une grande partie du travail réalisé porte sur la fabrication du dispositif expérimental par des techniques de micro- et nano-fabrication, ainsi que sur l'optimisation du greffage des aimants moléculaires sur la surface du nanotube. Dans un second temps, nous nous intéressons à l'étude du système et à son comportement à très basse température (100 mK). En effet, la proximité des aimants moléculaires TbPc2 modifie de façon spectaculaire les propriétés de transport d'un nanotube. En particulier, nous présentons la réalisation d'un dispositif dont la réponse est analogue à une vanne de spin classique, où les molécules magnétiques jouent le rôle de polariseur ou d'analyseur de spin. Grâce à ce système, nous avons réussi à affiner nos connaissances sur TbPc2. Entre autres résultats, nous sommes parvenus à isoler et à caractériser le retournement du moment magnétique d'un seul ion de terbium. Enfin, la dernière partie de cette thèse est consacrée à l'étude de l'interaction hyperfine au sein du terbium. En réalisant un dispositif qui n'est couplé qu'à deux molécules, nous avons mis en évidence qu'il est possible de réaliser une lecture directe de l'état d'un spin nucléaire unique.

Au cours de ce travail, nous avons applique la technique de la diffraction de neutrons polarises a l'etude de la densite de spin dans des composes magnetiques moleculaires. Nous avons tout d'abord etudie deux nitronyl nitroxydes presentant une structure en chaine via des liaisons hydrogenes : la densite de spin est majoritairement localisee dans une orbitale antiliante * construite sur les fonctions 2#p des atomes d'azote et d'oxygene des groupes nitroxydes. Par ailleurs, nous avons mis en avant le role primordial des liaisons hydrogenes dans le couplage ferromagnetique intermoleculaire, role confirme par des calculs theoriques ab initio (methode de la fonctionnelle de la densite). Nous nous sommes ensuite interesses a un complexe cuivrique d'une enaminocetone nitroxyde. Nous avons pu determiner qu'une dimerisation quasi totale avait lieu a basse temperature entre deux fonctions nitroxydes, de deux molecules differentes, se faisant face et distantes de 3. 40a. Puis, nous presentons l'etude d'un ferroaimant moleculaire (radical nitronyl nitroxyde). La densite de spin est en grande partie concentree sur le fragment oncno du cycle nit dans une orbitale magnetique moleculaire antiliante *, le couplage ferromagnetique intermoleculaire modifiant l'orientation relative des orbitales magnetiques des deux atomes d'oxygene des fonctions -no, rotation s'accompagnant d'une hybridation sp. La derniere etude presentee dans ce manuscrit a trait a un alkyl nitroxyde presentant un ordre ferromagnetique. Nous avons verifie la localisation majoritaire de l'electron non apparie sur le groupe -no et la nature antiliante * de l'orbitale moleculaire magnetique. Les populations individuelles de spin obtenues sur l'ensemble de la molecule, nous ont permis de proposer un mecanisme expliquant les couplages ferromagnetiques intermoleculaires.

Cette thèse porte sur l'utilisation de plusieurs techniques de diffusion de neutrons polarisés pour la conduite d'expériences de diffraction et de spectroscopie inélastique à haute résolution. Nous décrivons de façon exhaustive l'option à écho de spin neutronique résonant ZETA, installée sur le spectromètre triple axe thermique CRG IN22 à l'Institut Laue Langevin. Grâce à elle, nous étudions la structure nucléaire et la dynamique de spin de plusieurs systèmes modèles. Dans un premier temps, nous nous intéressons à la série BaM2(XO4)2 (M = Co, Ni; X = As, P) dont les membres sont de bons exemples de systèmes magnétiques quasi-bidimensionnels. L'effet de la mise en ordre magnétique sur leurs paramètres de maille est révélé par diffraction de Larmor. De plus, nous montrons que l'évolution thermique de la durée de vie du mode de magnon optique dans BaNi2(PO4)2 est fortement affecté par la présence de défauts dans sa structure. Ensuite, nous abordons le composé à chaînes et échelles de spin 1/2 Sr14Cu24O41. Nous nous focalisons d'abord sur l'étude du pic inélastique associé au gap de spin des échelles et présentons une méthode capable de montrer de façon directe la dégénérescence de la transition concernée. Ensuite, nous évaluons sa largeur énergétique intrinsèque et observons l'effet des différentes mises en ordre de charge sur la structure cristallographique du matériau. Finalement, nous adaptons l'instrumentation disponible pour mener des expériences de réflectométrie résolues en temps, par le biais de la méthode MIEZE, sur une multicouche magnétique pouvant posséder des propriétés intéressante pour des applications en spintronique
This thesis is mainly concerned with the use of several polarized neutron scattering techniques for carrying high resolution diffraction and inelastic spectroscopy experiments. We describe exhaustively our neutron resonant spin-echo option ZETA, installed on the thermal triple-axis spectrometer CRG IN22 at Institut Laue Langevin. Through it, we study the nuclear structure and spin dynamics of several model systems. First, we are interested in the BaM2(XO4)2 (M = Co, Ni; X = As, P)-family which members are good prototypes of quasi-2D magnetic systems. The effect of magnetic ordering on lattice constants is revealed thanks to Larmor diffraction. Moreover, we show that the thermal evolution of optic magnon lifetime in BaNi2(PO4)2 is strongly affected by the presence of defects in its structure. Then, we address the spin-chain and -ladder compound Sr14Cu24O41. We first focus on the study of the inelastic peak associated with the spin gap in the ladders spectrum and introduce a method capable of showing directly the degeneracy of the associated spin transition. We also evaluate its intrinsic linewidth and observe the effect of different charge ordering process on the material crystallographic structure. Ultimately, we adapt our instrumentation to perform time-resolved reflectometry experiments on a magnetic multilayer which can possess interesting properties for spintronics applications, through the so-called MIEZE technique

