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DNA is a potential component in molecular electronics. To explore this end, there has been an incredible amount of research on how well DNA conducts and by what mechanism. There has also been a tremendous amount of research to find new uses for it in nanoscale electronics. DNA's self-assembly and recognition properties have found a unique place in this area. We predict, using a tight-binding model, that spin-dependent transport can be observed in short DNA molecules sandwiched between ferromagnetic contacts. In particular, we show that a DNA spin-valve can be realized with magnetoresistance values of as much as 26% for Ni and 16% for Fe contacts. Spin-dependent transport can broaden the possible applications of DNA as a component in molecular electronics and shed new light into the transport properties of this important biological molecule. / Master of Science

Neste trabalho investigamos pela primeira vez flutuações dependentes de spin em correntes eletrônicas polarizadas através de estruturas magnéticas. Nosso sistema físico consiste de uma heteroestrutura com tunelamento ressonante formada por um poço ou \"ponto\" quântico contendo Mn, confinado entre duas barreiras de potencial. Usamos um modelo semiclássico baseado em equações de taxa para calcular as ocupações dos estados ressonantes up e down. Estas equações são derivadas de uma equação mestra que descreve a probabilidade de ocupação dos estados de spin em um dado tempo. Funções correlação corrente-corrente são expressas em termos das funções correlação hop-hop (associadas à transições entre os níveis ressonantes up e down) e o shot noise dependente de spin é determinado em termos da matriz variância do sistema, também derivada da equação mestra. Quando consideramos um feixe polarizado e tempos distintos (&#964 &#8593 &#8595 &#8800 &#964 &#8595 &#8593), podemos obter ambas \"correlações positivas\" (&#8249 &#916 &#8593 &#916 &#8595 &#8250 &#8805 0) e/ou \"negativas\" (&#8249 &#916 &#8593 &#916 &#8595 &#8250 &#8804 0) no nosso modelo. A generalização e reinterpretação do modelo de ilhas nos possibilitou (i) investigar flutuações dependentes de spin em correntes polarizadas; (ii) observar aumento e atenuação do shot noise; (iii) verificar que processos de \"spin-flip\" com (&#964 &#8593 &#8595 &#8800 &#964 &#8595 &#8593) são relevantes na atenuação do shot noise e (iv) verificar que o ruído contém informações sobre o processo de \"spin-flip\" / In this work we investigate for the first time spin-dependent fluctuations in spin-polarized electronic currents through magnetic structures. Our physical model consists of a resonant-tunneling heterostructure formed by a Mn-based quantum well or \"point\", confined between a double-barrier potential. We used a semiclassical model based on rate equations to calculate the occupations of the spin-up and spindown resonant states. These equations are derived from a master equation describing the probability of occupation of the spin states at a given time. Current-current correlation functions are expressed in terms of hop-hop correlation functions (for hops between islands representing the up and down states) and the spin-dependent shoi noise is determined in terms of the variance matrix of the system; also derived from the master equation. When we consider a polarized beam and distinctive times (&#964 &#8593 &#8595 &#8800 &#964 &#8595 &#8593), we can obtain both \"positive correlations\" (&#8249 &#916 &#8593 &#916 &#8595 &#8250 &#8805 0) and \"negative correlations\" (&#8249 &#916 &#8593 &#916 &#8595 &#8250 &#8804 0) in our model. The generalization and reinterpretation of the island model allowed us (i) to investigate spin-dependent fluctuations in spinpolarized electronic currents; (ii) to observe enhancement and suppression of shot noise; (iii) to verify that spin-flip processes with (&#964 &#8593 &#8595 &#8800 &#964 &#8595 &#8593) are relevant to shotnoise suppression and (iv) to verify that noise contains information about spin-flip processes

