The spin-splitting energy in wurtzite structure semiconductors had been investigated by linear combination of atomic orbital method (LCAO), atomic bond orbital method and two-band k¡Ep method. In order to explain the large zero field spin splitting in wurtzite GaN, a different mechanism (£GC1¡V£GC3 coupling) was proposed, which originated from the intrinsic wurtzite effects (band folding and wurtzite bulk inversion asymmetry). The band-folding effect generates two conduction bands (£GC1 and £GC3), in which p-wave probability has tremendous change when kz approaches the anticrossing zone. The spin-splitting energy induced by the£GC1¡V£GC3 coupling and wurtzite bulk inversion asymmetry is much larger than theory calculation of Kane model. When we apply the coupling to GaN/AlN quantum wells, we find that the spin-splitting energy is sensitively controllable by an electric field. It is also found that ideal wurtzite bulk inversion asymmetry yields not only a spin-degenerate line (along the kz axis; time reversal axis) but also a minimum-spin-splitting surface, which can be regarded as a spin-degenerate surface in the form of bkz2- k//2=0 (b≈4) near the £F point. This phenomenon is referred to as the Dresselhaus effect (defined as the cubic-in-k term) in bulk wurtzite materials because it generates a term £^wz(bkz2- k//2)(£mxky-£mykx)=0 in the two-band k¡Ep Hamiltonian. And it is also demonstrated that in the k.p scheme, the spin splitting vanishes to cubic order in k. Consequently, the D¡¦yakonov-Perel¡¦ (DP) spin relaxation mechanism can be effectively suppressed for all spin components in [001] wurtzite quantum wells (QWs) at a resonance condition through device design with appropriate strain, gate voltage or optical illumination. In conclusion: (1) the spin-splitting energy is enhance by wurtzite bulk inversion asymmetry; (2) the spin-splitting energy in wurtzite quantum well is sensitively controllable by electric field; (3) there exist a spin degenerate surface for wurtzite materials in k¡Ep scheme. Therefore, wurtzite QWs (e.g., InGaN/AlGaN and InN/AlInN) are potential candidates for spintronic devices such as the resonant spin lifetime transistor.

Breaking inversion symmetry in crystalline solids enables the formation of spin-polarized electronic states by spin-orbit coupling without the need for magnetism. A variety of interesting physical phenomena related to this effect have been intensively investigated in recent years, including the Rashba effect, topological insulators and Weyl semimetals. In this work, the interplay of inversion symmetry breaking and spin-orbit coupling and, in particular their general influence on the character of electronic states, i.e., on the spin and orbital degrees of freedom, is investigated experimentally. Two different types of suitable model systems are studied: two-dimensional surface states for which the Rashba effect arises from the inherently broken inversion symmetry at the surface, and a Weyl semimetal, for which inversion symmetry is broken in the three-dimensional crystal structure. Angle-resolved photoelectron spectroscopy provides momentum-resolved access to the spin polarization and the orbital composition of electronic states by means of photoelectron spin detection and dichroism with polarized light. The experimental results shown in this work are also complemented and supported by ab-initio density functional theory calculations and simple model considerations. Altogether, it is shown that the breaking of inversion symmetry has a decisive influence on the Bloch wave function, namely, the formation of an orbital angular momentum. This mechanism is, in turn, of fundamental importance both for the physics of the surface Rashba effect and the topology of the Weyl semimetal TaAs. / Wird die Inversionssymmetrie kristalliner Festkörper gebrochen, ermöglicht dies die Ausbildung von spinpolarisierten elektronischen Zuständen durch Spin-Bahn-Kopplung ohne die Notwendigkeit von Magnetismus. In den vergangenen Jahren wurde eine Vielzahl interessanter physikalischer Phänomene diskutiert, die mit diesem Effekt zusammenhängen, darunter der Rashba-Effekt, topologische Isolatoren sowie Weyl-Halbmetalle. In dieser Arbeit wird das Zusammenspiel von Inversionssymetriebruch und Spin-Bahn-Kopplung sowie insbesondere deren Einfluss auf die Eigenschaften der elektronischen Zustände, also auf die Spin- und Orbital-Freiheitsgrade, experimentell untersucht. Zwei verschiedene Arten geeigneter Modellsysteme werden dazu betrachtet: zweidimensionale Oberflächenzustände, in denen der Rashba-Effekt aufgrund der an der Oberfläche inhärent gebrochenen Inverisonssymetrie auftritt, und ein Weyl-Halbmetall, dessen dreidimensionale Kristallstruktur kein Inversionszentrum besitzt. Winkelaufgelöste Photoelektronenspektroskopie bietet einen impulsaufgelösten Zugang zur Spinpolarisation sowie zur orbitalen Zusammensetzung der elektronischen Zustände mittels Photoelektronenspindetektion und Dichroismus mit polarisiertem Licht. Die in dieser Arbeit gezeigten experimentellen Ergebnisse werden außerdem durch ab-initio Dichtefunktionaltheorierechnungen sowie einfachen Modellbetrachtungen ergänzt und untermauert. Insgesamt zeigt sich, dass das Brechen von Inversionssymmetrie einen entscheidenden Einfluss auf die Bloch-Wellenfunktion hat, nämlich die Ausbildung eines orbitalen Bahndrehimpulses. Dieser Mechanismus ist wiederum von grundlegender Bedeutung sowohl für die Physik des Oberflächen- Rashba-Effekts als auch für die Topologie desWeyl-Halbmetalls TaAs.