Assujettir des atomes ou des molécules à des impulsions lasers de fortes intensités done lieu à une variété de phénomènes hautement non-linéaires, tels que par exemple l'ionisation des électrons et la radiation de photons de hautes fréquences. Les distributions des vitesses des électrons ionisés ou des fréquences des photons radiés encodent des informations pertinentes sur les atomes ou les molécules ciblés à l'échelle temporelle naturelle des électrons, l'attoseconde-qui est un millionième, d'un millionième, d'un millionième d'une seconde. Comprendre la dynamique des électrons ionisés ainsi qu'identifier les mécanismes de radiation de hautes fréquences sont des étapes essentielles afin d'interpréter et décoder les informations cryptées dans les mesures expérimentales.Dans cette thèse, des atomes soumis à des impulsions lasers de fortes intensités polarisées elliptiquement dans le régime infra-rouge sont étudiés théoriquement. Malgré leur nature fondamentalement quantique dans les atomes, les électrons manifestent certains comportements classiques lorsqu'ils sont sujets à des impulsions lasers de fortes intensités. Nous exploitons ces traits classiques pour comprendre et illustrer, à l'aide des trajectoires, les mécanismes physiques en jeu afin d'interpréter les résultats expérimentaux. Après ioniser, le mouvement des électrons est analysé en utilisant des techniques issues de la dynamique non-linéaire. Ce travail de thèse démontre la complémentarité de la mécanique quantique et de la dynamique non-linéaire pour comprendre et illustrer des mécanismes impliqués lorsque des atomes sont sujets à des impulsions lasers de fortes intensités polarisées elliptiquement
Subjecting atoms or molecules to intense laser pulses gives rise to a variety of highly nonlinear phenomena, such as for instance the ionization of electrons and the radiation of high-frequency photons. The distributions of the velocity of the ionized electrons of the frequency of the radiated photons measured at the detector encode relevant informations on the target atoms and molecules at the natural time scale of the electrons, the attosecond-that is, million, million, millionths of a second. Understanding the dynamics of the ionized electrons and identifying the mechanisms of high-frequency radiation are essential steps toward interpreting and decoding the informations encrypted in the experimental measurements.In this thesis, atoms subjected to intense and elliptically polarized laser fields in the infrared regime are theoretically studied. Despite their fundamental quantal nature in atoms, electrons display some classical behaviors when subjected to intense laser pulses. We exploit these classical features to understand and picture, with the help of trajectories, the physical mechanisms at play in order to interpret experimental measurements. After ionizing, the motion of the electrons is analyzed using techniques from nonlinear dynamics. This thesis work demonstrates the complementarity of quantum mechanics and nonlinear dynamics for understanding and illustrating the mechanisms involved when atoms are subjected to intense and elliptically polarized laser pulses

Nous avons développé un modèle et un programme d'affinement joint des densités de charge et spin. Lors des premiers tests plusieurs difficultés sont apparues et ont été étudiées puis résolues notamment par la mise en place de contraintes. Après la mise en place d'un programme stable d'affinement joint nous avons testé celui-ci sur le complexe MnCu(pba)...(H2O)3...2H2O, ou pba représente le 1,3-propylenbis(oxamato) en réutilisant les données provenant d'une expérience de diffraction de neutrons polarisés et en effectuant une nouvelle expérience de diffraction des rayons X à 10K, température à laquelle l'expérience de diffraction des neutrons polarisés a été conduite. Cette étude a permis de tester trois schémas de pondération, ainsi que les contraintes. Ces tests ont montré que l'affinement joint permet de retrouver les résultats des différents affinements séparés mais aussi d'aller plus loin en autorisant un affinement de la densité de spin avec plus de paramètres pertinents. Suite à ces premiers tests nous nous sommes intéressés à un complexe azido cuivre (Cu2L2(N3)2 avec L=1,1,1-trifluoro-7-(dimethylamino)-4-méthyle-5-aza-3-heptène-2-onato). L'affinement joint a permis d'avoir accès, pour la première fois, à la densité de valence expérimentale résolue en spin et d'affiner également des paramètres de contraction/dilatation différents pour la valence avec un spin up ou un spin down. Dans le dernier chapitre nous avons étudié un complexe de cobalt qui présentait des propriétés magnétiques intéressantes. Cependant la particularité magnétique du composé venant d'une forte anisotropie magnétique a rendu l'étude par affinement joint délicate dans un premier temps, c'est pourquoi nous avons étudié ce composé uniquement d'un point de vue de la densité de charge. Cette étude a tout de même permis de mettre en évidence expérimentalement à 100K un angle de torsion de 39° entre les axes principaux des atomes de cobalt, prédit par la théorie
We developed a model and a refinement program for charge and spin densities. During the first tests several difficulties have arisen and have been investigated and solved by implementation of constraints. After the establishment of stable joint refinement program, we tested it on the MnCu(pba)...(H2O)3...2H2O, with pba = 1,3-propylenbis(oxamato) complex reusing data from an experiment of polarized neutron diffraction and making a new experience of X-ray diffraction at 10K. This study tested three weighting schemes and constraints. These tests showed that the joint refinement give access to the same results as the separated refinements but also allow us to go further by refining the spin density with more pertinent parameters. Following these initial tests, we were interested in a copper azido complex (Cu2L2(N3)2 with L=1,1,1-trifluoro-7-(dimethylamino)-4-methyl-5-aza-3-hepten-2-onato). The joint refinement give us access for the first time to the experimental spin-resolved valence density and also to refine the parameters of contraction / expansion for spin up or spin down separately. In the last chapter we studied a cobalt complex which shows interesting magnetic properties. However, the magnetic properties of the compound come from a high magnetic anisotropy which complicates a study by joint refinement. That is why we studied only the charge density of this compound. This study still allowed to show experimentally a torsion angle of 39° between the principal axes of the cobalt atoms, which was predicted by a previous theoretical study

