The design and synthesis of stable organic radicals with delocalized spin density distribution and low energy optical and redox processes is central to the development of magneto-conducting materials. Towards this end, a generalized synthetic methodology has been developed allowing for the synthesis of a series of annelated benzonitronyl nitroxide (BNN) radicals. The BNN radicals exhibited remarkably low reduction potentials (~0.0 V vs SCE) and a near-infrared absorption attributed to a HOMO–SOMO charge-transfer excitation. The annelated BNN radicals were found to be less stable than the closely related tetramethyl nitronyl nitroxide radicals, particularly in solution. A series of π-delocalized and heteroaromatic radicals were synthesized to determine if the instability was due to the delocalization of electron density onto the carbon skeleton or the low reduction potential. DFT calculations with the EPR-II basis gave rise to calculated electronic structures that were consistent with EPR spectroscopy and suggested changes in spin density distribution are occurring with perturbation of the annelated ring. Cyclic voltammetry revealed the heteroaromatic and π-delocalized radicals had reduction potentials lower than BNN, with some systems reducing at potentials of 0.2 V vs SCE, comparable to that of 7,7,8,8-tetracyanoquinodimethane. The distribution of spin throughout the molecular framework and the low reduction potential of the annelated nitronyl nitroxide radicals were both found to contribute to the lowered stability of the annelated nitronyl nitroxides relative to the far less redox active tetramethyl nitronyl nitroxides. The low reduction potential of the BNN radicals suggested that incorporation into acceptor–donor (A–D) systems would allow for investigation of the role of charge transfer interactions on the electronic structure and properties of neutral open-shell A–D radicals. Two D–A–D radicals were prepared using metal catalyzed coupling and furoxan condensation methodologies which resulted in incorporation of a second donor in the C5 position of the BNN moiety. The radical D1–A–D2 triads, where D1 = thiophene and D2 = thiophene or phenyl, exhibited three intramolecular charge-transfer excitations (λmax = 550, 580 and 1000 nm) that were investigated by variable temperature absorption spectroscopy. Structural characterization of the triads in the solid state by single crystal and powder X-ray diffraction revealed slipped π stacks that arise from intermolecular π– π and D–A interactions, providing pathways for antiferromagnetic (AFM) and ferromagnetic (FM) exchange. While the phenyl substituted triad (Th–BNN–Ph) exhibited antiferromagnetic interactions and a room temperature conductivity of σRT = 10−7 S cm−1, the thienyl substituted derivative (Th–BNN–Th) exhibited short-range FM interactions and increased conductivity (σRT = 10−5 S cm−1), giving rise to an organic semiconductor exhibiting FM exchange. The differences in conductivity and magnetic behavior were rationalized by the degree of slippage dictated by an interplay between π– π and intermolecular D−A interactions. Finally, a series of BNN–D radicals were investigated where the donor ability of D was systematically varied from Eox = 2.30 V vs SCE (benzene) to 0.32 V vs SCE (tetrathiafulvalene). Calculations of the near-infrared charge transfer excitation suggested that the HOMO–SOMO gap could be significantly decreased with increasing donor ability, consistent with charge transfer theory. A subset of the series of BNN–D radicals with D = anisole, benzo[b]thiophene, N-methylindole, N-ethylcarbazole, and N,Ndiphenylaniline were synthesized. Solution state spectroscopic studies of the series by EPR and electronic absorption spectroscopy revealed spin-delocalized structures with extremely low reduction potentials (~0 V vs SCE). The solid state properties of the BNN–D radicals were investigated by magnetometry and room temperature conductivity measurements. Due to primarily steric interactions, weak D–A coupling was observed, with weak intermolecular interactions in the solid state leading to paramagnetic and insulating behaviour. The BNN-(N,N-diphenylaniline) radical structure was characterized by single crystal XRD and found to exist as well isolated radical moieties with extremely weak intermolecular interactions, consistent with magnetometry and conductivity measurements. / Graduate / 0490 / 0794