A spintrônica oferece um paradigma de uma eletrônica baseada no spin do elétron, ao invés de sua carga, para manipular e transportar informação. Para que o conceito de um transistor de spin possa se traduzir em um dispositivo prático, é necessário que se conheça a influência causada pelo movimento dos elétrons polarizados na dinâmica de magnetização de spins em um canal de transporte. Neste sentido, foi estudado um gás de elétrons bidimensional confinado em um poço quântico de GaAs, dopado simetricamente com Si, em um sistema composto de duas sub-bandas. A injeção e detecção de polarização de spin na amostra foi realizada opticamente em uma configuração de bombeio-prova, através da técnica de microscopia de rotação de Kerr com resolução espacial e temporal. Induzindo deriva de spins pela aplicação de voltagens no plano da amostra, foram encontradas mobilidades de spin com valores próximos à mobilidade de carga e que podiam ser modificadas por voltagens aplicadas em um eletrodo de porta. Através de medidas dos campos spin-órbita gerados na amostra, foi possível a avaliação de sua relação com as velocidades de deriva, em comparação com o previsto por um modelo teórico. Dessas medidas puderam ser obtidos os coeficientes spin-órbita das interações de Rashba e Dresselhaus, em que ambos mostraram um comportamento de dependência com as velocidades de deriva não descrito pelo modelo utilizado, o que sugere que a magnitude das interações spin-órbita seja influenciada por essas velocidades. / Spintronics offers a paradigm of an electronics based on the electron spin, rather than its charge, for the manipulation and transport of information. In order that the concept of a spin transistor can be translated into a practical device, it is necessary to know the influence caused by the movement of spin-polarized electrons in the spin magnetization dynamics in a transport channel. In this sense, it was studied two-dimensional electron gases confined in a GaAs quantum well, symmetrically doped with Si, in a system composed of two subbands. Injection and detection of spin polarization in the sample was performed optically in a pump-probe configuration, using space and time resolved Kerr rotation microscopy technique. By the application of in-plane voltages inducing spin drift in the sample, spin mobilities were found with values close to the charge mobility and that could be modified by gate voltages. Through measures of the spin-orbit fields generated in the sample, it was possible to evaluate their relation with the drift velocities, in comparison with that predicted by a theoretical model. From these measures the spin-orbit coefficients of Rashba and Dresselhaus interactions could be obtained, in which both showed a dependence behavior with the drift velocities not described by the model used, suggesting that the magnitude of the spin-orbit interactions is influenced by these velocities.