Magnetic fields influence the rate and/or yield of chemical reactions that proceed via spin correlated radical pair intermediates. The field of spin chemistry centres around the study of such magnetic field effects (MFEs). This thesis is particularly concerned with the effects of the weak magnetic fields B₀ ~ 1mT relevant in the ongoing debates on the mechanism by which animals sense the geomagnetic field and on the putative health effects of environmental electromagnetic fields. Relatively few previous studies have dealt with such weak magnetic fields. This thesis presents several new theoretical tools and applies them to interpret experimental measurements. Chapter 1 surveys the development and theory of spin chemistry. Chapter 2 introduces the use of Tikhonov and Maximum Entropy Regularisation methods as a new means of analysing MARY field effect data. These are applied to recover details of the diffusive motion of reacting pyrene and N,N-dimethylaniline radicals. Chapter 3 gives a fresh derivation and appraisal of an approximate, semiclassical approach to MFEs. Monte Carlo calculations allow the elucidation of several "rules of thumb" for interpreting MFE data. Chapter 4 discusses recent optically-detected zero-field EPR measurements, adapting the gamma-COMPUTE algorithm from solid state NMR for their interpretation. Chapter 5 explores the role of RF polarisation in producing MFEs. The breakdown in weak fields of the familiar rotating frame approximation is analysed. Chapter 6 reviews current knowledge and landmark experiments in the area of animal magnetoreception. The origins of the sensitivity of European robins Erithacus rubecula to the Earth’s magnetic field are given particular attention. In Chapter 7, Schulten and Ritz’s hypothesis that avian magnetoreception is founded on a radical pair mechanism (RPM) reaction is appraised through calculations in model systems. Chapter 8 introduces quantitative methods of analysing anisotropic magnetic field effects using spherical harmonics. Chapter 9 considers recent observations that European robins may sometimes be disoriented by minuscule RF fields. These are shown to be consistent with magnetoreception via a radical pair with no (effective) magnetic nuclei in one of the radicals.

Double perovskite materials exhibit alterations in magnetic order through manipulation oftheir crystal structure. Certain ultra thin metallic bilayers can create an exotic magnetic stateof confined spin textures called skyrmions. In both cases, new atomic arrangements leadto new electrical and magnetic properties. The following work comprises two studies, bothof which examine the magnetic properties of transition metals in either powder or thin filmsamples. The first part is dedicated to a series of muon spin rotation and relaxation (muSR)experiments on a LaSrNiReO6, double perovskite, powder sample. In the muSR technique, aspin polarized muon beam is focused onto a powder envelope in low pressure and temperatureconditions. The spins of the implanted muons evolve depending on the intrinsic or externallyapplied magnetic field according to Larmor precession. The measurement is based onthe detection of decay positrons that carry this precession information on their preferreddecay directions. Measurements that were realized in wTF, ZF and LF setups, reveal asecond transition to magnetic order at Tc ≃ 22K, below a transition that was observed at T =261K from magnetic susceptibility measurements. The experimental results point to threemagnetic phases, paramagnetic for T > 261K, dilute ferrimagnetic for 22 < T < 261K and amagnetically ordered state for T < 22K, that may implicate ferro- and antiferromagnetismfrom Ni sublattices and Ni-Re interactions. The second part follows an attempt to produce and characterize ultra thin bilayer filmsfor the observation of interfacial chiral structures and skyrmions. Co/Fe/MgO (100) andW/Ni/Cu (100) bilayers were grown with magnetron sputter deposition in various layerthicknesses and their structure was determined by X-ray reflectometry (XRR). The XRRscans presented a relatively thick-layered Co/Fe/MgO film, while extremely thin and roughW/Ni/Cu bilayers, for the purposes of studying films with broken interfacial inversionsymmetry. This study was concluded with indicative magneto-transport measurements thatalso point to the reconfiguration of the growth procedure.