<div>Optical signals are a necessary tool for quantum technologies to carry information both for long-range and on-chip application. The scope of their use is determined by their ability to effectively interact with qubits. The deep-subwavelength interaction volume demands the understanding of the properties of optical fields in the near-field and light-matter interaction in this regime. Recent studies have unraveled the rich characteristics in the physical quantity known as the near-field photonic spin density (PSD). Photonic spin density is the spatial distribution of light's spin angular momentum. It is characterized by the degree of circular polarization of an optical field in deep-subwavelength volumes. In this thesis we study the properties of PSD in the near-field regime and demonstrate a platform for coherent light-spin-qubit interaction based on PSD. We show that nitrogen-vacancy (NV) centers in diamond can coherently interact with an optical beam where the interaction strength is determined by PSD in the nanoscale. To understand the near-field characteristics of PSD we study the evanescent waves and spin-momentum locking of light.</div><div><br></div><div>Evanescent electromagnetic waves possess spin-momentum locking, where the direction of propagation (momentum) is locked to the inherent polarization of the wave (transverse spin). We study the optical forces arising from this universal phenomenon and show that the fundamental origin of recently reported optical chiral forces is spin-momentum locking. For evanescent waves, we show that the direction of energy flow, direction of decay, and direction of spin follow a right hand rule for three different cases of total internal reflection, surface plasmon polaritons, and HE₁₁ mode of an optical fiber.</div><div>Furthermore, we explain how the recently reported phenomena of lateral optical force on chiral and achiral particles is caused by the transverse spin of the evanescent field and the spin-momentum locking phenomenon. Our work presents a unified view on spin-momentum locking and how it affects optical forces on chiral and achiral particles. </div><div><br></div><div>To probe the near-field properties of PSD, we propose and employ a single NV center in diamond as a nanoscale sensor. NV centers have emerged as promising room-temperature quantum sensors for probing condensed matter phenomena ranging from spin liquids, two-dimensional (2D) magnetic materials, and magnons to hydrodynamic flow of current. Here, we demonstrate that the NV center in diamond can be used as a quantum sensor for detecting the photonic spin density. We exploit a single spin qubit on an atomic force microscope tip to probe the spinning field of an incident Gaussian light beam. The spinning field of light induces an effective static magnetic field in the single spin qubit probe. We perform room-temperature sensing using Bloch sphere operations driven by a microwave field (XY8 protocol). This nanoscale quantum magnetometer can measure the local polarization of light in ultra-sub-wavelength volumes. We also put forth a rigorous theory of the experimentally measured phase change using the NV center Hamiltonian and perturbation theory involving only virtual photon transitions. </div><div><br></div><div>In order to study the wavelength dependence of the optically induced magnetic field, we demonstrate this effect for an ensemble of NV centers. We characterize the wavelength dependence of the effective static magnetic field caused by the interaction of PSD and the spin qubit. We show that the strength of the field is inversely dependent on the detuning between the frequency of the optical beam and the optical transition of the NV centers. We show an optically induced rotation of over 10 degrees in the spin qubit of NV centers at room temperature. The direct detection of the photonic spin density at the nanoscale using NV centers in diamond opens interesting quantum metrological avenues for studying exotic phases of photons, nanoscale properties of structured light as well as future on-chip applications. </div><div><br></div>

An O-methylated analog of protonated phenazine-di-N-oxide radical anion abstracts hydrogen from primary and secondary alcohols in a slow (k1 < 500 M-1 s-1) bimolecular reaction. No kinetic evidence has been found for the unimolecular release of free methoxyl radicals through the homolytic N-OMe bond cleavage in these species. DFT calculations at the UB3LYP 6-31G(d) level indicate that protonated and O-alkylated radical anions of pyrazine, quinoxaline and phenazine di-N-oxides are close analogues of aromatic nitroxyl radicals with the highest spin density localized on the oxygen and nitrogen of the nitrone moiety.

No description available.

Guanine is generally accepted as the most easily oxidized DNA base when cells are subjected to ionizing radiation, photoionization or photosensitization. At pH 7, the midpoint reduction potential is on the order of 0.2 – 0.3 V higher than those of the radicals of e.g. tyrosine, tryptophan cysteine and histidine, so that the radical “repair” (or at least, a thermodynamically favorable reaction) involving these amino acids is feasible. Computational quantum studies have been done on tyrosine, tryptophan, cysteine and histidine side chains as they appear in histones. Density functional theory was employed using B3LYP/6-31G+ (d, p) basis set to study spin densities on these amino acids side chains as they pair with the guanine radical cation. The amino acid side chains are positioned so as not to disrupt the Watson-Crick base pairing. Our results indicate that, these side chains of amino acid with reducing properties can repair guanine radical cation through electron transfer coupled with proton transfer.

