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Ce mémoire présente les travaux réalisés durant ces trois années de thèse. Ils se sont orientés autour de l'étude des propriétés électroniques et magnétiques des matériaux pour la spintronique. Ce domaine d'avenir doit encore trouver les matériaux adaptés qui permettraient de réaliser les concepts liés à la spintronique. Nos résultats sont les suivants. Nous avons proposé un nouveau modèle qui décrit l'arrangement magnétique dans les couches minces ferromagnétiques possédant une anisotropie uniaxiale. Le modèle apporte une meilleure description des largeurs de domaine en fonction de l'épaisseur ainsi qu'une bonne estimation de l'épaisseur critique à partir de laquelle les domaines de Weiss ne sont plus stables. L'ensemble d'équations en unité réduite découlant du modèle apporte quant à lui un outil supplémentaire aux expérimentateurs. L'étude ab-Initio fait sur les semi-Conducteurs magnétiques dilués à mis en évidence l'importance de la corrélation forte et de l'effet Jahn-Teller dans les matériaux tel que le (Ga,Mn)N et le (Zn,Cr)S. Ces calculs confirment l'ensemble des données expérimentales existantes sur le (Ga,Mn)N. La modélisation analytique apporte un complément aux calculs ab-Initio en faisant le lien entre les paramètres expérimentaux et la théorie de Vallin, très largement utilisée pour interpréter les mesures optiques du (Zn,Cr)S. Nos calculs ab-Initio ont montré que le métacinabre est un isolant topologique robuste, qui se distingue par la présence d'un cône de Dirac extrêmement anisotrope. Les effets de passivation à l'hydrogène influencent les états de surface des isolants topologiques de la série HgX (X : S, Se, Te). / This thesis contains the scientific work of three years. The main topic can be defined as follows : study of electronic and magnetic properties of materials for spintronics. That technology of the future has still to find the necessary materials to realize new concepts. Our results are the following : We propose a new model to describe the magnetic configuration in thin ferromagnetic film with uniaxial anisotropy. It gives a better description of domain widths in function of film thickness and permits to obtain a good evaluation of the critical thickness where domains of Weiss type are no longer stable. The set of equations in reduced units is a useful tool to analyze experimental data. The ab-Initio study of diluted magnetic semiconductors has demonstrated the combined effect of strong correlations and Jahn-Teller distortion in (Ga,Mn)N and (Zn,Cr)S. The calculations confirm the experimental results of (Ga,Mn)N. We develop an analytical model that is complementary to the ab-Initio calculations and permits to create a link between several experimental parameters, the ab-Initio calculations and the former theory of Vallin. It has been used to interpret the optical measurements of (Zn,Cr)S. Our ab-Initio studies show that metacinnabar is a strong topological insulator with one peculiarity, it has a highly anisotropic Dirac cone. The passivation with hydrogen atoms influences the surface states of the topological insulator in the series HgX (X : S, Se, Te). When the dangling bonds are saturated by hydrogen, the trivial surface states disappear.