L'électronique moléculaire est l'un des domaines les plus intrigants de la recherche moderne. Ce domaine pourrait produire un système de construction modulaire et évolutif pour des applications spintroniques à l'échelle nanométrique. Un exemple particulièrement prometteur est celui des aimants à une seule molécule, qui se sont déjà avérés être appropriés pour des la réalisation de spin valve et de qubit de spin. L'un des plus grands défis du domaine est l'intégration de ces objets de taille nanométrique dans des circuits complexes afin de permettre la détection et la manipulation d'états de spin moléculaires. Comme l'ont montré ces dernières années le groupe NanoSpin, les nanotubes de carbone (CNTs) peuvent servir de support pour les aimants à une seule molécule, en combinant les caractéristiques des deux constituants.Une pierre angulaire de ce projet de thèse a donc été le développement d'une technique de fabrication fiable pour des dispositifs de CNTs de haute qualité, contrôlables par de multiples électrodes de grille locales afin de permettre le contrôle local des systèmes hybrides moléculaires. Un procédé basé sur la fabrication conventionnelle à un substrat a été développé à partir de zéro, pour lequel l'optimisation de la conception des échantillons, les techniques de lithographie et de dépôt ainsi que les choix de matériaux ont dû être soigneusement incorporés afin de respecter les restrictions imposées par les conditions de croissance. Nous avons d'abord réussi à produire des échantillons CNT propres, permettant de mettre en évidence une configuration à double boite quantique, tout en ajustant des caractéristiques de type p à n. Les segments créés de cette manière peuvent être contrôlés de manière stable sur toute la longueur du dispositif et devraient donc constituer une base appropriée pour l'étude de la physique moléculaire.La matière topologique non triviale constitue une plate-forme séduisante pour étudier à la fois les principes fondamentaux et les applications possibles de la spintronique au calcul quantique. Les isolants cristallins topologiques, avec tellurure d'étain (SnTe) comme exemple principal, représentent un nouvel état au sein de ce zoo des matériaux topologiques 3D. Peu de temps après les premières réalisations expérimentales, des suggestions ont été faites sur la possibilité d’un type de supraconductivité non conventionnelle hébergé à l'interface entre la matière topologique et les supraconducteurs classiques. Les implications possibles de ces systèmes comprennent l'appariement de Cooper avec une quantité de mouvement finie dans la phase FFLO ou l’ordinateur quantique topologique, basé sur des excitations particulières, appelé quasi-particule Majorana.Ce projet de thèse visait à participer à l'enquête sur les signes de supraconductivité non conventionnelle dans SnTe. Les expériences de transport sur des couches pures dans les géométries de la barre de Hall et des dispositifs hybrides supraconducteurs, réalisés à la fois comme jonctions Josephson et SQUID, sont discutés. Un couplage étonnamment fort de SnTe au supraconducteur a été trouvé et dépendances de la supraconductivité sur les géométries des échantillons, la température et le champ magnétique ont été étudiées. La relation courant-phase a été analysée dans la limite d’effets cinétiques forts. Le couplage électrostatique et l'exposition à des micro-ondes ont été explorée, mais la physique prédominante dans de telles configurations s'est avéré être de type purement conventionnel, soulignant l’importance des améliorations sur le côté matériaux.Des mesures de champ magnétique dans le plan ont donné lieu à la signature d’un φ0-SQUID avec des transitions 0-π accordables, fournissant des preuves de possibles de transitions contrôlées de la supraconductivité triviale aux régimes de couplage non conventionnels dans SnTe
Molecular electronics is one of the most intriguing fields of modern research, which could bring forth a modular and scalable building system for nanoscale spintronics applications. A particularly promising example are single-molecule magnets, which have already successfully shown to be suitable for spin valve or spin qubit operations. One of the biggest challenges of the field is the integration of these nanometer-sized objects in complex circuits in order to allow for detection and manipulation of moleculear spin states. As shown in recent years by the NanoSpin group, carbon nanotubes (CNTs) can serve as such type of carrier for the single-molecule magnets, combining features of both constituents.A corner stone of this thesis project was hence the development of a dependable fabrication technique for high-quality CNT devices, controllable by multiple local gate electrodes in order to enable local control of molecular hybrid systems. A process based on conventional one-chip fabrication was developed from scratch, for which optimization of sample design, lithography and deposition techniques as well as material choices had to be carefully incorporated, in order to accomodate the restrictions imposed by the CNT growth conditions on the prevention of leakage currents. We succeeded in producing clean CNT devices, which could support a double dot configuration, tunable from p- to n-type characteristics. The segments created in this way can be stabily controlled over the entire device length and should hence provide a suitable backbone to study molecular physics.Topological matter constitutes an enticing platform to investigate both fundamental principles as well as possible applications from spintronics to quantum computation. Topological crystalline insulators, with tin telluride ( SnTe ) as a prime example, represent a new state of matter within this zoo of 3D topological materials. Soon after first experimental realizations, suggestions were made about the possibility of an unconventional type of superconductivity hosted at the interface between topological matter and conventional superconductors. Possible implications of such systems include Cooper pairing with finite momentum, the FFLO phase, or topological quantum computing, based on peculiar excitations, called Majorana bound states.This thesis project aimed to participate in the investigation of signs of unconventional superconductivity in SnTe . Transport experiments on bare films in Hall bar geometries and superconducting hybrid devices, realized as both Josephson junctions and SQUIDs, are discussed. A surprisingly strong coupling of SnTe to Ta superconductor was found and dependencies of superconductivity on sample geometries, temperature and magnetic field were investigated. The current-phase relation was analyzed in the limit of strong kinetic effects. Electrostatic gating and rf exposure was explored, but predominant physics in such configurations turned out to be of purely conventional type, pointing out the importance of improvements on the material side.In-plane magnetic field measurements gave rise to the manifestation of ϕ0-SQUIDs with tunable 0−π-transitions, providing evidence for possible controlled transitions from trivial superconductivity to unconventional coupling regimes in SnTe