A spintrônica se beneficia do uso do grau de liberdade quântico que o elétron possui, o spin, para criar novos dispositivos eletrônicos exibindo novas funcionalidades. Para isso, foi estudado um gás de elétrons bidimensional confinado em um poço quântico de GaAs, dopado simetricamente com Si, contendo um sistema composto de duas subbandas no regime de espalhamento inter-subbanda forte. Utilizando-se técnicas de magnetotransporte, foi possível obter a mobilidade dos portadores de carga \\mu = 2:2 10^6cm^2/Vs bem como a densidade total ns = 6:9 10^11/cm^2. A amostra foi caracterizada opticamente através da técnica de rotação de Kerr com resolução temporal e espacial utilizando-se um esquema de bombeio-prova. Após a polarização de spin ser criada opticamente, ao aplicar um campo elétrico no material é produzida deriva de spins. Através de um modelo de deriva que incorpora a interação de Rashba e de Dresselhaus, além dos coeficientes intersubbanda, a dependência do campo spin-órbita com a velocidade de deriva foi avaliada. Encontramos um valor para a interação de Rashba dado por \\alpha = 0:7 meV Å e a influência do termo cúbico de Dresselhaus \\beta_3 na deriva de spins, como consequência do aquecimento da amostra devido à aplicação de altas correntes elétricas, nos possibilitou correlacionar a interação de Dresselhaus com a velocidade de deriva, obtendo-se um comportamento linear. / Spintronics takes advantage of the quantum spin degree of freedom to create new electronic devices with new functionalities. Therefore, it was studied a two-dimensional electron gas confined in a GaAs quantum well, symmetrically doped with Si, producing a two-subband system in the strong intersubband scattering regime. The sample was characterized using magnetotransport techniques, where we obtained the electron mobility as \\mu = 2:2 10^6 cm^2/Vs and total charge densities as ns = 6:9 10^11/cm^2. The sample was optically characterized using time and space resolved Kerr rotation through a pump-probe scheme. Moreover, after optically creating a spin polarization, spin drift is produced by applying an electric field on the material. Using a drift model incorporating Rashba and Dresselhaus term and inter-subband spin-orbit couplings, the spin-orbit fields and drift velocity dependence were evaluated. We found a value for Rashba interaction given by \\alpha = 0:7 meV Å and the influence of cubic Dresselhaus term \\beta_3 in spin drift, as consequence of sample heating due to high electrical currents applied, allowing to correlate Dresselhaus interaction with drift velocity, obtaining a linear behaviour.

A spintrônica se beneficia do uso do grau de liberdade quântico que o elétron possui, o spin, para criar novos dispositivos eletrônicos exibindo novas funcionalidades. Para isso, foi estudado um gás de elétrons bidimensional confinado em um poço quântico de GaAs, dopado simetricamente com Si, contendo um sistema composto de duas subbandas no regime de espalhamento inter-subbanda forte. Utilizando-se técnicas de magnetotransporte, foi possível obter a mobilidade dos portadores de carga \\mu = 2:2 10^6cm^2/Vs bem como a densidade total ns = 6:9 10^11/cm^2. A amostra foi caracterizada opticamente através da técnica de rotação de Kerr com resolução temporal e espacial utilizando-se um esquema de bombeio-prova. Após a polarização de spin ser criada opticamente, ao aplicar um campo elétrico no material é produzida deriva de spins. Através de um modelo de deriva que incorpora a interação de Rashba e de Dresselhaus, além dos coeficientes intersubbanda, a dependência do campo spin-órbita com a velocidade de deriva foi avaliada. Encontramos um valor para a interação de Rashba dado por \\alpha = 0:7 meV Å e a influência do termo cúbico de Dresselhaus \\beta_3 na deriva de spins, como consequência do aquecimento da amostra devido à aplicação de altas correntes elétricas, nos possibilitou correlacionar a interação de Dresselhaus com a velocidade de deriva, obtendo-se um comportamento linear. / Spintronics takes advantage of the quantum spin degree of freedom to create new electronic devices with new functionalities. Therefore, it was studied a two-dimensional electron gas confined in a GaAs quantum well, symmetrically doped with Si, producing a two-subband system in the strong intersubband scattering regime. The sample was characterized using magnetotransport techniques, where we obtained the electron mobility as \\mu = 2:2 10^6 cm^2/Vs and total charge densities as ns = 6:9 10^11/cm^2. The sample was optically characterized using time and space resolved Kerr rotation through a pump-probe scheme. Moreover, after optically creating a spin polarization, spin drift is produced by applying an electric field on the material. Using a drift model incorporating Rashba and Dresselhaus term and inter-subband spin-orbit couplings, the spin-orbit fields and drift velocity dependence were evaluated. We found a value for Rashba interaction given by \\alpha = 0:7 meV Å and the influence of cubic Dresselhaus term \\beta_3 in spin drift, as consequence of sample heating due to high electrical currents applied, allowing to correlate Dresselhaus interaction with drift velocity, obtaining a linear behaviour.