Neste trabalho de mestrado apresentamos um estudo sobre a manifestação do efeito Rashba em isolantes topológicos na ausência de simetria de inversão estrutural. Os cálculos das propriedades atomísticas, energéticas e as estruturas eletrônicas são abordados através de métodos de primeiros princípios baseados na teoria do funcional da densidade. E seus resultados foram utilizados para o desenvolvimento de hamiltoniana efetiva baseado no modelo de Zhang. Realizamos o estudo de dois sistemas: 1) Bi$_2$Se$_3$ com átomos de Sn depositados na superfície: Este sistema pode ser entendido através da manifestação do efeito Rashba sobre um isolante topológico dada a quebra de simetria de inversão estrutural. Para um sítio de deposição específico, os átomos de Sn causam uma reconstrução da superfície e um terceiro cone de Dirac é observado na estrutura eletrônica. Este terceiro cone é não localizado na superfície e pode ser entendido como a manifestação do efeito Rashba. 2) PbBiI: Reportado aqui como um novo isolante topológico 2D com efeito Rashba. Descobrimos este sistema por um estudo sistemático sobre uma família de materiais formados por átomos tipo IV, V, e VII, cuja estrutura cristalina é hexagonal e não centrossimétrica. Mostramos que o PbBiI possui: i) Estabilidade mecânica, ii) Spin-splitting Rashba de 60 meV, iii) um gap de energia não trivial de 0.14 eV, iv) retroespalhamento proibido entre os estados de borda e v) retroespalhamento proibido entre os estados do bulk no entorno do nível de Fermi. Estas propriedades fazem do PbBiI um candidato para construção de dispositivos de spintrônica que atenua a perda de energia. / In this work, were studied the Rashba effect in topological insulators without structural inversion symmetry. We performed a first principles study based on density functional theory to calculate the atomistic properties, formation energy and electronic structure. These results were used to development a effective Hamiltonian based on Zhang model. They were studied two systems: 1) Bi$_2$Se$_3$ with Sn atoms deposited on the surface: This system can be seen as the Rashba effect manifestation on a topological insulator due to the structural inversion symmetry breaking. For a specific deposition site, the Sn atoms cause a reconstruction of the surface and display a third Dirac cone in the electronic structure. This third cone is not located on the surface and can be understood as the giant Rashba effect manifestation. 2) We propose a new non-centrosymmetric honeycomb-lattice QSH insulator family formed by the IV, V, and VII elements. The system formed by Bi, Pb and I atoms is reported here as a new 2D topological insulator with Rashba effect. We show that this system has: i) Mechanical stability, ii) spin-splitting Rashba of 60 meV, iii) nontrivial energy gap of 0.14 eV, iv) backscattering forbidden for both edge and bulk conductivity channels in the nanoribbon band structure. These properties make PbBiI a good candidate to construct spintronic devices with less energy loss.

The development and characterization of experimental setups for angle-resolved photoelectron spectroscopy (ARPES) and spin- and angle-resolved photoelectron spectroscopy (SARPES) is described. Subsequently, the two techniques are applied to studies of the electronic band structure in topologically non-trivial materials. The laser-based ARPES setup works at a photon energy of 10.5 eV and a typical repetition rate in the range 200 kHz to 800 kHz. By using a time-of-flight electron energy analyzer electrons emitted from the sample within a solid angle of up to ±15 degrees can be collected and analyzed simultaneously. The SARPES setup is equipped with a traditional hemispherical electron energy analyzer in combination with a mini-Mott electron polarimeter. The system enables software-controlled switching between angle-resolved spin-integrated and spin-resolved measurements, thus providing the possibility to orient the sample by mapping out the electronic band structure using ARPES before performing spin-resolved measurements at selected points in the Brillouin zone. Thin films of the topological insulators (TIs) Bi2Se3, Bi2Te3 and Sb2Te3 are grown using e-beam evaporation and their surface states are observed by means of ARPES. By using a combination of low photon energies and cryogenic sample temperatures the topological states originating from both the vacuum interface (surface) and the substrate interface are observed in Bi2Se3 films and Bi2Se3/Bi2Te3 heterostructures, with total thicknesses in the ultra-thin limit (six to eight quintuple layers), grown on Bi-terminated Si(111) substrates. Band alignment between Si and Bi2Se3 at the interface creates a band bending through the films. The band bending is found to be independent of the Fermi level (EF) position in the bulk of the substrate, suggesting that the surface pinning of EF in the Si(111) substrate remains unaltered after deposition of the TI films. Therefore, the type and level of doping of the substrate does not show any large influence on the size of the band bending. Further, we provide experimental evidence for the realization of a topological crystalline insulator (TCI) phase in the narrow-band semiconductor Pb1−xSnxSe. The TCI phase exists for temperatures below the transition temperature Tc and is characterized by an inverted bulk band gap accompanied by the existence of non-gapped surface states crossing the band gap. Above Tc the material is in a topologically trivial phase where the surface states are gapped. Thus, when lowering the sample temperature across Tc a topological phase transition from a trivial insulator to a TCI is observed. SARPES studies indicate a helical spin structure of the surface states both in the topologically trivial and the TCI phase. / <p>QC 20130507</p>