A system with two magnetic molecules embedded in a junction between non-magnetic leads was studied. In this system electrons tunnel from the localized energy level in region one to the localized energy level in region two generating a flow of electric charge through the quantum dot system. The current density and thus the conductance changes depending on the molecular spin moment. In this work we studied molecules with either spin "up" or spin "down" and with symmetric coupling strengths. The results indicate that the coupling strength between energy level and molecule together with the tunneling rate through the insulating layer play a major role when switching from parallel to anti-parallel molecular spin, for a specific combination of the coupling strength and tunneling rate we could observe a decrease in the current by 99.7% in the non-gated system and 99.4% in the gated system.

We carried out experiments on a spin polarized 3He- 4He mixture with 3He concentration x 3 = 6.26 × 10−4, and on pure 3He liquid. Spin polarization affects the transport properties of these Fermi systems. The effect on momentum transport was studied by using a vibrating-wire viscometer to measure viscosity of the 3He-4He mixture over the temperature range 6.09 mK–100 mK in 7.96 T and 1.00 T magnetic fields. A large viscosity increase was observed upon application of the 7.96 T magnetic field for temperature T < TF(TF = 19.5 mK is the Fermi temperature). The observed viscosity is in very good agreement with theoretical calculations for a dilute Fermi gas by Jeon and Mullin [1988, 1989] and Mullin and Jeon [1992]. The polarization effect on spin transport was investigated by measuring the transverse diffusion coefficient D ⊥ in pure 3He liquid at saturated vapor pressure and at 15.85 bar over the temperature range 4.5 mK–159 mK in a 7.96 T magnetic field. We used a pulsed NMR spin echo technique in a field gradient of 16.0 G/cm to do the measurements and fits to the Leggett equations [1970] to obtain D⊥. For T < 20 mK, we found D⊥ is less than measured in earlier experiments at lower magnetic fields. D⊥ does not increase with decreasing temperature as 1/T2, but appears to approach a constant as T → 0 while it is expected that the longitudinal spin diffusion coefficient D∥ ∝ 1/ T2. This is called spin diffusion anisotropy and it was observed for the first time in our 3He liquid experiments. The anisotropy temperature we determined for 3He liquid was Ta = 16.4 ± 2.2 mK at saturated vapor pressure and in a 7.96 T magnetic field. The transverse spin diffusion in 3 He results agree qualitatively with theories proposed by Meyerovich and Musaeflan [1992, 1994]. They also agree qualitatively with theories proposed by Golosov and Ruckenstein [1995, 1998] by extrapolation of the dilute theory to a strongly interacting system.

In this work the electronic and magnetic structure of the crystals Sr2FeMoO6, Fe0.5Cu0.5Cr2S4, LuFe2O4 and the molecules FeStar, Mo72Fe30, W72Fe30 are investigated by means of X-ray spectroscopic techniques. These advanced materials exhibit very interesting properties like magnetoresistance or multiferroic behaviour. In case of the molecules they also could be used as spin model systems. A long standing issue concerning the investigation of these materials are contradicting results found for the magnetic and electronic state of the iron (Fe) ions present in these compounds. Therefore this work focuses on the Fe state of these materials in order to elucidate reasons for these problems. Thereby the experimental results are compared to multiplet simulations.

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2009.
Title from title screen (site viewed July 6, 2010). PDF text: x, 133 p. : ill. (some col.) ; 3 Mb. UMI publication number: AAT 3366064 . Includes bibliographical references. Also available in microfilm and microfiche formats.

At present, the possibility to employ Single Molecule Magnets (SMMs) properties in real technological devices is still a challenge. In order to reach this target a significant extension of their spin life-time and a rational tuning of their properties on demand is mandatory. In this Ph.D. thesis a comprehensive theoretical and computational assessment of the main open questions related to the microscopic quantum origins of SMMs properties has been done together with the development of an ab initio protocol based on both DFT and post HF schemes able to describe SMMs electronic structure in any sort of chemical environment. The multi-spin origin of the ground state in polynuclear SMMs has been discussed and the several involved spin terms assessed. A fundamental extension of spin relaxation theory has been developed in order to account for the real complexity of the spin environment made by phonons and other SMMs spins. Spin relaxation phenomena have been addressed tracking down the origin of the spin-flip effective barrier reduction, observed experimentally and never interpreted before. The determination of the main contribution of molecular internal degrees of freedom at the origin of spin relaxation has also been pointed out for the first time, paving the ground for a rational design of molecular structures to extend relaxation time-scales. Finally, the issues related to the conservation of SMMs properties once adsorbed on a metallic substrate have also been addressed. A 2combination of different computational schemes made possible to highlight the striking importance of electronic and structural rearrangements of SMMs once deposited. This effect, largely underestimated in literature, has been observed both for what concerns substrate/SMM and SMM/SMM interactions, demonstrating, for instance, the possibility to modulate the molecular orientation playing with the SMMs organic scaffold degrees of freedom.