Les systèmes électroniques bidimensionnels sont d'une grande actualité tout particulièrement depuis la découverte de la supraconductivité à haute température. Ici, on se restreint à l'étude d'un modèle de Hubbard étendu, à la limite d'un couplage faible. En général, le gaz électronique subit une instabilité supraconductrice même sans phonons. Cependant, dans le cas spécial d'une bande demi-remplie, la surface de Fermi est emboîtée et se trouve à une singularité de Van Hove. Cette situation conduit à une compétition entre six instabilités différentes. Outre la supraconductivité en onde $s$ et $d$, on trouve des ondes de densités de spin et de charge ainsi que deux phases qui sont caractérisées par des courants circulaires de charge et de spin respectivement. Le formalisme du groupe de renormalisation est présenté en reliant l'idée de la "< sommation parquet "> au concept plus moderne de l'action effective de Wilson. Comme résultat on obtient un diagramme de phases riche en fonction de l'interaction du modèle. Ce diagramme de phase est exact dans la limite d'une interaction infiniment faible, puisque dans ce cas les lignes de transitions sont fixées par des symétries du modèle. Les comportements à basse température de la susceptibilité de spin ainsi que de la compressibilité de charge complètent l'image physique de ces instabilités. Il s'avère que la surface de Fermi à une tendence générale de se déformer spontanément, mais l'emboîtement n'est pas détruit. En résumé, le modèle de Hubbard à couplage faible reproduit deux propriétés essentielles des cuprates: une phase antiferromagnetique à demi remplissage et la supraconductivité en onde $d$ dans le cas dopé. Mais elle n'éxplique pas les propriétés inhabituelles de l'état métallique dans le régime sous-dopé. Une extension systématique de l'approche perturbative pourrait aider à mieux comprendre ces propriétés, mais reste difficile puisque les techniques nécessaires ne sont pas encore complètement développées.

<p>Theoretical studies based on first-principles theory are presented for a number of different magnetic systems. The first part of the thesis concerns spin dynamics and the second part concerns properties of magnetic multilayers. The theoretical treatment is based on electronic structure calculations performed by means of density functional theory.</p><p>A method is developed for simulating atomistic spin dynamics at finite temperatures, which is based on solving the equations of motion for the atomic spins by means of Langevin dynamics. The method relies on a mapping of the interatomic exchange interactions from density functional theory to a Heisenberg Hamiltonian. Simulations are performed for various magnetic systems and processes beyond the reach of conventional micromagnetism. As an example, magnetization dynamics in the limit of large magnetic and anisotropy fields is explored. Moreover, the method is applied to studying the dynamics of systems with complex atomic order such as the diluted magnetic semiconductor MnGaAs and the spin glass alloy CuMn. The method is also applied to a Fe thin film and a Fe/Cr/Fe trilayer system, where the limits of ultrafast switching are explored. Current induced magnetization dynamics is investigated by calculating the current induced spin-transfer torque by means of density functional theory combined with the relaxation time approximation and semi-classical Boltzmann theory. The current induced torque is calculated for the helical spin-density waves in Er and fcc Fe, where the current is found to promote a rigid rotation of the magnetic order.</p><p>Properties of magnetic multilayers composed of magnetic and nonmagnetic layers are investigated by means of the Korringa-Kohn-Rostocker interface Green's function method. Multilayer properties such as magnetic moments, interlayer exchange coupling and ordering temperatures are calculated and compared with experiments, with focus on understanding the influence of interface quality. Moreover, the influence on the interlayer exchange coupling of alloying the nonmagnetic spacer layers with small amounts of a magnetic impurity is investigated.</p>

Theoretical studies based on first-principles theory are presented for a number of different magnetic systems. The first part of the thesis concerns spin dynamics and the second part concerns properties of magnetic multilayers. The theoretical treatment is based on electronic structure calculations performed by means of density functional theory. A method is developed for simulating atomistic spin dynamics at finite temperatures, which is based on solving the equations of motion for the atomic spins by means of Langevin dynamics. The method relies on a mapping of the interatomic exchange interactions from density functional theory to a Heisenberg Hamiltonian. Simulations are performed for various magnetic systems and processes beyond the reach of conventional micromagnetism. As an example, magnetization dynamics in the limit of large magnetic and anisotropy fields is explored. Moreover, the method is applied to studying the dynamics of systems with complex atomic order such as the diluted magnetic semiconductor MnGaAs and the spin glass alloy CuMn. The method is also applied to a Fe thin film and a Fe/Cr/Fe trilayer system, where the limits of ultrafast switching are explored. Current induced magnetization dynamics is investigated by calculating the current induced spin-transfer torque by means of density functional theory combined with the relaxation time approximation and semi-classical Boltzmann theory. The current induced torque is calculated for the helical spin-density waves in Er and fcc Fe, where the current is found to promote a rigid rotation of the magnetic order. Properties of magnetic multilayers composed of magnetic and nonmagnetic layers are investigated by means of the Korringa-Kohn-Rostocker interface Green's function method. Multilayer properties such as magnetic moments, interlayer exchange coupling and ordering temperatures are calculated and compared with experiments, with focus on understanding the influence of interface quality. Moreover, the influence on the interlayer exchange coupling of alloying the nonmagnetic spacer layers with small amounts of a magnetic impurity is investigated.

