• Energy Research

This thesis describes electrical measurements performed on low dimensional p- type devices, fabricated from GaAs/AlGaAs heterostructures. The Coulomb interaction between holes is similar to that between electrons. However, the kinetic energy is suppressed, which makes interaction effects particularly important. Holes may also be used to study band structure effects which arise from spin-orbit coupling in the valence band. The effects of Coulomb interactions in low dimensional electron systems are currently being studied extensively. Experiments presented in this thesis indicate the possible importance of Coulomb exchange interactions in both one and two dimensional hole systems (1DHSs,2DHSs). Tilted magnetic field studies of 2DHSs in the quantum Hall regime indicate that Landau levels at even filling factors will not cross. For high filling factor, this is attributed to a spin-orbit mixing effect which arises from the low symmetry of the system. At lower filling factor, activation-energy measurements verify that the energy gaps decrease and then increase as the field is tilted. However, the energy gap versus field dependences do not exhibit the curvature that might be expected from a perturbative anticrossing. It is speculated that the origin of this effect is a phase transition driven by the exchange interaction. Balanced arguments contrasting the relative strengths of the mixing and interactions theories are provided. The second part of this thesis describes a new method for the fabrication of ballistic 1DHSs, which exhibit clear conductance quantization. The quantization changes from even to odd multiples of e2/h as a function of the magnetic field in the plane of the heterostructure, as "spin splitting" causes the 1D subbands to cross. Measurements of the 1D subband energy spacings are used together with the magnetic fields at which the crossings occur to calculate the in-plane g factors of the 1D subbands. These are found to increase as the number of occupied 1D subbands decreases. This enhancement of the g factor is attributed to exchange interactions; possible mixing explanations are also discussed. At higher magnetic fields, the pattern of quantization features shows that the subbands have crossed many times, and that the 1DHS can be strongly magnetized.

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.

De nombreux niveaux de Sm I ont ete identifies a l'aide de leur deplacement isotopique et de leur facteur de Lande. Les calculs theoriques des energies et des facteurs de Lande des niveaux de l'ensemble des sous-configurations 4f6(7F) 6s6p + 4f5 (6H— 6F) 5d6s2 ont permis l'interpretation de 127 niveaux experimentaux avec un ecart quadratique moyen de 105 K. Les vecteurs propres, donnes dans le couplage LS, montrent le melange de 4f6 6s6p et 4f 55d6s2 des 18 000 K. Les valeurs des parametres d'interaction electrostatique et de spin-orbite ont ete ajustees par la methode des moindres carres.

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.

Magnetic nanorings are the object of increasing scientific interest because they possess the vortex (stray field free) state which ensures lower magnetostatic interactions between adjacent ring elements in high packing density memory devices. In addition, they have other potential applications such as single magnetic nanoparticle sensors, microwave-frequency oscillators and data processing. The stabilization of magnetization state, types of domains and domain wall structures depends on the competing energies such as magnetostatic, exchange and anisotropy. The nucleation/ pinning of domain walls depends on the local inhomogeneity in shape such as roughness, notches etc, which play an important role in stabilizing domain configurations that can be controlled by magnetic field/spin polarized current etc. The information gained by the study of magnetization reversal in the nanoring devices could help in understanding the possible stable magnetization states, which can be incorporated into the development of magnetic logic and recording devices in a NR-based architecture. The magnetization reversal and the stable states in the symmetric cobalt nanorings (NRs) attached with nanowires (NWs) (at diametrically opposite points), is studied through magnetoresistance (MR) measurements by application of in-plane magnetic field (H). Here, a strong in-plane shape anisotropy is introduced in cobalt thin films by patterning them into NR and NWs. The presence or absence of a DW in the device is detected utilizing the AMR property of the material, where the presence of DW leads to a decrease in the resistance of the probed section of the device. It is demonstrated that the magnetization reversal of the device with smaller width, proceeds through four distinct magnetization states, one of these is the stabilized vortex state that persists over a field range of 0.730 kOe. The effect of width (from 70 nm to 1 µm) and diameter (from 2 µm to 6 µm) on the switching behavior is demonstrated. The magnetization states observed in the MR ...

Transition metal dichalcogenides (TMDs) are a new class of two-dimensional layered materials characterized by a MX₂ chemical formula, where M (X) stands for a transition metal (chalcogen). MoS₂, MoSe₂ and MoTe₂ are semiconducting TMDs, which at the monolayer limit possess bandgaps >1 eV, rendering them attractive as possible channel material for scaled transistors. The bandstructures of monolayers feature coupled spin and valley degrees of freedom, thanks to large spin-orbit interaction, and large effective masses (m*), suggesting that electron-electron interaction effects are expected to be important in these semiconductors. In this dissertation we discuss the fabrication and electrical characterization of TMD-based electronic devices, with a focus on their electronic properties, including scattering mechanisms contributing to the mobility, carriers' effective mass, band offset in heterostructures, electronic compressibility, and spin susceptibility. We begin studying the four-point field-effect mobilities of few-layers MoS₂, MoSe₂ and MoTe₂ field effect transistors (FETs), in top-contact, bottom-gate architectures. Using hexagonal boron-nitride dielectrics, we fabricate FETs with an improved bottom-contact, dual-gate architecture to probe transport at low temperatures in monolayer MoS₂, and mono- and bilayer MoSe₂. From conductivity and carrier density measurements we determine the Hall mobility, which shows strong temperature dependence, consistent with phonon scattering, and saturates at low temperatures because of impurity scattering. High mobility MoSe₂ samples probed in perpendicular magnetic field, at low temperatures show Shubnikov-de Haas oscillations. Using magnetotransport we probe carriers in spin split bands at the K point in the conduction band and extract their m* = 0.8m [subscript e]; m [subscript e] is the bare electron mass. Quantum Hall states emerging at either odd or even filling factors are explained by a density dependent, interaction enhanced Zeeman splitting. Gated graphene-MoS₂ heterostructures reveal a saturating electron branch conductivity at the onset of MoS₂ population. Magnetotransport measurements probe the graphene electron density, which saturates and decreases as MoS₂ populates, a finding associated with the negative compressibility of MoS₂ electrons, modeled by a decreasing chemical potential, where many-body contributions dominate. Using a multi-gate architecture in monolayer MoTe₂ FETs, that allows for independent contact resistance and threshold voltage tuning, we integrate reconfigurable n- and p-FETs, and demonstrate a complementary inverter. ; Electrical and Computer Engineering

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.