In this work, I present nuclear probe spectroscopy studies, in detail, µ+SR and 57Fe-Mössbauer spectroscopy on solid-state systems with localized magnetic moments of 3d transition-metal ions supported by density functional theory calculations. Local probes are able to extract local quantities, e.g. the spin dynamics of the 57Fe site or the local, mostly interstitial µ+ site to distinguish between di_erent magnetic phases. The density functional theory calculations help to identify the muon site position from which the local quantity depends. My µ+SR studies on frustrated 3d magnets with quenched disorder concern the physics of phase transitions, avoided order-by-disorder, quantum uctuations or the appearance of spin-liquid-by-disorder. µ+SR is able to identify quantum spinliquid-like ground states without symmetry breaking or static magnetic order by the magnetic field at the muon site. BaTi0.5Mn0.5O3 is a magnetically highly-frustrated double perovskite with quenched disorder.It shows no freezing temperature or no frequency dependence of x1as expected for a spin glass. Microscopically, it is proposed that local interactions between magnetic orphan spins, dimers, and magnetic trimers of Mn4+ play an important role. The µ+SR experiment on BaTi0.5Mn0.5O3 shows an increase of the dynamical muon spin relaxation rate below 3 K which saturates down to 0.019 K coexisting with residual short-range magnetic order (<20% of the signal). A clear difference is observed in comparison with the classical cluster-spin glass SrTi0.5Mn0.5O3 which shows a peak of the zero-field muon spin relaxation rate: a persistent low-energy spin dynamics is present in BaTi0.5Mn0.5O3 down to 20 K. My DFT calculations propose a positive muon site insight the Ba plane close to O atoms. Here, a slight preference of the muon site close to Mn4+ is possible which could put the muon close the orphan spins, dimers, and magnetic trimers, respectively, avoiding the nonmagnetic Ti4+ face-sharing octahedra. Theoretically, a specific ground state of BaTi0.5Mn0.5O3 is not proposed. A clear discrimination between a quantum spin liquid ground state and a mimicry state with the appearance of spin-liquid-by-disorder is not possible from the existing data. I present a µ+SR study on the bond-disordered magnetically highly frustrated pyrochlore fluoride NaCaCo2F7. Neutron spectroscopy studies on NaCaCo2F7 revealed static short-range order consistent with a continuous manifold of cluster-like states being a superposition of noncoplanar ψ2(m3z2-r2) and coplanar ψ3(mx2-y2) states with a correlation length of around 16Å. No evidence for static magnetic long-range order is found in NaCaCo2F7 probed by µ+SR confirming the absence of an order-by-disorder mechanism. The experimental results are not consistent with a classical local-planar XY cluster-spin glassiness. In these µSR experiments, two muon sites are observed. The relative occupancy of both muon sites is nearly temperature independent. Muon site I is a collinear diamagnetic F-µ+-F bound state pulling two F- close towards the muon revealed by the muon spin time evolution. To investigate the pure F-µ+-F bound state in a broad temperature range I have performed an additional µ+SR study on CaF2. This study solved open questions of muon diffusion around 290 K which was observed in NaCaCo2F7 as well. The F-µ+-F spin relaxation indicates the slowing down of the magnetic Co2+ spin fluctuations upon cooling towards the NMR spin freezing temperature Tf≈ 2.4 K. The relaxation rate saturates below 800 mK and remains constant down to 20 mK. The dominant part of the magnetic short-range relaxation signal is a dynamical relaxation as probed by longitudinal magnetic-field experiments. Muon site II exhibits a strong dynamical relaxation rate at 290 K and below and shows persistent µ+ spin dynamics down to 20 mK. Qualitatively, muon site II shows persistent µ+ spin dynamics with one order of magnitude higher dynamical relaxation rates compared to muon site I. DFT calculations of a comparison of the unperturbed unit cells of NaCaCo2F7 and NaCaNi2F7, which has shown just one muon site experimentally, are consistent with a decrease of the energy differences of energy minima and support the experimentally observed muon site ambivalence. In summary, the µ+SR studies propose NaCaCo2F7 as a quantum cluster-spin glass candidate. I present a systematic 57Fe-Mössbauer study on highly diluted Fe centers in Li2(Li1-xFex)N as a function of temperature and magnetic field applied transverse and longitudinal with respect to the single-ion anisotropy axis. Here, Fe is embedded in an α-Li3N matrix. The oxidation state of Fe and possible ferromagnetic nature are in controversial discussions in the literature. Below 30 K the Fe centers exhibit a giant magnetic hyperfine field of BA=70.25(2) T parallel to the axis of strongest electric field gradient Vzz=-154.0(1) V / Å 2. This observation is consistent with a Fe1+d7 charge state with unquenched orbital moment and J=7/2. Fluctuations of the magnetic hyperfine field are observed between 50 K and 300 K and described by the Blume two-level relaxation model consistent with single-atomic magnetism as proven by the invariance of Blume relaxation parameters for the concentration tuning x< 0.025 excluding a ferromagnetic nature. From the temperature dependence of the fluctuation rate an Orbach spin-lattice relaxation process is deduced. An Arrhenius analysis yields a single thermal-activation barrier of EA=570(6) K and an attempt frequency v0=309(10) GHz. Mössbauer spectroscopy studies with applied transverse magnetic fields up to 5 T reveal a large increase of the fluctuation rate by two orders of magnitude. In longitudinal magnetic fields a splitting of the fluctuation rate into two branches is observed. The experimental observations are qualitatively reproduced by a single-ion spin Hamiltonian analysis. It demonstrates that for dominant magnetic quantum tunneling relaxation processes a weak axial single-ion anisotropy D of the order of a few Kelvin can cause a two orders of magnitude larger energy barrier for longitudinal spin fluctuations.