La spintronique classique utilise généralement des matériaux magnétiques pour produire un courant de spin à partir d’un courant de charge. Un autre moyen, plus récemment étudié, consiste à utiliser le couplage spin-orbite (SOC). Il permet de produire un courant de spin pur selon une direction transverse au courant de charge en tenant compte des principes de la mécanique quantique relativiste. Dans les matériaux à fort couplage spin-orbite, les courants de spin ainsi produits sont suffisamment importants pour imaginer les utiliser pour la commutation magnétique dans les dispositifs spintroniques. Le couplage spin-orbite, correspondant à une correction relativiste dans les équations du mouvement de l’électron, particule de spin 1/2, peut être grand dans des matériaux contenant des atomes lourds. Cela signifie qu’une conversion du courant de charge en courant de spin peut être obtenue en utilisant les propriétés de systèmes à fort SOC tel que le platine (Pt), le tungstène (W) ou le tantale (Ta), par exemple. Depuis peu, des systèmes électroniques bidimensionnels (2DEG), obtenus au niveau d’interfaces ou de surfaces particulières, ont démontré des propriétés permettant des effets d’inter-conversion particulièrement efficaces. En particulier des états Rashba ou des systèmes d’isolants topologiques, suscitent actuellement un fort engouement dans la communauté de la spintronique pour cette faculté d’inter-conversion spin-charge.Dans ce cadre particulier, depuis une dizaine d’années, les isolants topologiques ont été étudiés pour leurs propriétés électroniques non conventionnelles qui prennent racine dans la définition théorique de l’effet Hall quantique entier donnée par Thouless, ainsi que dans les travaux de Haldane dans le graphène et de Kane dans des systèmes semi-conducteurs à faible bande interdite pourvus d’un SOC fort. Ces systèmes 2D présentent des propriétés électriques intrigantes : ils sont isolants en volume et conducteurs en surface. Ces états de conductions sont pourvus d’une dispersion linéaire en énergie en fonction du vecteur d’onde k, comme dans le cas du graphène, avec une hélicité en spin déterminée.De nombreuses questions restent néanmoins ouvertes quant à la compréhension des mécanismes à l’origine de ces états de conduction en surface, mais également quant à la manière la plus simple de détecter ces états topologiques. En vue de leur intégration dans des dispositifs spintroniques et de la réalisation d’interface TI/Matériaux ferromagnétiques un certain nombre de questions se posent : comment préserver la nature des états topologiques à l’interface ? Quels matériaux utiliser et quelle est la nature atomique de l’interface (diffusion atomique) ? Quels sont les échanges électroniques à l’interface ? Etc.L’une des applications utilisant les propriétés des isolants topologiques, est d’utiliser les propriétés de conversion du courant de charge en courant de spin (et vice versa) afin de modifier ou commuter l’aimantation d’un élément ou mémoire ferromagnétique déposé directement (ou séparé par une couche tampon) sur le matériau topologique lui-même. Un tel système de bicouches ou multi-couches devrait être capable de s’intégrer dans une mémoire vive magnétique (MRAM) ou d’accroître le potentiel des disques électroniques (SSD) en raison du caractère permanent et non volatile de l’état d’aimantation du matériau. C’est dans ce cadre que s’inscrit cette thèse. / Conventional spintronics generally uses magnetic materials to produce a spin current from a current of charge. Another means, more recently studied, is the use of spin-orbit coupling (SOC). It makes possible to produce a pure current of spin in a direction transverse to the charge current, taking into account the principles of relativistic quantum mechanics. In materials with strong spin-orbit coupling, the spin currents are large enough to imagine using them for magnetic switching in spintronic devices. The spin-orbit coupling, corresponding to a relativistic correction in the equations of motion of the electron, a spin 1/2 particle, can be large in materials containing heavy atoms. This means that a conversion from charge current to spin current can be obtained using the properties of SOC systems such as platinum (Pt), tungsten(W) or tantalum (Ta) for example. Recently 2 dimensionnal electronic gas (2DEG), obtained at particular interfaces or surfaces, have demonstrated properties allowing particularly effective inter-conversion effects. In particular Rashba states or topological insulator systems, are currently arousing a strong interest in the spintronics community for this faculty of spin-charge conversion.In this particular context, over the last ten years or so, topological insulators have been studied for their electronic properties which are rooted in the theoretical definition of the integer quantum Hall effect given by Thouless, as well as in the work of Haldane in graphene and Kane in low bandgap semiconductor systems with a strong SOC. These systems have intriguing electrical properties: they are insulating in volume and conductive on the surfaces. These conductivity states have a linear energy dispersion as a function of the k-wave vector, as in the case of the graphene, with a determined spin helicity.Nevertheless, many questions remain open as the understanding of the mechanisms at the origin of these states of surface conduction, but also as to the simplest way to detect these topological states. In order to integrate in spintronic devices and to realize TI/Ferromagnetic materials interface, a number of questions arise: how to preserve the nature of the topological states at the interface? What materials should be used and what is the atomic nature of the interface (inter-mixing) ? What are the electronic exchanges at the interface? Etc.One of the applications using the properties of topological insulators, is to use the conversion properties of the charge current to spin current in order to modify or switch the magnetization of a ferromagnetic element or memory deposited directly (or separated by a buffer layer) on the topological material itself. Such a two-layer system or multilayer should be capable of integration into a magnetic random access memory (MRAM) or of increasing the potential of disks (SSD) due to the permanent and non-volatile nature of the magnetisation state of the material. This is framework of this thesis.

