Academic literature on the topic 'Facteur de Landé effectif g*'

Les dichalcogénures des métaux de transition sont constitués d'un empilement de monocouches atomiques liées entre elles par des liaisons faibles de type Van der Waals. Lorsqu'une monocouche de ce matériau est isolée, la symétrie d'inversion du cristal est brisée et la présence d'un couplage spin-orbite fort introduit une levée de dégénérescence des états électroniques ayant des spins différents. Le facteur de Landé effectif (g*) qui intervient dans l'énergie Zeeman est un paramètre qui caractérise, entre autres, la structure de bande du matériau. Il est exceptionnellement grand dans le système WSe_2 en raison de la présence de tungstène et des interactions électroniques. Sa détermination au travers des mesures de résistance électrique sous champ magnétique intense est l'objet de cette thèse. Dans un premier temps, des monocouches de WSe_2 sont produites par l'exfoliation mécanique du matériau massif et leur adressage électrique à l'échelle micrométrique est réalisé par des procédés technologiques de salle blanche impliquant la lithographie électronique. La magnétorésistance des échantillons produits est ensuite étudiée dans des conditions extrêmes de basse température et de champ magnétique intense. La densité de porteur de charges, des trous dans le cas cette thèse, peut être ajustée in-situ par effet de champ. Dans les monocouches de WSe_2, la quantification de l'énergie des niveaux de Landau modifiée par l'effet Zeeman est révélée par la présence d'oscillations complexes de la magnéto-résistance (oscillations de Shubnikov-de Haas). Le développement d'un modèle théorique dédié, où le désordre est pris en compte par un élargissement Gaussien des niveaux de Landau, est nécessaire afin d'interpréter quantitativement les résultats expérimentaux. Il simule l'évolution des composantes du tenseur de résistivité où les paramètres d'ajustement sont la mobilité électronique, l'énergie des bords de mobilité des niveaux de Landau ainsi que le facteur de Landé effectif. L'ajustement théorique aux résultats expérimentaux permet d'extraire l'évolution de g* des trous en fonction de leur densité dans une gamme variant de 5.10^12 à 7,5.10^12 cm^-2, qui s'inscrit dans la continuité des résultats issus de la littérature. Au-delà des approches novatrices sur le plan des conditions expérimentales et de modélisation, cette étude confirme l'importance des interactions électroniques dans la compréhension des propriétés électroniques de ce matériau
Transition metal dichalcogenides are made up of a stack of atomic monolayers bound together by weak Van der Waals interactions. When a single layer of this material is isolated, the crystal inversion symmetry is broken, leading to the degeneracy lifting of the electronic states having different spins in the presence of strong spin-orbit coupling. The effective Landé factor (g*) which arises in the Zeeman energy is a parameter which characterizes, among others, the band-structure of the material. It is exceptionally large in WSe_2 monolayers thanks to the presence of heavy tungsten atoms as well as electronic interactions. Its experimental determination through electrical resistance measurements under intense magnetic field constitutes the objective of this thesis. First, WSe_2 monolayers are produced by mechanical exfoliation of the mother material and their electrical addressing at the micrometric scale is achieved by clean room processes involving electron-beam lithography. Their magneto-resistance is studied under extreme conditions of low temperature and high magnetic field. The charge carrier density, holes in the thesis, can be varied in situ thanks to field effect. In WSe_2 monolayers, the quantization of the Landau level energy modified by the Zeeman effect is revealed by the presence of complex magneto-resistance oscillations (Shubnikov-de Haas oscillations). A dedicated theoretical model, where disorder is introduced through a Gaussian broadening of the Landau levels, is necessary for a quantitative understanding of the experimental results. The components of the resistivity tensor are simulated by this model where the main fitting parameters are the electronic mobility, the mobility edge of the Landau levels and the effective Landé factor. The fitting of the experimental results allows the extraction of g* for a hole density ranging from 5.10^12 to 7.5.10^12 cm^-2, which follows the trend reported in the literature. Beyond the innovative approaches in terms of experimental conditions and modelling, this study confirms the importance of electronic interactions in understanding the electronic properties of this material

