The ultimate goal in the rapidly burgeoning field of spintronics is to realize semiconductor-based devices that utilize the spin degree of freedom of a single charge carrier (electron or hole) or an ensemble of such carriers to achieve novel and/or enhanced device functionalities such as spin based light emitting devices, spin transistors and femto-Tesla magnetic field sensors. These devices share a common feature: they all rely on controlled transport of spins in semiconductors. A prototypical spintronic device has a transistor-like configuration in which a semiconducting channel is sandwiched between two contacts (source and drain) with a gate electrode sitting on top of the channel. Unlike conventional charge-based transistors, the source electrode of a spin transistor is a ferromagnetic (or half-metallic) material which injects spin polarized electrons in the channel. During transit, the spin polarizations of the electrons are controllably rotated by a gate electric field mediated spin-orbit coupling effect. The drain contact is ferromagnetic (or half-metallic) as well and the transmission probability of an electron through this drain electrode depends on the relative orientation of electron spin polarization and the (fixed) magnetization of the drain. When the spins of the electrons are parallel to the drain magnetization, they are transmitted by the drain resulting in a large device current (ON state of spin FET). However, these electrons will be completely blocked if their spins are antiparallel to the drain magnetization, and ideally, in this situation device current will be zero (OFF state of spinFET). Thus, if we vary the gate voltage, we can modulate the channel current by controlling the spin orientations of the electrons with respect to the drain magnetization. This is how transistor action is realized (Datta-Das model). However, during transport, electrons' velocities change randomly with time due to scattering and hence different electrons experience different spin-orbit magnetic fields. As a result, even though all electrons start their journey with identical spin orientations, soon after injection spins of different electrons point along different directions in space. This randomization of initial spin polarization is referred to as spin relaxation and this is detrimental to the spintronic devices. In particular, for Datta-Das transistor, this will lead to inefficient gate control and large leakage current in the OFF state of the spinFET. The aim of this work is to understand various spin relaxation processes that are operative in semiconductor nanostructures and to indicate possible ways of minimizing them. The theoretical aspect of this work (Chapters 2-5) focuses on the D'yakonov-Perel' process of spin relaxation in a semiconductor quantum wire. This process of spin relaxation occurs because during transport electron spin precesses like a spinning top about the spin-orbit magnetic field. We show that the conventional drift-diffusion model of spin transport, which has been used extensively in literature, completely breaks down in case of a quantum confined system (e.g. a quantum wire). Our approach employs a semi-classical model which couples the spin density matrix evolution with the Boltzmann transport equation. Using this model we have thoroughly studied spin relaxation in a semiconductor quantum wire and identified several inconsistencies of the drift-diffusion formalism.The experimental side of this work (Chapters 6-8) deals with two different issues: (a) performing spin transport experiments in order to extract spin relaxation length and time in various materials (e.g. Cu, Alq3) under one-dimensional confinement, and (b) measurement of the ensemble spin dephasing time in self-assembled cadmium sulfide quantum dots using electron spin resonance technique. The spin transport experiment, as described in Chapter 7 of this dissertation, shows that the spin relaxation time in organic semiconductor (Alq3) is extremely long, approaching a few seconds at low temperatures. Alq3 is the chemical formula of tris- 8 hydroxy-quinoline aluminum, which is a small molecular weight organic semiconductor. This material is extensively used in organic display industry as the electron transport and emission layer in green organic light emitting diodes. The long spin relaxation time in Alq3 makes it an ideal platform for spintronics. This also indicates that it may be possible to realize spin based organic light emitting diodes which will have much higher internal quantum efficiency than their conventional non-spin counterparts. From spin transport experiments mentioned above we have also identified Elliott-Yafet mode as the dominant spin relaxation mechanism operative in organic semiconductors. Electron spin resonance experiment performed on self-assembled quantum dots (Chapter 8) allows us to determine the ensemble spin dephasing time (or transverse spin relaxation time) of electrons confined in these systems. In quantum dots electrons are strongly localized in space. Surprisingly, the ensemble spin dephasing time shows an increasing trend as we increase temperature. The most likely explanation for this phenomenon is that spin dephasing in quantum dots (unlike quantum wells and wires) is dominated by nuclear hyperfine interaction, which weakens progressively with temperature. We hope that our work, which elaborates on all of the above mentioned topics in great detail, will be a significant contribution towards the current state of knowledge of subtle spin-based issues operative in nanoscale device structures, and will ultimately lead to realization of novel nano-spintronic devices.