Monolayers and heterostructures of two-dimensional (2D) electronic materials with spin-orbit interactions offer the promise of observing many novel physical effects. While theoretical predictions of 2D layered materials based on density functional theory (DFT) are many, the DFT approach is limited to small simulation sizes (several nanometers), and thus inhomogeneous strain and boundary effects that are often observed experimentally cannot be simulated within a reasonable time. The aim of this thesis is (i) to study effects of strain on 2D materials beyond graphene using first-principles and tight-binding methods and (ii) to investigate the effects of cuts--"kirigami"-- on 2D materials using molecular dynamics and machine learning approach. The first half of this thesis focuses on the effects of strain on manipulating spin and valley degrees of freedom for two classes of 2D materials--monochalcogenide and lead chalcogenide monolayers--using DFT. A tight-binding (TB) approach is developed to describe the electronic changes in lead chalcogenide monolayers due to strains that often persist in real devices. The strain-dependent TB model allows one to establish a relationship between the Rashba field and the out-of-plane strain or electric polarization from a microscopic view, a connection that is not well understood in the ferroelectric Rashba materials. This framework connecting strain fields and electronic changes is important to overcome the size and computational limitations associated with DFT. The second part of the thesis focuses on defect engineering and design of 2D materials via the "kirigami" technique of introducing different patterns of cuts. A machine learning (ML) approach is presented to provide physical insights and an effective model to describe the physical system. We demonstrate that a machine learning model based on a convolutional neural network is able to find the optimal design from a training data set that is much smaller than the design space.

Im Rahmen dieser Dissertation wird die Untersuchung von 3d-Übergangsmetallphthalocyanin- Molekülen (ÜMPc) – quadratisch-planaren organischen Molekülen, welche im Zentrum ein 3d-Übergangsmetallion besitzen – auf metallischen Oberflächen vorgestellt. Der Fokus dieser Arbeit liegt dabei auf dem Einfluss der d-Orbitalbesetzung auf die magnetischen, elektronischen und strukturellen Eigenschaften der adsorbierten Moleküle, die mit Hilfe der Rastertunnelmikroskopie und -spektroskopie charakterisiert wurden. Die gewonnen Ergebnisse werden zum Teil mit theoretischen Berechnungen analysiert und interpretiert. Die erste Hälfte der experimentellen Auswertung behandelt die Untersuchung dieser Moleküle auf Ag(001) in Hinblick auf die Existenz einer magnetischen Wechselwirkung, bei der ein unkompensiertes magnetisches Moment des Moleküls durch die Substratelektronen abgeschirmt wird. Dieser Effekt wird als Kondo-Abschirmung bezeichnet und erzeugt in der Zustandsdichte des Moleküls eine Resonanz am Fermi-Niveau. Die Messungen zeigen, dass diese Resonanz ausschließlich am Zentralion von MnPc vorgefunden wird, wohingegen sie bei allen anderen 3d-Übergangsmetallphthalocyanin-Molekülen, die eine höhere d-Orbitalbesetzung besitzen, nicht vorhanden ist. Anhand theoretischer Berechnungen kann die Ursache für dieses Verhalten darauf zurückgeführt werden, dass von allen d-Orbitalen einzig das dz2-Orbital mit dem Substrat geeignet hybridisiert, um eine Kondo-Abschirmung zu erzeugen. Da ausschließlich MnPc einen unkompensierten Spin in diesem Orbital besitzt, kann die An- bzw. Abwesenheit des Kondo-Effekts auf die unterschiedliche Besetzung des dz2-Orbitals zurückgeführt werden. Neben der eben erwähnten Kondo-Resonanz ist bei MnPc ein weiteres Merkmal am Fermi- Niveau überlagert. Durch die Analyse der räumlichen Verteilung, den Vergleich mit anderen Molekülen und der Manipulation des MnPc-Moleküls kann gezeigt werden, dass es sich bei diesem Merkmal um einen d-Orbitalzustand handelt. Die Manipulation des Moleküls durch gezieltes Entfernen von Wasserstoffatomen ermöglicht darüber hinaus die Stärke der Kondo-Abschirmung zu beeinflussen. In der zweiten Hälfte der experimentellen Auswertung werden Moleküle auf bismutinduzierten Oberflächenlegierungen der Edelmetalle Cu(111) und Ag(111) untersucht. Diese Legierungen zeichnen sich durch einen