Neste trabalho, apresentamos estudos de magnetotransporte em um sistema quase tridimensional de elétrons produzido em amostras contendo poços quânticos parabólicos (PQW, Parabolic Quantum Well ) formados em heteroestruturas de AlGaAs crescidos sobre substratos de GaAs pela técnica de epitaxia por feixe molecular (MBE). Na primeira parte do nosso trabalho realizamos medidas de magnetoresistência, efeito Hall e efeito Shubnikov-de Haas em PQWs com larguras de 1000 Å a fim de investigar as propriedades eletronicas tais como a concentração e a mobilidade dos elétrons nas amostras. Através de cálculos autoconsistentes determinou-se os perfis de potencial, os níveis de energia e as concentrações de cada uma das sub-bandas ocupadas no poço. Uma análise através da transformada de Fourier também permitiu determinar as concentrações eletrônicas nas sub-bandas. Em uma segunda parte estudou-se a influência da aplicação de potenciais externos através de uma porta metálica com barreira em uma amostra contendo um PQW de largura 3000 Å na presença de campos magnéticos perpendicular e paralelo à superfície da amostra. Encontrou-se que para uma tensão de porta de Vg = 0, 55V forma-se uma barreira de potencial ainda sem ter depleção de cargas no poço. Apresenta-se a idealização do dispositivo transistor válvula de spin, a partir do fato que aplicando uma tensão de porta é possível deslocar espacialmente os elétrons e mudar a sua orientaçãp de spin.
Results from magnetic transport studies made on quasi-three-dimensional electron systems are presented in this work. AlGaAs heterostructures grown on GaAs subtrates through molecular beam epitaxy (MBE) enable the existence of this type of systems by means of parabolic quantum wells (PQW) formation. This work was developed in two main parts. First, we studied magnetoresistence phenomena, such as Hall effect and Shubnikov-de Haas, on 1000 Å width PQWs. This permits to know the electronic concentration and mobility values of this type of samples, among other electrical properties. Then, self-consistent calculations gave an outline of the size and shape of the potentials, and gave the values for the energy levels and the electronic concentration on each occupied sub-band of the quantum well. Through Fourier transform analysis was also possible to obtain and confirm the electronic concentrations of the occupied sub-bands. In the second part of the work, we studied the effects of applying an external potential through a barrier gate to a 3000 Å width PQW sample in the presence of magnetic fields parallel and perpendicular to the sample surface. For a V g = 0, 55 V gate voltage, it was found that a potential barrier was formed even without charge depletion in the well. An idealization for the spin valve transistor device, based on the fact that applying a gate potential spatially dislocates the electrons and changes their spin orientation, is presented.

This theoretical thesis presents detailed study of negatively charged excitons - trions - confined in single quantum well in presence of perpendicular magnetic field. Complex valence band of GaAs/GaAlAs compound is described within Luttinger Hamiltonian framework. Singlet and triplet states of negative trion are introduced. Advanced theoretical analysis of Zeeman effect for different states of trion is performed. Landau gauge of magnetic field and unusual wavefunctions basis is chosen and its accuracy is tested. Evolution of ground state energy and photoluminescence spectra with magnetic field is evaluated for different values of Landé g-factors. Probability of occurrence of electrons with respect to the hole position and their spatial correlation function are investigated.

The use of aero engine design methods and experience including higher temperature materials and protective coatings have significantly increased thermal efficiency, and output capability of large industrial gas turbines such as the F, G and H class. As a result the gas path components operate at much higher gas temperatures over significantly longer maintenance intervals, as compared to aero engine experience. Therefore, it is essential that the hardware durability can effectively endure longer periods of attack by oxidation, creep and fatigue because of longer operating intervals between scheduled maintenance periods. Another factor that has become increasingly important is the need for greater flexibility in power plant operation. Specifically, the power plants must operate reliably under more frequent cyclic operation, including partial load cycling. This is in addition to the normal dispatch cycle of the power plant (i.e. daily-start-stop, weekly-start-stop, etc). Gas Turbine reliability is directly dependent on hardware performance and durability. Therefore, the gas turbine must have sufficient design margin to sustain the synergistic effect of higher firing temperature, and the operational challenges associated with greater partial load cycling. This paper discusses Mitsubishi’s approach for achieving the above mentioned objectives so that the overarching goals of higher reliability and durability of hot components are achieved in large advanced gas turbines.