Gotowa bibliografia na temat „Electron gas states”

This thesis is based on two low temperature experiments in spintronics - physics and engineering of electronic spins. The measurements were performed on a GaAs/AlGaAs two-dimensional electron gas with geometries defined by tunable surface gates. The first experiment is about detection of electrons in a quantum dot. A quantum point contact (QPC) and a quantum wire (QW) is coupled to a single-lead few-electron quantum dot. By measuring the conductance of the QPC and the QW, one can gain information on the average number of electrons in the dot as well as energy-level structure of the dot. The second experiment investigates anisotropy of spin-orbit interaction in GaAs/AlGaAs heterostructure by measuring spin polarization in a narrow channel. Polarized electrons are injected into the channel through a spin-selective injector QPC and diffuse towards the end of the channel. This diffusion generates a pure spin current and the spin polarization 25 microns away is measured by a detector QPC. A periodic spin-orbit field induced by motion of the electrons in the channel causes the spins to resonate with external magnetic field. Spin-orbit anisotropy is demonstrated by the different resonance strength observed in channels aligned along two different crystal axes.

The continuing refinement of crystal growth techniques has made possible the fabrication of semiconductor superlattices where the period can be as small as one lattice constant. Prediction of many of the properties of such systems requires a detailed description of their electronic structure. In this thesis, a self-consistent pseudopotential method which includes a parametrization scheme has been used to calculate the electronic properties of (GaAs)n(AlAs)n superlattices with n ranging from 1 to 4. The parametrization scheme is used to reproduce energy gaps at the principal symmetry points for the bulk constituents and the resulting parameter set is employed in all subsequent calculations. The n=l superlattice is found to be indirect with the conduction band minimum at R (equivalent to the zincblende L point) and all the thicker systems are pseudodirect in good agreement with experimental results. The lowest conduction band state at the zone centre for all systems is found to be mainly X-derived reflecting the importance of zone translating effects here. By analysing the states near to the band edges, the observed pattern of confinement in states of the n=l superlattice shows the band offsets to have at most a small role, in contrast to the thicker systems where a definite relationship was established. Moreover, the results suggest that Dingle's "15% rule" is consistently violated and that a valence band offset of about 30-40% is obtained which changes little with layer thickness. Attempts to study the effects of hydrostatic pressure on the n=3 superlattice were in part successful and predicted quite complex behaviour for the electronic states. Much of the discrepancy between the results obtained and the experimental data was attributed to the inadequacies of the empty-core pseudopotential to model the ions.

Experimental physics in the low temperature limit has consistently produced major advances for condensed matter research. Likewise, scanning probe microscopy offers a unique view of the nanometer scale features that populate the quantum landscape. This work discusses the merger of the two disciplines via the development of the Ultra Low Temperature Scanning Probe Microscope, the ULT-SPM. We focus on the novel characterization of an exotic condensed matter system: a deeply buried two dimensional electron gas with a cleaved edge overgrowth geometry. By coupling the dynamics of the force sensing probe microscope to the electrostatics of the electron gas, we can remotely and non-invasively measure charge transport features which are normally only observable using physically contacted electrodes. Focusing on the quantum Hall regime, we can exploit the high sensitivity of the local force sensor to study spatially dependent phenomena associated with electronic potential distributions. The instrument shows promise for many exciting experiments in which low temperatures, high magnetic fields, and local measurements are critical.Designed for operation at 50 mK, in magnetic fields reaching 16 T, many components of the instrument are not commercially available and were therefore designed and constructed in- house. As such, the intricate details of its design, construction and operation are documented thoroughly. This includes: the microscope assembly, the modular components such as the scan head and coarse motors, the electronics developed for controlling the instrument, and the general integration into the low temperature infrastructure. A quartz tuning fork is used as the force sensor in this instrument, enabling a wide selection between different modes of operation, the most relevant being electrostatic force microscopy. Noise limits are investigated and matched sources of experimental noise are identified. Detailed schematics of the instrument are also included.<br>La physique expérimentale aux limites des basses températures contribue constamment à des percées majeures dans le domaine de la matière condensée. Pour sa part, la microscopie à balayage de sonde offre la possibilité unique d'observer les éléments nanométriques qui car- actérisent le paysage quantique. Ce projet allie les avantages de ces deux disciplines par le développement d'un microscope à balayage de sonde opérant à très basse température (Ultra Low Temperature Scanning Probe Microscope), le « ULT-SPM. » Nous étudions en particulier un système exotique de la matière condensée : un gaz d'électrons bidimensionnel profondément enfoui, comportant une croissance latérale sur le bord clivé. Le couplage des forces dynamiques de la sonde du microscope et électrostatiques du gaz à électrons, nous permet de mesurer à distance et de façon non invasive, les caractéristiques de transport des charges, qui ne sont normalement observables qu'à l'aide d'électrodes et donc, par un contact physique. Dans le régime de l'effet Hall quantique, nous pouvons exploiter la grande sensibilité du capteur de force local pour étudier des phénomènes spatiodépendants associés aux distribu- tions de potentiel électronique. L'instrument se révèle prometteur pour la poursuite de nom- breuses expériences passionnantes où les conditions de basse température, champ magnétique élevé et mesures locales sont essentielles. Comme il est conçu pour fonctionner à 50 mK et sous un champ magnétique pouvant at- teindre 16 T, plusieurs composantes du microscope ne sont pas disponibles commercialement et ont donc été entièrement conçues et fabriquées sur place. Les détails intrinsèques de la con- ception, de la construction et du fonctionnement sont ainsi documentés à fond. Ceci inclut : l'assemblage du microscope, les composantes modulaires comme la tête de balayage et les mo- teurs, l'électronique des contrôles de l'instrument et l'intégration à l'infrastructure opérant à basse température. Dans cet instrument, un diapason de quartz fait office de capteur, ce qui permet une grande flexibilité quant aux différents modes d'opération, le plus utile étant la mi- croscopie de force électrostatique. Les limites de bruit sont étudiées et comparées aux sources de bruit expérimentales. Les schémas détaillés de l'instrument sont également inclus.