ausgeprägten Rashba-Effekt aus, der durch eine Aufspaltung der Parabeldispersion und Aufhebung der Spin-Entartung im zweidimensionalen Elektronengas der Oberflächenlegierung charakterisiert ist. Das Wachstumsverhalten von CuPc und MnPc auf diesen Oberflächen zeigt ein sehr gegensätzliches Verhalten. Während bei MnPc die Substrat-Molekül-Wechselwirkung dominant ist, wodurch diese Moleküle immer einen festen Adsorptionsplatz auf der Oberfläche besitzen, ist diese Wechselwirkung bei CuPc schwach ausgeprägt. Aus diesem Grund wandern die CuPc-Moleküle zu den Stufenkanten und bilden Cluster. Das unterschiedliche Wachstumsverhalten der Moleküle lässt sich auf die partiell-gefüllten d-Orbitale von MnPc zurückführen, die aus der Molekülebene ragen, mit dem Substrat hybridisieren und damit das Molekül an das Substrat binden. Bei CuPc hingegen sind diese d-Orbitale gefüllt und die Hybridisierung kann nicht stattfinden. Im letzten Abschnitt werden die elektronischen und magnetischen Eigenschaften von MnPc auf diesen Substraten behandelt, die einige Besonderheiten aufweisen. So bildet sich durch die Adsorption des Moleküls auf den Oberflächen eine Grenzschichtresonanz aus, die eine partielle Füllung erkennen lässt. Spektroskopiedaten, aufgenommen am Ort der Grenzschichtresonanz, weisen eine symmetrisch um das Fermi-Niveau aufgespaltene Resonanz auf. Die Intensität der unter- und oberhalb der Fermi-Energie befindlichen Resonanz zeigen dabei ein komplementäres Verhalten bzgl. der jeweiligen Lage auf der Grenzschichtresonanz: An den Orten, an denen die Resonanz unterhalb des Fermi-Niveaus ihre maximale Intensität besitzt, ist die Resonanz oberhalb des Fermi-Niveaus nicht vorhanden und umgekehrt. Diese experimentellen Beobachtungen werden mit einem Modellansatz erklärt, welcher die Wirkung eines effektiven Magnetfeldes und eine Spin-Filterung postuliert. / In the framework of this thesis, the investigation of 3d-transition metal phthalocyanine molecules (TM Pc) on metallic surfaces is presented. These molecules possess a square planar structure with a 3d transition metal ion in their center. The main focus of this work concentrates on the influence of the d-level-occupation on the magnetic, electronic and structural properties of the molecules, which are characterized by scanning tunneling microscopy and spectroscopy. The achieved results are partly analyzed and interpreted by theoretical calculations. The first half of this thesis deals with the investigation of TMPc molecules on Ag(001) and the existence of the so-called Kondo effect. This magnetic interaction, which is caused by the screening of an uncompensated magnetic moment of the molecule by the conduction electrons of the substrate, creates a resonance in the density of states close to the Fermi level. The results show, that this resonance is only present at the central metal ion of MnPc, whereas it is absent in the case of all the other 3d transition metal phthalocyanine molecules with a higher d-level occupation. Theoretical calculations indicate that the origin of this behavior can be explained by the fact that out of five d-orbitals only the dz2-orbital can sufficiently hybridize with the substrate to form a Kondo screening channel. As MnPc is the only molecule with an uncompensated spin in this orbital, the presence and absence of a Kondo resonance can be explained by the different occupation of the dz2-orbital. Besides the aforementioned Kondo resonance, another superimposed feature close to the Fermi energy was observed for MnPc. By analyzing the spatial distribution of the features, by comparing the spectroscopy curves of different molecules and by manipulating the MnPc molecule, this feature can be assigned to a d-orbital state. With the manipulation of the MnPc, which was achieved by removing hydrogen atoms of the molecule, the strength of the Kondo screening can be tuned. The second half of the experimental analysis deals with the molecular investigation on bismuth–induced surface alloys of the noble metal crystals Cu(111) and Ag(111). These surface alloys exhibit a pronounced Rashba effect, which splits the parabolic dispersion and lifts the spin degeneracy of the two-dimensional electron gas. On these surfaces, the growth behavior of CuPc and MnPc is very different. While the substrate-molecule–interaction dominates in the case