Neste trabalho investigamos pela primeira vez flutuações dependentes de spin em correntes eletrônicas polarizadas através de estruturas magnéticas. Nosso sistema físico consiste de uma heteroestrutura com tunelamento ressonante formada por um poço ou \"ponto\" quântico contendo Mn, confinado entre duas barreiras de potencial. Usamos um modelo semiclássico baseado em equações de taxa para calcular as ocupações dos estados ressonantes up e down. Estas equações são derivadas de uma equação mestra que descreve a probabilidade de ocupação dos estados de spin em um dado tempo. Funções correlação corrente-corrente são expressas em termos das funções correlação hop-hop (associadas à transições entre os níveis ressonantes up e down) e o shot noise dependente de spin é determinado em termos da matriz variância do sistema, também derivada da equação mestra. Quando consideramos um feixe polarizado e tempos distintos (&#964 &#8593 &#8595 &#8800 &#964 &#8595 &#8593), podemos obter ambas \"correlações positivas\" (&#8249 &#916 &#8593 &#916 &#8595 &#8250 &#8805 0) e/ou \"negativas\" (&#8249 &#916 &#8593 &#916 &#8595 &#8250 &#8804 0) no nosso modelo. A generalização e reinterpretação do modelo de ilhas nos possibilitou (i) investigar flutuações dependentes de spin em correntes polarizadas; (ii) observar aumento e atenuação do shot noise; (iii) verificar que processos de \"spin-flip\" com (&#964 &#8593 &#8595 &#8800 &#964 &#8595 &#8593) são relevantes na atenuação do shot noise e (iv) verificar que o ruído contém informações sobre o processo de \"spin-flip\" / In this work we investigate for the first time spin-dependent fluctuations in spin-polarized electronic currents through magnetic structures. Our physical model consists of a resonant-tunneling heterostructure formed by a Mn-based quantum well or \"point\", confined between a double-barrier potential. We used a semiclassical model based on rate equations to calculate the occupations of the spin-up and spindown resonant states. These equations are derived from a master equation describing the probability of occupation of the spin states at a given time. Current-current correlation functions are expressed in terms of hop-hop correlation functions (for hops between islands representing the up and down states) and the spin-dependent shoi noise is determined in terms of the variance matrix of the system; also derived from the master equation. When we consider a polarized beam and distinctive times (&#964 &#8593 &#8595 &#8800 &#964 &#8595 &#8593), we can obtain both \"positive correlations\" (&#8249 &#916 &#8593 &#916 &#8595 &#8250 &#8805 0) and \"negative correlations\" (&#8249 &#916 &#8593 &#916 &#8595 &#8250 &#8804 0) in our model. The generalization and reinterpretation of the island model allowed us (i) to investigate spin-dependent fluctuations in spinpolarized electronic currents; (ii) to observe enhancement and suppression of shot noise; (iii) to verify that spin-flip processes with (&#964 &#8593 &#8595 &#8800 &#964 &#8595 &#8593) are relevant to shotnoise suppression and (iv) to verify that noise contains information about spin-flip processes

On étudie les propriétés de spin des états évanescents d'un semi-conducteur dépourvu de centre d'inversion. La topologie particulière des bandes évanescentes qui résulte de l'interaction spin-orbite est à l'origine d'un l'effet tunnel anormal. La nature même du processus tunnel devient très dépendante de l'orientation cristallographique de la barrière. Deux cas typiques sont analysés : tunneling sous incidence oblique sur une barrière orientée selon la direction [001] et tunneling sous incidence normale au travers d'une barrière orientée dans la direction [110]. Dans le premier cas, un processus tunnel quasi-classique peut être restauré de façon assez subtile et des effets de filtres à spin sont mis en évidence. Dans le second cas, la situation est particulièrement originale. La notion de courant de probabilité, qui joue un rôle central, est réexaminée et les conditions de discontinuité aux interface de la dérivée de la fonction d'onde sont établies. Selon cette direction où la levée de dégénérescence de spin de la bande de conduction est maximum, il n'y a curieusement aucun filtrage de spin mais l'onde transmise subit un déphasage dont le signe dépend de l'orientation du spin. On prédit des effets de précession de spin autour d'un champ effectif complexe régnant dans la barrière. Ces résultats permettent de concevoir, par ingénierie spin-orbite d'hétérostructures, des dispositifs tunnel résonnant capables de manipuler le spin.

<p>The first measurements of the two- and three-body photodisintegration of longitudinally</p><p>polarized 3He with a circularly-polarized gamma-ray beam were carried out at the High Intensity gamma-ray Source facility located at Triangle Universities Nuclear Laboratory (TUNL). A high pressure 3He target, polarized via spin exchange optical pumping with alkali metals, was used in the experiments. The protons from the two-body photodisintegration experiment were detected using seventy two silicon surface barrier detectors of various thicknesses while the neutrons from the three-body photodisintegration were detected with sixteen 12.7 cm diameter liquid scintillator detectors. The spin-dependent cross sections and the contributions from the two- and three-body photodisintegration to the 3He Gerasimov-Drell-Hearn sum rule integrand were extracted and compared with state-of-the-art three-body calculations at the incident photon energies of 29.0 MeV (two-body) and 12.8, 14.7, and 16.5 MeV (three-body).</p><p>These are the first measurements of the contributions of the two- and three-body photodisintegration of 3He to the GDH integrand. These measurements were found to be in good agreement with the theoretical calculations which include the Coulomb interaction between protons in the final state. They also reveal-for the first time-the importance of the three-nucleon forces and the relativistic single-nucleon charge corrections which are responsible in the calculations for the observed difference</p><p>between the spin-dependent cross sections.</p> / Dissertation