Les isolants topologiques constituent une nouvelle classe de matériaux caractérisés par l'association d'un volume isolant et de surfaces conductrices. Avec des propriétés électroniques similaires au graphene, notamment un transport régit par des particules à énergie de dispersion linéaire couramment appelés fermions de Dirac ainsi qu'une protection topologique empêchant tout phénomène de rétrodiffusion, ces matériaux suscitent un intérêt grandissant dans la quête d'une électronique de faible consommation. En effet, la production de courants de spin non-dissipatifs et polarisés ainsi que la formation de courants de spin purs en l'absence de matériaux magnétiques constituent une partie des attentes de ces matériaux topologiques.L'objectif de cette thèse a été de démontrer expérimentalement le potentiel de l'isolant topologique HgTe pour des applications notamment dans le domaine de la l'électronique de spin ou spintronique.Pour ce faire, d'importants efforts ont été mis en œuvre pour améliorer le procédé de croissance par épitaxie par jets moléculaires.La composition chimique, la contrainte ainsi que la qualité des interfaces de la couche de HgTe ont été identifiées comme des axes majeurs de travail et d'optimisation afin d'obtenir une structure de bande inversée, l'ouverture d'un gap de volume, ainsi que pour protéger les propriétés électroniques des états de surface topologiques. Fort de ces caractéristiques, notre matériau possède à priori toutes les qualités nécessaires pour permettre de sonder les propriétés topologiques. Accéder à ces propriétés particulières est en particulier possible par des mesures d'effet Hall quantique sur des structures de type barres de Hall. La fabrication de ces dispositifs a néanmoins requis une attention particulière à cause de la forte volatilité du mercure et a nécessité le développement d'un procédé de nanofabrication à basses températures.Des mesures d'effet Hall quantique à très basses températures ont ensuite été réalisées dans un cryostat à dilution. Tout d'abord des couches épaisses de HgTe ont été mesurées et ont démontrées des mécanismes de transport très complexes mêlant les états de surface topologiques à d'autres contributions attribuées au volume et aux états de surface latéraux. La réduction de l'épaisseur des couches de HgTe a permis de limiter l'impact de ces contributions en les rendant négligeable pour les couches les plus fines. Dans ces conditions, ces structures ont affiché les propriétés attendues de l'effet Hall quantique avec notamment une annulation de la résistance. Avec ces propriétés, l'analyse en température de l'effet Hall quantique a permis de démontrer la nature des porteurs circulant sur les états de surface topologiques et de les identifier à des fermions de Dirac.Avec la mise en évidence de la nature topologique de notre système, l'étape suivante a été d'utiliser les propriétés topologiques et plus particulièrement le blocage entre le moment et le spin d'un électron pour tester le potentiel du système 3D HgTe/CdTe pour la spintronique. Premièrement, des mesures de pompage de spin ont été réalisées et ont mis en exergue la puissance de ces structures pour l'injection et la détection de spin. Deuxièmement, ces structures ont été implémentéessous la forme de jonction p-n dans l'idée de réaliser un premier dispositif de spintronique qui présente à ce jour des premiers signes de fonctionnement. / With graphene-like transport properties governed by massless Dirac fermions and a topological protection preventing from backscattering phenomena, topological insulators, characterized by an insulating bulk and conducting surfaces, are of main interest to build low power consumption electronic building-blocks of primary importance for future electronics.Indeed, the absence of disorder, the generation of dissipation-less spin-polarized current or even the possibility to generate pure spin current without magnetic materials are some of the promises of these new materials.The objective of this PhD thesis has been to experimentally demonstrate the eligibility of HgTe three dimensional topological insulator system for applications and especially for spintronics.To do so, strong efforts have been dedicated to the improvement of the growth process by molecular beam epitaxy.Chemical composition, strain, defect density and sharpness of the HgTe interfaces have been identified as the major parameters of study and improvement to ensure HgTe inverted band structure, bulk gap opening and to emphasize the resulting topological surface state electronic properties. Verification of the topological nature of this system has then been performed using low temperature magneto-transport measurements of Hall bars designed with various HgTe thicknesses. It is worth noting that the high desorption rate of Hg has made the nanofabrication process more complex and required the development of a low temperature process adapted to this constraint. While the thicker samples have evidenced very complex transport signatures that need to be further investigated and understood, the thickness reduction has led to the suppression of any additional contributions, such as bulk or even side surfaces, and the demonstration of quantum Hall effect with vanishing resistance. Consequently, we have managed to demonstrate direct evidences of Dirac fermions by temperature dependent analysis of the quantum Hall effect. The next step has been to use the topological properties and especially the locking predicted between momentum and spin to test the HgTe potential for spintronics. Spin pumping experiments have demonstrated the power of these topological structures for spin injection and detection. Moreover, the implementation of HgTe into simple p-n junction has also been investigated to realize a first spin-based logic element.

