Le pompage optique laser des mélanges gazeux d'hélium peut fournir, dans un champ magnétique faible, de fortes valeurs de polarisation nucléaire. Une technique thermique efficace qui permet la circulation des atomes d'hélium 3 dans la cellule expérimentale rend possible le transfert, en régime stationnaire, de la polarisation nucléaire du gaz à un échantillon liquide à basse température. La relaxation sur les parois de la cellule est efficacement réduite par l'utilisation de césium métallique. Dans de tels échantillons, fortement aimantés et anisotropes, c'est le champ dipolaire qui contrôle l'évolution de l'aimantation. Des modes amortis d'aimantation sont observés par une technique de RMN pulsée. L'analyse détaillée de leur fréquence et de leur taux d'amortissement donne des informations sur la densité d'aimantation et le coefficient de diffusion de spin dans des mélanges polarisés. Les expériences relatées ici sont réalisées à des températures supérieures à 0.2K dans des mélanges dont la concentration en hélium 3 est au moins de l'ordre de quelques pour cents. Quand survient la séparation de phase, la phase riche en hélium 3 renferme une forte densité d'aimantation. Des résultats préliminaires concernant l'observation de la séparation de phase et la mesure du potentiel chimique de l'hélium 4 sont également donnés dans cette thèse.

Cette thèse présente une étude expérimentale de la dynamique de mouillage d'un film d'4He liquide superfluide sur une surface de césium, lors de la traversée en température de la transition de prémouillage. Pour imager le film d'hélium, nous avons adapté aux basses températures une méthode de contraste interférentiel différentiel, qui donne accès selon une direction au gradient de couverture locale d'hélium. Les couches de césium ont été fabriquées par évaporation in situ à froid. Elles montrent des angles de contact de l'hélium équivalents à ceux des autres groupes utilisant le même procédé de fabrication. L'épaisseur de films d'hélium a été mesuré en fonction de la désaturation sur ces surfaces et est compatible avec les valeurs attendues. Suivant les surfaces de césium, deux types de comportement du film durant la transition de prémouillage ont été observés. Sur un premier type de surface, l'envahissement par le film s'effectue en passant par un état intermédiaire qui apparaît comme un mélange de surface sèche et mouillée à une échelle inférieure à la résolution. Nous n'avons eu accès alors qu'à la proportion de surface sèche à l'avant du film, moyennée sur la tache de diffraction. L'évolution de la couverture moyenne avec la température peut s'interpréter dans le cadre du modèle d'Ising à champ aléatoire à température nulle sans nucléation, avec un paramètre de désordre important. Sur un deuxième type de surface, les longueurs de corrélation du film sont bien plus importantes et l'invasion de la surface par le film s'effectue par une avancée observable du bord du film. Celle-ci se fait à température constante, le film progressant par avalanches successives activées thermiquement. La vitesse d'avancée du front du film a été mesuré en fonction de la température et de l'épaisseur du film.

Nanostructure systems possessing certain desirable features can arise from the self-organization of fundamental building blocks. In this thesis we explore two types of controlled self-assembly mechanisms in hetero-epitaxy: (a) classical assembly of atom vacancies into quasi one-dimensional line structures and (b) quantum-driven assembly of atoms into atomically-smooth two-dimensional thin films. In the classical assembly phenomenon, adatom vacancies, created via elastic strain-relaxation in compressively strained atom chains on a silicon substrate, self-organize into meandering vacancy lines. The average spacing between these line defects can be varied by adjusting the chemical potential μ of the adsorbed atoms. We implemented a lattice model that quantitatively connects density functional theory calculations for perfectly ordered structures to the fluctuating disorder seen in experiment and the experimental control parameter μ. The quantum-mechanical thin-film assembly explored in this thesis has an electronic origin. It is made possible by strong quantum size effects at the nanoscale and can be controlled experimentally by tuning the quantum mechanical boundary conditions and free carrier density of an ultrathin metal film. This is accomplished via atomic-scale template modification and chemical doping, respectively. Our investigations focused on the formation and structure-property relationship of these engineered quantum films, and specifically on the emergence of collective phenomena such as superconductivity and plasmon excitations. We succeeded in growing atomically-smooth Pb1-xGax (x = 0.06) alloy films on a Si(111)-7 × 7 substrate through quantum confinement, a remarkable observation because Pb and Ga are totally immiscible in the bulk. The resulting films exhibit large uniform-depth holes which turn out to be responsible for the exceptionally large critical current density in these films. Remarkably, the critical current density increases with temperature up to 3.25 K, a phenomenon that has not been seen before and that can be attributed to the unusual quantum-growth morphology of this material. The alloying experiments furthermore elucidate the likely origin of the Tc suppression generally observed in thin films. Finally, we demonstrate the existence of quantized plasmon modes in ultrathin metal films. Controlled self-organization experiments thus enable stabilization of novel nanophase materials, which in turn leads to discovery and understanding of novel collective properties.