of MnPc, leading to a specific and robust adsorption site of the molecule, this interaction is only weakly present in the case of CuPc. As a result, the CuPc molecules are able to move to the step edges and form clusters. This difference can be attributed to the partial filling of the d-orbitals in the case of MnPc, which protrude out of the molecular plane, hybridize with the substrate and bind the molecule to the substrate. Contrary, in the case of CuPc these orbitals are completely filled, which prevents the hybridization between the d-orbital and the substrate. In the last section, the electronic and magnetic properties of MnPc will be presented, which show some peculiar features. Due to adsorption of the molecule to the surface, an interface resonance with a partial occupancy is created. The spectroscopic data taken at the interface resonance indicate the existence of a split resonance arranged symmetrically with respect to the Fermi energy. The intensity of the occupied and unoccupied resonance show a complementary behavior regarding different positions of the interface resonance. At the positions, where the resonance in the occupied energy regime shows a maximum in intensity, the resonance in the unoccupied states is absent and vice versa. These experimental findings will be explained by a model approach, which postulates the influence of an effective magnetic field and a spin-filtering component.

The spin-splitting energies in strained bulk wurtzite aluminum nitride (AlN) are studied using the linear combination of atomic orbital method. It is found that strain and crystal field induce not only a linear-k (£\wz ) but also two cubic-k terms (£^¡¦and £f¡¦ ) in the two-band k¡Dp Hamiltonian Hso=(£\wz-£^¡¦k2//+£f¡¦k2z)(£mxky-£mykx)+H0so, where H0so=(-£^0k2//+£f0k2z)(£mxky-£mykx) is for ideal wurtzite and generates a cone-shaped minimum-spin-splitting (MSS) surface. As biaxial strain increases, the shape of the MSS surface changes from a hexagonal hyperboloid of two sheets in unstrained AlN to a hexagonal cone, and eventually becomes a hyperboloid of one sheet. The spin-splitting energies of first conduction band for A-plane and M-plane wurtzite are calculated by the sp3 linear combination of atomic orbital (LCAO). The results show the spin-splitting energies are dominated by linear-k term but contribution of cubic-k terms can not be neglected for larger k//. The parameter of linear-k and cubic-k terms are evaluated from the LCAO calculated spin-splitting energies fitting to two band k¡Ep model as increasing well width. The coefficients of linear-k and cubic-k terms decrease.

Submitted by Geyciane Santos (geyciane_thamires@hotmail.com) on 2015-08-06T14:46:34Z No. of bitstreams: 1 Dissertação - Elcivan dos Santos Silva.pdf: 6224760 bytes, checksum: 94c537842fa4d66158cc9db90e8afec2 (MD5) / Approved for entry into archive by Divisão de Documentação/BC Biblioteca Central (ddbc@ufam.edu.br) on 2015-08-07T13:37:35Z (GMT) No. of bitstreams: 1 Dissertação - Elcivan dos Santos Silva.pdf: 6224760 bytes, checksum: 94c537842fa4d66158cc9db90e8afec2 (MD5) / Approved for entry into archive by Divisão de Documentação/BC Biblioteca Central (ddbc@ufam.edu.br) on 2015-08-07T13:41:14Z (GMT) No. of bitstreams: 1 Dissertação - Elcivan dos Santos Silva.pdf: 6224760 bytes, checksum: 94c537842fa4d66158cc9db90e8afec2 (MD5) / Made available in DSpace on 2015-08-07T13:41:15Z (GMT). No. of bitstreams: 1 Dissertação - Elcivan dos Santos Silva.pdf: 6224760 bytes, checksum: 94c537842fa4d66158cc9db90e8afec2 (MD5) Previous issue date: 2013-03-08 / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / Quantum dots have been studied using two electrodes, represented by two conduction bands, coupled to an Anderson impurity. This impurity can be empty, occupied by an electron with energy f or two electrons with energy 2 f + U, where U is the Coulomb interaction between the electrons. The calculation of thermodynamic and transport properties in this model is rather complex, since the interaction U between the orbital electrons of the impurity induce many body interaction via the hybridization of the orbital levels with the conduction bands. Interactions of this type require sophisticated methods of many body calculation, usually numeric, with great computational demand In this Dissertation we use the above model