Le modèle du jellium est l'un des modèles fondamentaux de la matière condensée.Il est constitué d'un ensemble d'électrons et d'un fond uniforme qui assure la neutralité globale.À température nulle et sans champ extérieur, la densité électronique est le seul paramètre du système.Malgré la simplicité de ce modèle, l'état fondamental du jellium en fonction de la densité reste un problème ouvert.Nous avons étudié le modèle du jellium à 2 et 3 dimensions dans l'approximation de Hartree-Fock par une méthode numérique de descente.En utilisant des états périodiques, le nombre d’inconnues est grandement réduit et le nombre d’électrons peut atteindre le million.À type de réseau et polarisation fixés, nous montrons que le système forme un cristal de Wigner à basse densité puis, au dessus d'une densité de transition, occupe des états «métalliques» caractérisés par une structure cristalline avec une maille plus petite que celle du cristal de Wigner.Les états métalliques interpolent entre le cristal de Wigner et le gaz de Fermi, ce dernier n'étant retrouvé qu'à densité infinie.Ce comportement se retrouve à deux et trois dimensions, pour un gaz polarisé et non polarisé, et pour les différents type de réseaux considérés dans nos travaux.Le diagramme de phase à deux ou trois dimensions est alors très riche et comprend à basse densité diverses phases «cristal de Wigner» avec des symétries et polarisations différentes.À haute densité, les états métalliques non-polarisées déstabilisent le cristal de Wigner et le gaz de Fermi. Ces états métalliques s’interprètent comme une superposition d’ondes de densité de spin, prédite par Overhauser en 1962. / The jellium model is a fundamental model in condensed matter. It is formed by a set of electrons and a uniform background insuring global neutrality. At zero temperature and without external field, the ground-state depends only on the electronic density. Despite its simplicity, the jellium ground-state is still an open problem. We studied the jellium model in 2 and 3 dimensions within the Hartree-Fock approximation using a numerical descent method. Assuming periodic states, we greatly reduce the number of unknowns and the system may contain up to one million of electrons. At fixed lattice symmetry and polarization, the ground-state is a Wigner crystal at low density, and a «metallic state» above a critical density value. These metallic states are crystals with a lattice constant smaller than in Wigner phase, and interpolate between the latter and the Fermi gas. The metallic states exists in two and three dimensions, for a polarized and unpolarized gas, and for various lattice symmetries. Therefore, the jellium phase diagram at zero temperature is rich : it contains several Wigner crystal phases at low density, polarized and unpolarized, and an unpolarized metallic state at high density. These states are well described by a superposition of spin-density waves, as predicted by Overhauser in 1962.

Apresentamos uma teoria para obtenção de seções de choque de espalhamento inelástico de luz via mecanismos de flutuações de densidade de carga e spin em um gás de elétrons não-uniforme formado por um sistema multi-camadas de GaAs periodicamente &#948-dopadas com concentração eletrônica relativamente alta. Os cálculos, onde estão inclusos efeitos da interação coulombiana entre os portadores, efeitos de correlação e troca dinâmicos e o acoplamento com fônons LO, foram efetuados em condições de extrema ressonância com o gap de split-off do GaAs. Em tais condições, a estrutura detalhada dos níveis de energia dos buracos de spin-split torna-se extremamente importante e é fundamental para o surgimento do espectro de partícula independente apresentado nos espectros polarizados. Este comportamento é revelado através da seção de choque de espalhamento que consiste da parte imaginária de uma função resposta constituída de um termo de caráter de partícula independente e outro de caráter coletivo. De forma a levar em conta o amortecimento das flutuações de densidade, propõe-se uma função espectral baseada na conservação da corrente local. Comparação com formas de linha experimentais disponíveis para o caso de espectros despolarizados mostram excelente concordância. No caso dos espectros polarizados a concordância se deu em nível semi-quantitativo, já que excitações de caráter coletivo obtidas experimentalmente apresentaram intensidade menor do que aquelas fornecidas pela teoria. Tal discrepância é atribuída a efeitos de desordem introduzidas no processo de dopagem e que implicam na quebra das regras de conservação de momentum. / We present a theory for the inelastic light scattering cross-section for the mechanisms of charge and spin-density fluctuations in the relatively high concentration of the non-uniform electron gas of a multi-layered &#948-doped GaAs system. The calculations are done in conditions of extreme resonance with the spin-split edge of GaAs and include the effects of Coulomb interactions between the carriers, dynamical exchange-correlations and coupling with LO phonons. In such conditions, the detailed energy level structure of the spin-split holes becomes extremely important and is responsible by the single-particle behavior presented in the polarized spectrum. This behavior revealed by the scattering cross-section derived from the imaginary part of a response function, consists of a term showing single particle character and another displaying collective character. To include the damping of the density fluctuations, a spectral function is proposed based on the foreknowledge that the local current must be conserved. Comparison with the available experimental line-shapes for the depolarized spectra show excellent agreement. In the case of polarized spectra the agreement was given in semi-quantitative terms as experimental collective excitations present less intensity than those calculated by the theory. Such difference is attributed to disorder effects produced during the doping process which results in break down of momentum conservation rules.