This thesis consists of six chapters. The first chapter gives an introduction to the field of low-dimensional magnetic and electronic systems and relevant numerical techniques. The recent developments in molecular magnets are highlighted. The numerical techniques are reviewed along with their advantages and disadvantages from the present perspective. Study of entanglement of a system can give a great insight into the system. At the last part of this chapter a general overview is given regarding entanglement, its measures and its significance in studying many-body systems. Chapter 2 deals with the technique that has been developed by us for the full symmetry adaptation of non-relativistic Hamiltonians. It is advantageous both computationally and physically/chemically to exploit both spin and spatial symmetries of a system. It has been a long-standing problem to target a state which has definite total spin and also belongs to a definite irreducible representation of a point group, particularly for non-Abelian point groups. A very general technique is discussed in this chapter which is a hybrid method based on valence-bond basis and the basis of the z-component of the total spin. This technique is not only applicable to a system with arbitrary site spins and belonging to any point group symmetry, it is also quite easy to implement computationally. To demonstrate the power of the method, it is applied to the molecular magnetic system, Cu6Fe8, with cubic symmetry. In chapter 3, the extension of the previous hybrid technique to electronic systems is discussed. The power of the method is illustrated by applying it to a model icosahedral half-filled electronic system. This model spans a huge Hilbert space (dimension 1,778,966) and is in the largest non-Abelian point group. All the eigenstates of the model are obtained using our technique. Chapter 4 deals with the thermodynamic properties of an important class of single-chain magnets (SCMs). This class of SCMs has alternate isotropic spin-1/2 units and anisotropic high spin units with the anisotropy axes being non-collinear. Here anisotropy is assumed to be large and negative, as a result, anisotropic units behave like canted spins at low temperatures; but even then simple Ising-type model does not capture the essential physics of the system due to quantum mechanical nature of the isotropic units. A transfer matrix (TM) method is developed to study statistical behavior of this class of SCMs. For the first time, it is also discussed in detail that how weak inter-chain interactions can be treated by a TM method. The finite size effect is also discussed which becomes important for low temperature dynamics. This technique is applied to a real helical chain magnet, which has been studied experimentally. In the fifth chapter a bipartite entanglement entropy of finite systems is studied using exact diagonalization techniques to examine how the entanglement changes in the presence of long-range interactions. The PariserParrPople model with long-range interactions is used for this purpose and corresponding results are com-pared with those for the Hubbard and Heisenberg models with short-range interactions. This study helps understand why the density matrix renormalization group (DMRG) technique is so successful even in the presence of long-range interactions in the PPP model. It is also investigated if the symmetry properties of a state vector have any significance in relation to its entanglement. Finally, an interesting observation is made on the entanglement profiles of different states, across the full energy spectrum, in comparison with the corresponding profile of the density of states. The entanglement can be localized between two noncomplementary parts of a many-body system by performing local measurements on the rest of the system. This localized entanglement (LE) depends on the chosen basis set of measurement (BSM). In this chapter six, an optimality condition for the LE is derived, which would be helpful in finding optimal values of the LE, besides, can also be of use in studying mixed states of a general bipartite system. A canonical way of localizing entanglement is further discussed, where the BSM is not chosen arbitrarily, rather, is fully determined by the properties of a system. The LE obtained in this way, called the localized entanglement by canonical measurement (LECM), is not only easy to calculate practically, it provides a nice way to define the entanglement length. For spin-1/2 systems, the LECM is shown to be optimal in some important cases. At the end of this chapter, some numerical results are presented for j1 −j2 spin model to demonstrate how the LECM behaves.