in a simplified form, in the zero limit of the conduction bands width, and introduced the Rashba spin-orbit interaction to the conduction electrons. Thus, the conduction bands are replaced by their respective Fermi levels that are coupled to a third level, which represents the quantum dot. The advantage of the above model is that we can treat it exactly, without making any approach concerning their parameters. As the model is represented by three energy levels and each level can be unoccupied, occupied by an electron with spin up or spin down, or two electrons, one with spin up and the other with spin down, the Hamiltonian can be represented by a 64x64 matrix, which makes it difficult to perform an exact diagonalization. To work around this issue, we find that the studied Hamiltonian conserves charge and parity. This allows us to rewrite the Hamiltonian in the form of matrices whose basis belong to subspaces of the same charge and parity. With this procedure, the 64x64 matrix is replaced by a 1x1 matrix in the zero charge subspace, two 3x3 matrices in the one charge subspace, two 3x3 matrices and one 9x9 matrix in the two charge subspace, two 1x1 matrices and two 9x9 matrices in the three charge subspace, two 3x3 matrices and one 9x9 matrix in the four charge subspace, two 3x3 matrices in the five charge subspace, and one 1x1 matrix in the six charge subspace. Knowing the eigenstates (eigenvalues and eigenvectors) of the Hamiltonian of the studied model, we determined their corresponding thermodynamic and transport properties. Thus, we present the behavior of the energy spectrum, the occupation number, the magnetic susceptibility, the specific heat and the electrical conductance as a function of the parameters of the model. / Os pontos quânticos têm sido estudados utilizando-se dois eletrodos, representados por duas bandas de condução, acoplados a uma impureza de Anderson. Essa impureza pode estar com seus níveis de energia vazio, ocupado com um elétron com energia f ou com dois elétrons, com energia 2 f +U, onde U é a interação Coulombiana entre os seus elétrons. O cálculo das propriedades termodinâmicas e de transportes nesse modelo é bastante complexo, uma vez que a interação U entre os elétrons do orbital da impureza induz interações de muitos corpos, via a hibridização desse orbital com os níveis das bandas de condução. Interações desse tipo exigem métodos sofisticados de cálculo de muitos corpos, em geral numéricos, com grande demanda computacional. Nesta dissertação utilizamos o modelo acima de uma forma simplificada, no limite da largura das bandas de condução zero, e introduzimos a interação spin-órbita de Rashba aos elétrons de condução. Dessa forma, as bandas de condução são substituídas pelos seus respectivos níveis de Fermi que se acoplam a um terceiro nível, que constitui o ponto quântico. A vantagem do modelo acima é que podemos tratá-lo exatamente, sem fazer nenhuma aproximação a respeito dos seus parâmetros. Como o modelo é representado por três níveis de energia e cada nível pode estar desocupado, ocupado com um elétron com spin para cima ou para baixo, ou com dois elétrons, um com spin para cima e outro com spin para baixo, o Hamiltoniano pode ser representado por uma matriz de dimensão 64x64, o que torna difícil sua diagonalização exata. Para contornar essa questão, verificamos que o Hamiltoniano estudado possui as propriedades de conservação de carga e de paridade. Isso nos permite reescrevê-lo na forma de matrizes cujas bases pertencem a subespaços de mesma carga e paridade. Com esse procedimento, a matriz de dimensão 64x64 é substituída por uma matriz 1x1 no subespaço de carga zero, duas matrizes 3x3no subespaço de carga 1, duas matrizes 3x3 e uma matriz 9x9 no subespaço de carga 2, duas matrizes 1x1 de carga 3, 2 matrizes 9x9 de carga 3, duas matrizes 3x3 e uma matriz 9x9 no subespaço de carga 4, duas matrizes 3x3 no subespaço de carga 5 e, finalmente, uma matriz 1x1 no subespaço de carga 6. Obtidos os autoestados (autovalores e autovetores) do Hamiltoniano do modelo estudado, passamos a determinar as suas correspondentes propriedades termodinâmicas e de transporte. Assim, apresentamos o comportamento do espectro de energia, o número de ocupação, a susceptibilidade magnética, o calor específico e a condutância elétrica em função dos parâmetros do modelo.