• Energy Research
• 2021-2025close

Solid-state qubits can be implemented with electrostatically confined quantum dots in semiconductors, allowing gate voltages to independently control the electrochemical potentials of each quantum dot. Quantum dots offer high levels of reliability and scalability. In this paper, along with our proposed approach based on the Generalized Hubbard model followed by Fermi's Golden rule, the charge stability diagram of a double quantum dots system with two electrons has been studied extensively. The validity of the presented approach is confirmed by experimental data. Using Fermi's Golden rule for mapping the charge stability diagram, we have deeply studied the temperature effects arising from both the Hamiltonian and transport. In addition, spin-exchange, pair-hopping, and the occupation-modulated hopping parameters on the states of the charge stability diagram are deeply discussed. Furthermore, we incorporate the Zeeman energies in the Hubbard model in order to theoretically study the spin splitting caused by an external magnetic field applied to the quantum dots. In particular, the aim of this paper is to rely on fundamental physical concepts in order to model and optimize the singlet-triplet qubit in quantum dots. In this study, the probabilities associated with singlet and triplet states have been modeled and analyzed under the impacts of intrinsic and extrinsic parameters. This will help us to find the optimal condition for coupling between double dots and provides us the design rules in terms of physical parameters to efficiently design, measure and sense, initialize, manipulate, and readout of the qubit state.

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, ...

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.; 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.

Electrostatically confined quantum dots in semiconductors hold the promise to achieve high scalability and reliability levels for practical implementation of solid-state qubits where the electrochemical potentials of each quantum dot can be independently controlled by the gate voltages.In this paper, the current and charge stability diagram of two-well potentials arising from electrostatically defined double quantum dot (DQD) are analytically realized. We propose to apply the Generalized Hubbard model to find the Hamiltonian of the system. The proposed analysis takes the tunnel coupling between the dots, Coulomb interaction, and Zeeman energy arising from an external magnetic field into account. Using quantum master equations to predict the probability of the final states in a DQD system, we study the tunneling current through two quantum dots coupled in series with two conducting leads, and therefore, the charge stability diagram is theoretically investigated. The impact of the tunnel coupling and Zeeman energy on the charge stability diagram is deeply discussed. The validity of the presented analysis is confirmed by experimental data as well as the classical capacitance model.

Данная работа посвящена изучению спектра свободных носителей (электронов) в разбавленных магнитных полупроводниках во внешнем постоянном магнитном поле. В работе описана микроскопическая модель обменного взаимодействия электрона зоны проводимости с некоторым числом атомов магнитной примеси. Модель получается упрощением полного гамильтониана системы, описывающего свободный электрон в полупроводнике с N атомами магнитной примеси, взаимодействие между которыми описывается обменным гамильтонианом Гейзенберга, до спинового гамильтониана при помощи приближения виртуального кристалла. Для конкретного случая N = 4 и полного момента j=1/2 точно решена задача нахождения собственных значений энергии и собственных функций полного спинового гамильтониана. Затем путём усреднения по распределению Гиббса получены точные выражения для средних значений проекции спина свободного электрона и проекции собственного момента примесного атома на направление магнитного поля. Для произвольного числа примесных атомов N и произвольного полного момента j каждого атома получены приближённые выражения. Показано, что выражения, используемые в теории среднего поля, являются пределами полученных приближённых выражений модели при больших N. Целью исследования микроскопической модели было определение возможного происхождения эффективных параметров в используемых для описания экспериментальных зависимостей формулах модели среднего поля. Сопоставление точных и приближённых выражений с выражением модели среднего поля позволяет говорить о том, что появление эффективных параметров может быть связано с пренебрежением недиагональными элементами спинового гамильтониана. The given work is devoted to the study of the energy spectrum of free carriers (electrons) in dilute magnetic semiconductors in an external permanent magnetic field. The paper describes a microscopic model of the exchange interaction of an electron of the conduction band with a certain number of atoms of a magnetic impurity. The model is obtained by reducing the complete Hamiltonian of a system of N atoms of a magnetic impurity and a free electron, the interaction between which is described by the Heisenberg exchange Hamiltonian, to the spin Hamiltonian using the virtual crystal approximation. For the specific case of N = 4 and intrinsic moment j=1/2, the problem of finding the eigenvalues of the energy and eigenfunctions of the full spin Hamiltonian is precisely solved. Using the obtained spectrum and Gibbs distribution, exact expressions were then obtained for the average values of the projections of the spin of the electron and the intrinsic moment of the impurity atom in the direction of the magnetic field. Approximate expressions are obtained for the case of an arbitrary number N and an arbitrary intrinsic moment of impurity atoms. It is shown that the expressions used in the theory of the mean field are the limits of the obtained approximate expressions of the model at large N. The purpose of the microscopic model study was to clarify the origin of the effective parameters in the formulas of the mean field model used to describe experimental dependencies. The comparison of exact and approximate expressions with the expression of the mean field model suggests that the appearance of effective parameters can be caused by neglecting the non-diagonal elements of the spin Hamiltonian.