Phase transitions in a number of models related to hightemperature superconductors are investigated, using scalingmethods and Monte Carlo simulations. This thesis considers twomain topics. The first topic is phase transitions, phase diagrams, andvortex motion in high temperature superconductors at finitetemperature, subject to magnetic fields and disorder. We studya vortex glass model at finite temperature, with stronguncorrelated vortex pinning and a magnetic field. We find thatthe vortex glass exists at finite temperature and calculate thecritical exponents of the transition. We also investigate hightemperature superconductors with columnar disorder in zero andapplied magnetic fields. Some of these studies are alsorelevant for the superfluid to Mott insulator transition ofbosons in two dimensions. We find that the unscreened Boseglass transition belongs to a new universality class. Wecalculate the critical exponents of the superconductingtransition with columnar defects in zero applied magneticfield. The transverse Meissner transition is studied, and wefind an exotic universality class with a correlation volumethat is infinitely anisotropic in all directions. The second topic is confinement-deconfinement transitions incompact Abelian Higgs models. We develop a new order parameter,related to a large Wilson loop for fractionalized charges, anduse it to study the concept of topological order. Thesetransitions may be relevant for strongly correlated electronsin two dimensions.

The growth of vertically aligned arrays of single-walled carbon nanotubes (SWNT) has provided an efficient route toward production of highly aligned SWNT that can be grown ultra-long. Here, the growth of such aligned SWNTs is demonstrated from a thin e-beam deposited catalyst layer composed of Fe-Mo with ratio (5:1) respectively, supported by Al2O3. Using C2H2 decomposition in a hot filament chemical vapor deposition apparatus, experiments indicate that the order of deposition of the respective Fe and Mo catalyst components significantly affects growth characteristics, especially evident during growth at elevated reaction pressures under high carbon flux. The role of temperature and pressure on features of the nanotube arrays, such as height, alignment, quality, volumetric density, and diameter distribution are compared for each case of Fe/Mo and Mo/Fe considered. In order to better understand this effect, atomistic modeling using the Bozzolo-Ferrante-Smith (BFS) method for alloys is employed along with the Monte Carlo-Metropolis method. The dependence of the growth on the order of co-catalyst deposition is observed to be a structural effect that can be explained in a straight-forward interpretation of the BFS strain and chemical energy contributions toward the formation of Fe-Mo catalyst prior to growth. The competition between the formation of metastable inner Mo cores and clusters of surface-segregated Mo atoms in Fe-Mo catalyst particles influences catalyst formation, and the role of Mo concentration and catalyst particle size is found to also be a factor influencing this formation process. Finally, this modeling procedure is demonstrated as a general technique that can be employed to study the structural stability and formation of binary catalyst particles in a reducing environment-providing a potentially cheap and efficient method for catalyst design.

Single-walled carbon nanotubes (SWNTs) are fascinating materials to study one-dimensional photophysics. Their optical properties are strongly affected by strong Coulomb interactions and are determined by "excitons" which represent the quantum of polarization in non-metallic solids. In this thesis dissertation we have experimentally investigated both the structure and the dynamics of excitons in non-metallic SWNTs. In particular, we have performed micro-photoluminescence spectroscopy of individual semiconducting SWNTs at low temperature to study their intrinsic optical properties and investigate the excitonic fine structure. Using magnetic field parallel to the tube axis we were able to directly observe theoretically predicted dark states for the first time in SWNTs. In addition, we found that the inter-valley and exchange energy, which determines the energy separation between the dark and the bright state, to be very sensitive to the surrounding environment of the nanotube. We have also studied the temperature dependent lineshape of SWNT photoluminescence in order to gain insight into the dynamics of exciton-phonon interaction, finding evidence for acoustic phonon scattering. For the rest of this thesis dissertation, we have developed a model based on reaction-diffusion processes to theoretically explain the observation of photoluminescence saturation in SWNTs. Our model shows that efficient exciton-exciton annihilation under high pumping conditions can explain this observed behavior quantitatively.