Cette thèse porte sur la caractérisation et l'étude du magnéto-transport sur les structures de type HgTe/CdTe sous contraintes développant un transport de surface topologique tout en étant isolant en volume ; on nomme cette nouvelle classe de matériau isolant topologique 3D.Je développerai dans cette thèse la caractérisation et définition d'un isolant topologique 2D/3D pour ensuite me focalise plus particulièrement sur les systèmes II-VI HgTe/CdTe.Une partie de la thèse développe les conditions de croissance réalisées au CEA/Leti ainsi que la caractérisation du matériau par rayon X. La structure de bande des surfaces est caractérisée par ARPES.Une troisième partie traite de la fabrication des barres de Hall nécessaires à la caractérisation du comportement topologique des surfaces. La partie développement expérimentale est également fournie.La dernière partie traite du magnétotransport réalisé avec ces barres de Hall à faible et fort champ magnétique. Le comportement ambipolaire, une phase de Berry non triviale, l'antilocalisation faible et l'effet Hall quantique entier dans ces structures sont abordés tout tentant de fournir une interprétation des résultats obtenus. / This report deal with caracterisation of magnetotransport in HgTe/CdTe structures bulk strained in that a topological surface transport is predicted. This new kind of material is a 3D topological insulator.In this thesis, I will explain what means 3D/2D topological insulator before focusing on II-VI system lijke HgTe/CdTe.Next, I will discuss about growing conditions performed in CEA/Leti and then material caracterisation by X-ray. Surfaces band structures were also, observed by ARPES, underligned in the report.A third part deal with Hall bars design and conception in order to emphasize topological behavior of these surfaces.The last part shows the results obtained on these Hall bars with magnetotransport at low and high magnetic field. Ambipolaire behaviour, non trivial Berry phase, weak antilocalization and the interger quantum hall effect in HgTe/CdTe structures are studied and a possible interpretation of these results are given.

Submitted by Bruna Rodrigues (bruna92rodrigues@yahoo.com.br) on 2016-09-20T13:42:59Z No. of bitstreams: 1 DissPCDM.pdf: 3450913 bytes, checksum: 84b54d1d49257840a2f202608f7dba57 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-21T13:02:19Z (GMT) No. of bitstreams: 1 DissPCDM.pdf: 3450913 bytes, checksum: 84b54d1d49257840a2f202608f7dba57 (MD5) / Approved for entry into archive by Marina Freitas (marinapf@ufscar.br) on 2016-09-21T13:02:26Z (GMT) No. of bitstreams: 1 DissPCDM.pdf: 3450913 bytes, checksum: 84b54d1d49257840a2f202608f7dba57 (MD5) / Made available in DSpace on 2016-09-21T13:02:33Z (GMT). No. of bitstreams: 1 DissPCDM.pdf: 3450913 bytes, checksum: 84b54d1d49257840a2f202608f7dba57 (MD5) Previous issue date: 2015-10-26 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / The present work is based on the literature related to the Bi2Se3 compound, which is present in current researches as a topological insulator and is also well known for thermoelectricity applications. In this work the electrochemical synthesis of a material containing Bi, Se and X (X = Cu, Fe or Co) was carried out searching for a composition of Bi and Se in accordance with the stoichiometry of Bi2Se3. The third element, X, was used in the electrodeposition baths considering the Bi2Se3 literature, which presents doping and interaction with other elements for this compound. The synthesis conditions were determined based on studies using cyclic voltammetry. The main material studied was the ternary compound containing Bi, Se and Cu. The films obtained were characterized using scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction and Raman spectroscopy. It was found out that baths containing sulfuric acid were adequate for obtaining films of Bi, Se and Cu but a bath containing glycerol and sodium citrate was necessary for including Fe on the material. Some magnetic properties of the films obtained were also studied and paramagnetic behaviour was observed for the samples. / O presente trabalho fundamente-se na literatura relacionada com o composto