Neste trabalho exploramos propriedades físicas dos cristais líquidos liotrópicos na sua fase lamelar e dentro desse utilizamos um sistema de spins quadrupolares para a criação e manipulação de estados coerentes de spin nuclear com técnicas de RMN. Os spins nucleares utilizados eram provenientes do núcleo de césio-133, com spin 7/2, presentes em uma molécula de pentadecafluoroctanoato de césio com estrutura líquido-cristalina. Sobre esse núcleo, aplicamos um novo conceito de pulsos fortemente modulados suaves para gerar os estados pseudo-puros correspondentes aos estados coerentes de spin nuclear. Com esses estados pudemos realizar experimentos de compressão de estado coerente, um conceito quântico muito importante quando vinculado ao conceito de emaranhamento. Outro estudo foi a observação de dinâmica clássica e efeitos de bifurcação nesse sistema quântico. Em ambas aplicações se destaca o controle dos spins nucleares no desenvolvimento dos protocolos tanto na implementação do conceito de estado coerente em sistemas de spin nuclear, quanto nas leituras dos estados quânticos via tomografia de estado quântico. / In this work we use a quadrupolar spin system inside a lyotropic liquid crystal in the lamellar phase and explore its physical properties to create and manipulate nuclear spin coherent states with NMR techniques. The nuclear spins come from the cesium-133 nucleus, spin 7/2, contained in the cesium-pentadecafluoroctanoate with liquid crystalline structure. On this nucleus, we apply a new concept of smooth strongly modulating pulses to create the pseudo-pure states corresponding to nuclear spin coherent states. With these coherent states we were able to perform coherent state squeezing, an important concept closely related to entanglement. In another study we observed the classical dynamics and bifurcation on this quantum system. Both applications highlight the quantum control of the nuclear spins in developing the protocols for the creation of nuclear spin coherent states as well as for performing the readout using the quantum state tomography procedure.

<p>This thesis presents quantum chemical calculations, applications of the response function formalism recently implemented within the framework of density functional theory (DFT) by our research group. The purpose of the calculations is to assess the performance of this perturbative approach to determining heavy atom effects on magnetic resonance parameters. Relativistic corrections can be generated by spin-orbit interactions or by scalar relativistic effects due to high velocity electrons in the atomic core region of heavy atoms. In this work, the evaluation of nuclear magnetic resonance (NMR) parameters is considered, the nuclear shielding tensor and the indirect nuclear spin-spin coupling tensor. For series of homologous compounds, it is found that both types of corrections to these parameters are increasing in size upon substitution of a constituent atom by a heavier element, but that their relative importance is system dependent. The obtained results are compatible with the ones provided by electron correlated <em>ab initio</em> methods, and a qualitative agreement with experimentally determined parameters is overall achieved. The methodology presented in this thesis aims to be a practical approach which can be applied in the study of molecular properties of large systems.</p><p>This thesis also addresses the calculation of hyperfine coupling constants, and evaluates a novel approach to the treatment of spin-polarization in spin restricted calculations without the spin contamination associated with spin unrestricted calculations.</p>