Nous avons étudié par des méthodes numériques et en théorie k.p avancée les propriétés tunnel d’électrons et de trous dans des systèmes modèles et hétérostructures composés de semi-conducteurs impliquant des interactions spin-orbite de volume. Nous démontrons que le couplage entre les interactions spin-orbite et d’échange à l’interface de jonctions tunnel résulte en un fort contraste de transmission de porteurs selon le signe de la composante de leur vecteur d’onde dans le plan de la jonction. Cet effet conduit à un effet tunnel anormal d’interface que nous appelons « Effet Hall Tunnel Anormal » (ATHE). De façon similaire, des processus tunnel non-conventionnels se manifestant sur des isolants topologiques ont été prédits par d’autres auteurs. Alors que l’ensemble de ces effets Hall anormaux sont liés aux interactions spin-orbite, les effets tunnel anormaux diffèrent des effets Hall tunnel, des effets Hall et des effets Hall de spin par la forte amplitude prédite ainsi que par des phénomènes de chiralité. Ces propriétés possèdent un lien nontrivial avec la symétrie du système. L’ensemble de ces résultats démontre l’existence d’une nouvelle classe d’effets tunnel qui devaient être étudiés expérimentalement dans un futur proche. En ce qui concerne la bande de valence, nous démontrons, en utilisant un Hamiltonien 14x14 prolongeant un modèle 2x2, que le calcul décrivant l’ATHE repose sur un traitement subtil des états dits « spurious » (états non-physiques) et nous donnons quelques éléments d’amélioration et de compréhension. Dans ce mémoire de thèse, nous développons deux méthodes numériques pour résoudre le problème des états spurious en développant en parallèle des méthodes k.p respectivement à 14 bandes et 30 bandes afin de décrire des matériaux semiconducteurs à gap indirect. Les calculs menés dans la bande de valence d’hétérostructures semiconductrice incluant interfaces et barrières tunnel (approches 6x6 et 14x14) sans centre de symétrie d’inversion mettent en évidence des propriétés d’asymétrie équivalente à celles obtenues dans la bande de conduction. De tels effets sont interprétés, dans le cadre de calculs de perturbation en transport basés sur des techniques de fonctions de Green, par des effets chiraux orbitaux lors du branchement tunnel des fonctions évanescentes dans la barrière. / We report on theoretical, analytical and computational investigations and k.p calculations of electron and hole tunneling, in model systems and heterostructures composed of exchange-split III-V semiconductors involving spin-orbit interaction (SOI). We show that the interplay of SOI and exchange interactions at interfaces and tunnel junctions results in a large difference of transmission for carriers, depending on the sign of their incident in-plane wave vector (k//): this leads to interfacial skew-tunneling effects that we refer to as Anomalous Tunnel Hall Effect (ATHE). In a 2x2 exchange-split band model, the transmission asymmetry (A) between incidence angles related to +k// and -k// wave vector components, is shown to be maximal at peculiar points of the Brillouin zone corresponding to a totally quenched transmission (A = 100%). More generally, we demonstrate the universal character of the transmission asymmetry A vs. in-plane wavevector component, for given reduced kinetic energy and exchange parameter, A being universally scaled by a unique function, independent of the spin-orbit strength and of the material parameters. Similarly, striking tunneling phenomena arising in topological insulators have just been predicted. While they all are related to the spin-orbit directional anisotropy, ATHE differs from the tunneling Hall effect, spontaneous anomalous, and spin Hall effects, or spin-galvanic effect, previously reported for electron transport, by its giant forward asymmetry and chiral nature. These features have non-trivial connection with the symmetry properties of the system. All these results show that a new class of tunneling phenomena can now be investigated and experimentally probed.When valence bands are involved, we show (using 14x14 Hamiltonian and within a 2x2 toy model) that ATHE accurate calculations rely on a subtle treatment of the spurious (unphysical) states and we give an insight into the topology of the complex band structure. We introduce two numerical methods to remove spurious states and successfully, include them in 30-band codes able to describe indirect bandgap group-IV semiconductors. Calculations performed in the valence bands of model heterostructures including tunnel barriers, in both 6x6 and 14x14 k.p Hamiltonians without inversion asymmetry, more astonishingly highlight the same trends in the transmission asymmetry which appears to be related to the difference of orbital chirality and to the related branching (overlap) of the corresponding evanescent wave functions responsible for the tunneling current. Besides, we built an analytical model and developed scattering perturbative techniques based on Green’s function method to analytically deal with electrons and holes and to compare these results with numerical calculations. The agreement between the different approaches is very good. In the case of holes, the asymmetry appears to be robust and persists even when a single electrode is magnetic.