Bi2Se3. Esse composto está presente na literatura atual de isolantes topológicos e também é conhecido por suas propriedades termoelétricas. Nesse trabalho foi realizada a síntese eletroquímica de um material contendo Bi, Se e X (X = Cu, Fe or Co) buscando obter composições de Bi e Se adequadas à estequiometria do Bi2Se3. O terceiro elemento, X, foi utilizado nos banhos de eletrodeposição considerando estudos da literatura para o Bi2Se3 que tratam de sua dopagem ou interação com outros elementos. As condições de síntese foram determinadas com base em estudos utilizando voltametria cíclica. O principal material estudado foi o composto ternário de Bi, Se e Cu. Os filmes obtidos foram caracterizados por microscopia eletrônica de varredura, espectroscopia dispersiva de raios X, difração de raios X e espectroscopia Raman. Foi verificado que banhos contendo ácido sulfúrico foram adequados para obter filmes de Bi, Se e Cu, mas um banho contendo glicerol e citrato de sódio foi necessário para incluir Fe no material. Algumas propriedades magnéticas dos filmes obtidos também foram estudadas e o comportamento paramagnético foi observado para as amostras.

Neste trabalho de mestrado apresentamos um estudo sobre a manifestação do efeito Rashba em isolantes topológicos na ausência de simetria de inversão estrutural. Os cálculos das propriedades atomísticas, energéticas e as estruturas eletrônicas são abordados através de métodos de primeiros princípios baseados na teoria do funcional da densidade. E seus resultados foram utilizados para o desenvolvimento de hamiltoniana efetiva baseado no modelo de Zhang. Realizamos o estudo de dois sistemas: 1) Bi$_2$Se$_3$ com átomos de Sn depositados na superfície: Este sistema pode ser entendido através da manifestação do efeito Rashba sobre um isolante topológico dada a quebra de simetria de inversão estrutural. Para um sítio de deposição específico, os átomos de Sn causam uma reconstrução da superfície e um terceiro cone de Dirac é observado na estrutura eletrônica. Este terceiro cone é não localizado na superfície e pode ser entendido como a manifestação do efeito Rashba. 2) PbBiI: Reportado aqui como um novo isolante topológico 2D com efeito Rashba. Descobrimos este sistema por um estudo sistemático sobre uma família de materiais formados por átomos tipo IV, V, e VII, cuja estrutura cristalina é hexagonal e não centrossimétrica. Mostramos que o PbBiI possui: i) Estabilidade mecânica, ii) Spin-splitting Rashba de 60 meV, iii) um gap de energia não trivial de 0.14 eV, iv) retroespalhamento proibido entre os estados de borda e v) retroespalhamento proibido entre os estados do bulk no entorno do nível de Fermi. Estas propriedades fazem do PbBiI um candidato para construção de dispositivos de spintrônica que atenua a perda de energia. / In this work, were studied the Rashba effect in topological insulators without structural inversion symmetry. We performed a first principles study based on density functional theory to calculate the atomistic properties, formation energy and electronic structure. These results were used to development a effective Hamiltonian based on Zhang model. They were studied two systems: 1) Bi$_2$Se$_3$ with Sn atoms deposited on the surface: This system can be seen as the Rashba effect manifestation on a topological insulator due to the structural inversion symmetry breaking. For a specific deposition site, the Sn atoms cause a reconstruction of the surface and display a third Dirac cone in the electronic structure. This third cone is not located on the surface and can be understood as the giant Rashba effect manifestation. 2) We propose a new non-centrosymmetric honeycomb-lattice QSH insulator family formed by the IV, V, and VII elements. The system formed by Bi, Pb and I atoms is reported here as a new 2D topological insulator with Rashba effect. We show that this system has: i) Mechanical stability, ii) spin-splitting Rashba of 60 meV, iii) nontrivial energy gap of 0.14 eV, iv) backscattering forbidden for both edge and bulk conductivity channels in the nanoribbon band structure. These properties make PbBiI a good candidate to construct spintronic devices with less energy loss.