Abstract This thesis reports simulations of direct observables in electron and nuclear spin relaxation experiments in an example paramagnetic system, as well as polarization transfer occurring in a spin-exchange optical pumping (SEOP) experiment. Studies of paramagnetic relaxation are important, e.g., in the development of agents used for enhanced contrast in magnetic resonance imaging. SEOP is used to produce hyperpolarized noble gases, which are then used to, e.g., enhance sensitivity in structural studies of matter with nuclear magnetic resonance. Presently the theory, available software and hardware for such computational modeling have reached a state in which quantitative reproduction of the experimentally observed magnetization decay is possible from first principles. The present multiscale computations are carried out from first principles combining molecular dynamics simulations of atomistic motion and quantum-chemical electronic structure calculations of the spin interaction parameters that enter the effective spin Hamiltonian. A time series of the spin Hamiltonian is then explicitly used to propagate spin dynamics in the system, and dynamical time constants of the magnetization are obtained through ensemble averaging. The complete decay of electron spin magnetization could be followed directly within the duration of the simulation, whereas the nuclear spin relaxation rates were extracted using Kubo’s theory regarding generalized cumulant expansion and stochastic processes. The extracted electron and nuclear spin relaxation rates for the chosen prototypic system, the aqueous solution of Ni²⁺, are in quantitative and semi-quantitative agreement, respectively, with the available experimental results. The simulations of polarization transfer corroborate the empirical observations on the importance of van der Waals complexes and binary collisions in the spin-exchange process. Long van der Waals complexes represent the overwhelmingly most significant kind of individual events, but the short binary collisions can also give a relatively important contribution due to their vast abundance. This thesis represents a first study in which first principles-calculated trajectories of individual events could be followed. The simulations reported in this thesis were run without any empirical parametrization and thus represent a significant step in first-principles computational modeling of magnetization dynamics.

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

Neste trabalho exploramos propriedades físicas dos cristais líquidos liotrópicos na sua fase lamelar e dentro desse utilizamos um sistema de spins quadrupolares para a criação e manipulação de estados coerentes de spin nuclear com técnicas de RMN. Os spins nucleares utilizados eram provenientes do núcleo de césio-133, com spin 7/2, presentes em uma molécula de pentadecafluoroctanoato de césio com estrutura líquido-cristalina. Sobre esse núcleo, aplicamos um novo conceito de pulsos fortemente modulados suaves para gerar os estados pseudo-puros correspondentes aos estados coerentes de spin nuclear. Com esses estados pudemos realizar experimentos de compressão de estado coerente, um conceito quântico muito importante quando vinculado ao conceito de emaranhamento. Outro estudo foi a observação de dinâmica clássica e efeitos de bifurcação nesse sistema quântico. Em ambas aplicações se destaca o controle dos spins nucleares no desenvolvimento dos protocolos tanto na implementação do conceito de estado coerente em sistemas de spin nuclear, quanto nas leituras dos estados quânticos via tomografia de estado quântico. / In this work we use a quadrupolar spin system inside a lyotropic liquid crystal in the lamellar phase and explore its physical properties to create and manipulate nuclear spin coherent states with NMR techniques. The nuclear spins come from the cesium-133 nucleus, spin 7/2, contained in the cesium-pentadecafluoroctanoate with liquid crystalline structure. On this nucleus, we apply a new concept of smooth strongly modulating pulses to create the pseudo-pure states corresponding to nuclear spin coherent states. With these coherent states we were able to perform coherent state squeezing, an important concept closely related to entanglement. In another study we observed the classical dynamics and bifurcation on this quantum system. Both applications highlight the quantum control of the nuclear spins in developing the protocols for the creation of nuclear spin coherent states as well as for performing the readout using the quantum state tomography procedure.