This thesis consists of six chapters. The first chapter gives an introduction to the field of low-dimensional magnetic and electronic systems and relevant numerical techniques. The recent developments in molecular magnets are highlighted. The numerical techniques are reviewed along with their advantages and disadvantages from the present perspective. Study of entanglement of a system can give a great insight into the system. At the last part of this chapter a general overview is given regarding entanglement, its measures and its significance in studying many-body systems. Chapter 2 deals with the technique that has been developed by us for the full symmetry adaptation of non-relativistic Hamiltonians. It is advantageous both computationally and physically/chemically to exploit both spin and spatial symmetries of a system. It has been a long-standing problem to target a state which has definite total spin and also belongs to a definite irreducible representation of a point group, particularly for non-Abelian point groups. A very general technique is discussed in this chapter which is a hybrid method based on valence-bond basis and the basis of the z-component of the total spin. This technique is not only applicable to a system with arbitrary site spins and belonging to any point group symmetry, it is also quite easy to implement computationally. To demonstrate the power of the method, it is applied to the molecular magnetic system, Cu6Fe8, with cubic symmetry. In chapter 3, the extension of the previous hybrid technique to electronic systems is discussed. The power of the method is illustrated by applying it to a model icosahedral half-filled electronic system. This model spans a huge Hilbert space (dimension 1,778,966) and is in the largest non-Abelian point group. All the eigenstates of the model are obtained using our technique. Chapter 4 deals with the thermodynamic properties of an important class of single-chain magnets (SCMs). This class of SCMs has alternate isotropic spin-1/2 units and anisotropic high spin units with the anisotropy axes being non-collinear. Here anisotropy is assumed to be large and negative, as a result, anisotropic units behave like canted spins at low temperatures; but even then simple Ising-type model does not capture the essential physics of the system due to quantum mechanical nature of the isotropic units. A transfer matrix (TM) method is developed to study statistical behavior of this class of SCMs. For the first time, it is also discussed in detail that how weak inter-chain interactions can be treated by a TM method. The finite size effect is also discussed which becomes important for low temperature dynamics. This technique is applied to a real helical chain magnet, which has been studied experimentally. In the fifth chapter a bipartite entanglement entropy of finite systems is studied using exact diagonalization techniques to examine how the entanglement changes in the presence of long-range interactions. The PariserParrPople model with long-range interactions is used for this purpose and corresponding results are com-pared with those for the Hubbard and Heisenberg models with short-range interactions. This study helps understand why the density matrix renormalization group (DMRG) technique is so successful even in the presence of long-range interactions in the PPP model. It is also investigated if the symmetry properties of a state vector have any significance in relation to its entanglement. Finally, an interesting observation is made on the entanglement profiles of different states, across the full energy spectrum, in comparison with the corresponding profile of the density of states. The entanglement can be localized between two noncomplementary parts of a many-body system by performing local measurements on the rest of the system. This localized entanglement (LE) depends on the chosen basis set of measurement (BSM). In this chapter six, an optimality condition for the LE is derived, which would be helpful in finding optimal values of the LE, besides, can also be of use in studying mixed states of a general bipartite system. A canonical way of localizing entanglement is further discussed, where the BSM is not chosen arbitrarily, rather, is fully determined by the properties of a system. The LE obtained in this way, called the localized entanglement by canonical measurement (LECM), is not only easy to calculate practically, it provides a nice way to define the entanglement length. For spin-1/2 systems, the LECM is shown to be optimal in some important cases. At the end of this chapter, some numerical results are presented for j1 −j2 spin model to demonstrate how the LECM behaves.

MnSi(111) films were grown on Si(111) substrates by solid phase epitaxy (SPE) and molecular beam epitaxy (MBE) to determine their magnetic structures. A lattice mismatch of -3.1% causes an in-plane tensile strain in the film, which is partially relaxed by misfit dislocations. A correlation between the thickness dependence of the Curie temperature (TC) and strain is hypothesized to be due to the presence of interstitial defects. The in-plane tensile strain leads to an increase in the unit cell volume that results in an increased TC as large as TC = 45 K compared to TC = 29.5 K for bulk MnSi crystals. The epitaxially induced tensile stress in the MnSi thin films creates an easy-plane uniaxial anisotropy. The magnetoelastic coefficient was obtained from superconducting quantum interference device (SQUID) magnetometry measurements combined with transmission electron microscopy (TEM) and x-ray diffraction (XRD) data. The experimental value agrees with the coefficient determined from density functional calculations, which supports the conclusion that the uniaxial anisotropy originates from the magnetoelastic coupling. Interfacial roughness obscured the magnetic structure of the SPE films, which motivated the search for a better method of film growth. MBE grown films displayed much lower interfacial roughness that enabled a determination of the magnetic structure using SQUID and polarized neutron reflectometry (PNR). Out-of-plane magnetic field measurements on MBE grown MnSi(111) thin films on Si(111) substrates show the formation of a helical conical phase with a wavelength of 2?/Q = 13.9 ± 0.1 nm. The presence of both left-handed and right-handed magnetic chiralities is found to be due to the existence of inversion domains that result from the non-centrosymmetric crystal structure of MnSi. The magnetic frustration created at the domain boundaries explains an observed glassy behaviour in the magnetic response of the films. PNR and SQUID measurements of MnSi thin films performed in an in-plane magnetic field show a complex magnetic behaviour. Experimental results combined with theoretical results obtained from a Dzyaloshinskii model with an added easy-plane uniaxial anisotropy reveals the existence of numerous magnetic modulated states that do not exist in bulk MnSi. It is demonstrated in this thesis that modulated chiral magnetic states can be investigated with epitaxially grown MnSi(111) thin films on insulating Si substrates, which offers opportunities to investigate spin-dependent transport in chiral magnetic heterostructures based on this system.