Dans cette thèse, j’étudie la compressibilité électronique de deux isolants topologiques tridimensionnels : Le tellurure de mercure (HgTe) sous contrainte et le séléniure de bismuth (Bi2Se3).Je présente des mesures d’admittance électronique à basse température résolues en phase sur une large gamme de fréquence. Cela permet d’extraire la capacité quantique associé à la densité d’états et la résistivité des matériaux étudiés.Nous montrons qu’un isolant topologique intrinsèque présente une réponse dominée par les états de surface topologiques sur une large gamme d’énergie qui s’étend au-delà du gap de transport du matériau massif. Ce régime, appelé « écrantage de Dirac », est caractérisé par une compressibilité électronique proportionnelle à l’énergie de surface et une haute mobilité.Dans la suite, nous nous intéressons à la limite de ce régime. Nous observons qu’à haute énergie et sous l’influence de forts champs électriques perpendiculaires, des états excités massifs de surface sont peuplés ce qui se manifeste expérimentalement de différentes façons : Une chute dans la constante de diffusion électronique, un pic de conductivité ainsi que l’apparition d’un deuxième type de porteurs en magnéto-transport et de métastabilité dans la relation charge-tension.Un modèle théorique basé sur un traitement quasi-relativiste du Hamiltonien de surface est présenté. Il permet d’identifier la dépendance en énergie et champ électrique des états massifs de surface.Cette thèse est complémenté par des résultats expérimentaux sur Bi2Se3 obtenu par croissance sur nitrure de bore mettent en évidence l’importance de la pureté des interfaces d’isolants topologiques. / This thesis discusses the electronic compressibility of two representative three dimensional topological insulators: Strained mercury telluride (HgTe) and bismuth selenide (Bi2Se3).I present low temperature phase-sensitive electron admittance data over a broad frequency range. This allows to extract the quantum capacitance related to the density of states and the resistivity of the investigated materials.We show that the response of an intrinsic topological insulator is dominated by topological surface states over a large energy range exceeding the bulk material’s transport gap. This regime, named “Dirac screening” is characterized by an electron compressibility proportional to the surface Fermi level and a high mobility.Subsequently, we investigate the limits of this regime. At high energy and large perpendicular electric fields we observe the population of excited massive surface states. Experimentally, these manifest themselves in multiple signatures: A drop in the electronic diffusion constant, a peak in the conductivity, appearance of a second carrier type in magneto-transport and meta-stability in the charge-voltage relation.A theoretical model based on a quasi-relativistic treatment of the surface Hamiltonian is presented. It allows to identify the electric field and energy dependence of the massive surface states.This thesis is complemented by experimental results on Bi2Se3 grown on boron nitride, where we demonstrate the importance of clean surfaces for the study of electronic properties in topological insulators.

Dans cette thèse, nous étudions l’effet du désordre magnétique sur les propriétés de transport électronique du graphène et des isolants topologiques 2D de type HgTe. Le graphène et les isolants topologiques sont des matériaux dont les excitations électroniques sont assimilées à des fermions de Dirac sans masse. L’influence des impuretés magnétiques sur les propriétés de transport du graphène est étudiée dans le régime de forts champs électriques. En conséquence de la production de paires électron-trou, la réponse devient non linéaire et dépend de la polarisation magnétique. Nous étudions une transition entre un isolant topologique bi-dimensionnel conducteur, caractérisé par une conductance G = 2 (en quantum de conductance) et un isolant de Chern avec G = 1, induite par des impuretés magnétiques polarisées. / In this thesis, we study the effect of a magnetic disorder on the electronic transport properties of graphene and HgTe-type 2D topological insulators. Graphene and topological insulators are materials whose electronic excitations are treated as massless Dirac fermions.The influence of magnetic impurities on the transport properties of graphene is investigated in the regime of strong applied electric fields. As a result of electron-hole pair creation, the response becomes nonlinear and dependent on the magnetic polarization.We investigate a transition between a two-dimensional topological insulator conduction state, characterized by a conductance G = 2 (in conductance quantum) and a Chern insulator with G = 1, induced by polarized magnetic impurities.