With a multitude of different phases and quantum critical points, heavy fermion materials should reign supreme as the prototype for competing order, a major contemporary theme in condensed matter physics. One key feature that differentiates the types of magnetic phases and critical points is the presence or absence of Kondo screening. This singlet formation is dramatically manifested in the Fermi surface, which may or may not include atomic f-orbital electron states. To provide a theoretical basis for the different types of magnetism, we have carried out asymptotically exact studies of the Kondo lattice model inside both the antiferromagnetic and ferromagnetic phases. A fundamental aspect of the approach is to map the magnetic Hamiltonian for the f-orbitals onto a quantum nonlinear sigma model (QNLsigmaM). The Kondo interaction results in an effective coupling between the QNLsigmaM fields and the conduction electrons. Renormalization group analyses show that the Fermi surface in the corresponding ordered states is small (not incorporating the f-orbitals) for both the ferromagnetic and antiferromagnetic cases. These results are of relevance to a number of materials, including YbRh2Si2 and CeRu2Ge2, where experimental measurements of magnetotransport and de Haas van Alphen effects have supplied evidence for small Fermi surface phases. The implications of our results for heavy fermion quantum critical points will also be discussed.

Two-dimensional electron (2DES) and hole (2DHS) systems have attracted intense research attentions in past decades. A 2DES or 2DHS modulated by one-dimensional or two-dimensional spatially periodic potential shows particular importance because the existence of modulation provides a tunable parameter for exploring interaction between electrons and scattering centers presenting on the two-dimensional systems. This thesis documents a systematic experimental study, in collaboration with Bell Labs, of electronic transport in very-high mobility 2DES and 2DHS in GaAs/AlGaAs quantum structures. Fabrication of triangular antidot lattice in 2DES, as well as low-temperature transport and photoconductivity properties in spatially modulated 2DES, has been studied. Strong Geometric resonance (GR), up to seven peaks resolved, is observed in the longitudinal magnetoresistance because of high mobility of 2DES after fabrication of antidot lattice. Photoresistance shows clear millimeterwave-induced resistance oscillations (MIRO) but with heavily damping amplitudes, and magnetoplasmon resonance (MPR) is also observed as well. GR, MIRO and MPR are decoupled from each other in our modulated 2DES. These experimental findings pave the way for studies of nonlinear transport in modulated 2DES. Magnetotransport measurements on a new material, the Carbon delta-doped 2DHG in GaAs/AlGaAs quantum well, indicate that the 2DHG has a transport scattering time compatible with those in very-high mobility 2DES. However, photoresistance measurement shows much weaker MIRO in the 2DHS than that in 2DES with compatible transport scattering time. Low-temperature transport measurements on Landau, Zeeman, and spin-orbital parameters imply that the C-doped 2DHS has small zero field spin splitting and large effective g-factor. As part of the thesis work, the thesis also presents a development of low-temperature/high magnetic field (300mK/12T) scanning Hall probe microscope (SHPM) technique for measuring small local magnetic fields at low temperature and an algorithm for calculating the current density from measured magnetic fields based on Fourier transformation technique. Integration of SHPM and the algorithm provides a practical tool for imaging the current distribution and a powerful method to explore electronic transport properties of 2DES and 2DHS.

Carbon allotropes such as nanotubes, graphene, and buckyball all preserve the same lattice structure. However, their electronic properties are very different depending on their dimensionality. These nanostructures can be functionalized with other chemical groups or can be used to form complicated molecular structures. By using scanning tunneling microscopy and Raman spectroscopy, the functionalization of carbon nanotubes with fluorine was studied as a potential route to tailoring the electrical and chemical properties of carbon nanotubes, and functionalization and exfoliation techniques of graphite without inducing basal plane defects were investigated. Also, building upon our previous research on nanocars, nanodragsters combining buckyball and p-carborane wheels were studied with scanning tunneling microscopy. Unlike its predecessors, a nanodragster can show interesting motions even at room temperature as well as at elevated temperatures.

Through polarization-dependent magneto-optical absorption spectroscopy, the magnetic susceptibility anisotropy for metallic single-walled carbon nanotubes has been extracted and found to be up to 4x greater than values for semiconducting single-walled carbon nanotubes. Consistent with theoretical predictions, this is the first experimental evidence of the paramagnetic nature arising from the Aharonov-Bohm-phase-induced gap opening in metallic nanotubes. We also compare our values with previous work for semiconducting nanotubes, which confirm a break from the prediction that the magnetic susceptibility anisotropy increases linearly with the diameter.