Abstract In this thesis, the magnetic resonance and magneto-optical rotation parameters are studied in single-layer carbon systems of two different dimensionalities. Based on electronic structure calculations, the spectral parameters are predicted for both extended (2D) and finite, molecular (0D) systems consisting of pure sp²-hybridised pristine graphene (G), as well as hydrogenated and fluorinated, sp³-hybridised graphene derivatives, graphane (HG) and fluorographene (FG), respectively. Nuclear magnetic resonance (NMR) parameters are calculated for G, HG and FG systems at their large-system limit. For their 0D counterparts, graphene flakes, qualitative spectral trends are predicted as functions of their size and perimeter type. The last group of studied carbon systems consists of 2D graphenes containing spin-1/2 paramagnetic defects. Electron spin resonance (ESR) parameters and paramagnetic NMR shieldings are predicted for four different paramagnetic systems, including the vacancy-defected graphane and fluorographene, as well as graphene with hydrogen and fluorine adatoms. The magneto-optic properties of G and HG flakes are studied in terms of Faraday optical rotation and nuclear spin optical rotation parameters, to investigate the effects of their finite size and also the different level of hydrogenation. All the different investigated parameters displayed characteristic sensitivity to the electronic and atomic structure of the studied graphenes. The parameters obtained provide an insight into the physics of these 0D and 2D carbon materials, and encourage experimental verification.

We studied how distribution of nuclear-spin states of H+ 3 ions or H2 molecules influence rate coefficients of H+ 3 -electron recombination or reactions of H2 with N+ or H+ , with regard to kinetic and internal temperatures of the reactants. Experiments were carried out in plasma environment or in an ensemble of ions in an ion trap. Main diagnostic methods were the Langmuir probe diagnos- tics, laser absorption- and mass spectroscopy. The distribution of nuclear spin states (para and ortho) was varied using a specially constructed para-hydrogen generator. We performed pioneer measurements of the rate coefficients for the nuclear-spin-state-selective binary and ternary H+ 3 -electron recombination in thermalised plasma. We performed studies of N+ + para/ortho-H2 reaction with high accuracy and interpreted the results as dependent on fine structure states of N+ ions. We measured a temperature dependence of the rate coeffi- cients for radiative and ternary channels of H+ + para/ortho-H2 association. 1

The results of the laboratory study of reaction rate coefficients of several ion- molecule reactions with atomic and molecular hydrogen and molecular deuterium at low temperatures are presented in the thesis. The reaction rate coefficients of the N+ and H+ reaction with H2 were measured with respect to the nuclear spin configuration and rotational excitation of H2. The reactions of anions were a subject of the isotope exchange and isotope effect study. The measurements of the rate coefficients of H2O and D2O formation in the reaction of O- with H2 and D2, isotope exchange reactions OH- + D2 and OD- + H2, and associative detachment and charge transfer channels of D- + H interaction were performed. Experiments were carried out using an AB-22PT instrument with an ion trap. It has producing, guiding, trapping, and detecting systems for ions and a separate source of atomic H. The cooling system allowed to measure the temperature dependencies of the reaction rate coefficients at temperatures relevant to astrochemistry (10 K - 300 K)

Title: Magnetic circular dichroism and aromatic compounds Author: Petr Štěpánek Department/Institute: Institute of Organic Chemistry and Biochemistry AS CR, v.v.i. Supervisor: prof. RNDr. Petr Bouř, DSc., Institute of Organic Chemistry and Biochemistry AS CR, v.v.i. Abstract: The thesis presents a series of studies concerning magnetic circular dichroism (MCD), a spectroscopic method, which experienced an intense theo- retical development in the recent years. New computational codes opened possi- bilities to calculate MCD spectra of larger and more varied molecules than was possible in the past. In the presented studies, we took the advantage of the new computational codes to broaden the possible span of applications of the MCD technique. As an example, we present MCD as a method useful for obtaining information about the structure of fullerenes. We also studied the influence of the molecular conformation and the explicit and implicit solvent models on the MCD spectra of aromatic amino acids using the newly implemented alterna- tive computational protocol based on sum-over-states calculations. We have also theoretically predicted spectra of the nuclear spin circular dichroism (NSCD), a potential new high-resolution spectroscopy. Keywords: magnetic circular dichroism, quantum-chemical calculations, density...