Rb-IV is the stable high-pressure phase of rubidium between 16 and 21 GPa. The structure of Rb-IV has long been known to be complex, but it is only recently that it has been solved as being an incommensurate host-guest composite structure, comprising a tetragonal host framework containing chains of "guest" atoms that form structures incommensurate with the host along their common c- axis. While similar composite structures have been observed in a number of other elemental metals, Rb-IV is unique in that on pressure decrease below 16.7 GPa at 300 K, the chains of guest atoms become disordered and liquid-like. This thesis is a detailed structural study of Rb-IV. High-pressure, combined with high-temperature powder diffraction techniques, have been used to map the P-T phase diagram of rubidium between 15 GPa and 20 GPa and between 298 K and 600 K. The results show that the guest order-disorder transition pressure is strongly temperature dependent, and that the disordered phase is observed to the highest temperatures. Technical developments, which have made it possible to extract reliable modulation reflection intensities from a Rb-IV single crystal, are described. The resulting data are used for a full modulated structure refinement of Rb-IV, revealing a saw-tooth shaped modulation of the guest structure, from which new information on the host-guest interactions has been extracted. Inelastic X-ray scattering techniques have been used to measure the longitudinal acoustic (LA) phonons in a Rb-IV single crystal. Two LA-like phonon branches, one for each of the two composite subsystems, are observed along the common c-axis. The sound velocities in the host and guest structures are determined and the pressure dependence is shown to differ by a factor of two. Finally, developments that will enable future combined high-pressure high- temperature single-crystal diffraction studies, and single-crystal diffraction studies at pressures above 100 GPa, will be presented.

<p> Recently the theoretical discovery of single channel quantum dragons has been reported. Quantum dragons are a class of nanodevices that may have strong disorder but still permit energy-independent total quantum transmission of electrons. This thesis illustrates that multi-channel quantum dragons also exit in rectangular nanotubes and provide an approach to construct multi-channel quantum dragons in rectangular nanotubes. Rectangular nanotube multi-channel quantum dragons have been validated by matrix method based quantum transmission calculation. This work could pave the way for constructing multi-channel quantum dragons from more complex nanostructures such as single-walled zigzag carbon nanotubes and single-walled armchair carbon nanotubes.</p>

<p> Studies were performed on two seemingly different topics, molecular thin films on graphite/graphene and metal induced changes in various cuts of silicon (Si) surfaces. However, both projects share the underlying theme of self-assembly. Since nature can rely upon self-assembly at the nano-scale, all that is needed is to discover functional means to create components for integrated circuits as well as electronic and photonic devices. </p><p> Scanning Tunneling Microscopy and Spectroscopy (STM/STS) studies were carried out to characterize the morphology of thin porphyrin films on graphite and the effects of Zn-Phthalocyanine (Zn-Pc) adsorption on the electronic properties of graphene. It was found that the metal atom complex of porphyrin molecules can determine the morphology, intermolecular forces and ability to create thin films on a graphite surface. Zn-Pc adsorption onto graphene shifts the position of the Dirac point with respect to Fermi level which leads to localized p- and n-type doping effects in the graphene substrate. </p><p> STM, STS and Low-Energy Electron Diffraction (LEED) measurements were carried out on iridium (Ir) modified Si(111) and Si(100) surfaces. The Ir-modified Si(111) surface exhibited a &radic;7&times;&radic;7 <i>R </i>19.1&deg; domain formation that was composed of Ir-ring clusters. LEED measurements showed that on Ir-modified Si(100), a <i>p</i>(2&times;2) structure arose after annealing at ~700&deg;C. The proposed model for the Ir-silicide nanowires shows that an Ir atom replaces every other Si dimer along the Si dimer rows of Si(100)-2&times;1.</p>

<p> This thesis reports on measurements of the switching distributions in Co-Ni nanopillars with perpendicular magnetic anisotropy. The Co-Ni nanopillars are incorporated into a spin-valve device - a two terminal device consisting of two ultrathin (1-3 nm) Co-Ni ferromagnets separated by a thin (4 nm) Cu spacer patterned into ellipses and circles with lateral sizes ranging from 40-300 nm. Magnetic fields applied along the uniaxial anisotropy axis can switch the alignment of the constituent ferromagnetic layers between anti-parallel and parallel. Electric currents flowing can also switch the nanopillar through the spin-transfer torque effect - an electric current transfers spin-angular momentum from conduction electrons to the background magnetization of a ferromagnet, ultimately manifesting as a torque on the magnetization. </p><p> Lateral geometry effects were studied on nanopillars with notches along the perimeter. Switching field measurements revealed an asymmetry between the anti-parallel (AP) to parallel (P) and P to AP switching field distributions. A phenomenological model that considers the spatially inhomogeneous dipole field from the polarizing layer explains this asymmetry. </p><p> In nanopillars with an 80 nm circular diameter, switching field measurements taken in a cryostat reveal non-uniform magnetization configurations during reversal. At the lowest temperatures (12 K), the transition between uniform states (P to AP) shows three consecutive hysteretic jumps. The thermal stability of the transition states was investigated for temperatures between 12 K and room temperature. </p><p> The thermally activated magnetization reversal model by N&eacute;el and Brown was tested on 75 nm diameter spin-valves between 20 and 400 K. The temperature dependence of the statistics of switching reflects enhanced thermal fluctuations and cannot be modeled by the N&eacute;el expression for the energy barrier. Taking into account the implicit temperature dependence of the energy barrier from the saturation magnetization and perpendicular anisotropy energy explains this discrepancy. </p><p> The effective barrier model for spin-torque thermally-activated switching of Co-Ni nanopillars was investigated. We extracted an effective energy barrier height for switching field distributions under several dc currents. The results mostly agree with the prediction that the current modifies the barrier height. However, rare switching events at the tails of the distributions reveal qualitative deviations from this model.</p>

<p> The fractional quantum Hall effect (FQHE) is the archetype of the strongly correlated systems and the topologically ordered phases. Unlike the integer quantum Hall effect (IQHE) which can be explained by single-particle physics, FQHE exhibits many emergent properties that are due to the strong correlation among many electrons. In this Thesis, among those emergent properties of FQHE, we focus on the guiding-center metric, the guiding-center Hall viscosity, the guiding-center spin, the intrinsic electric dipole moment and the orbital entanglement spectrum. </p><p> Specifically, we show that the discontinuity of guiding-center Hall viscosity (a bulk property) at edges of incompressible quantum Hall fluids is associated with the presence of an intrinsic electric dipole moment on the edge. If there is a gradient of drift velocity due to a non-uniform electric field, the discontinuity in the induced stress is exactly balanced by the electric force on the dipole. </p><p> We show that the total Hall viscosity has two distinct contributions: a "trivial'' contribution associated with the geometry of the Landau orbits, and a non-trivial contribution associated with guiding-center correlations. </p><p> We describe a relation between the intrinsic dipole moment and "momentum polarization'', which relates the guiding-center Hall viscosity to the "orbital entanglement spectrum(OES)''. </p><p> We observe that using the computationally-more-onerous "real-space entanglement spectrum (RES)'' in the momentum polarization calculation just adds the trivial Landau-orbit contribution to the guiding-center part. This shows that all the non-trivial information is completely contained in the OES, which also exposes a fundamental topological quantity &gamma; = c&tilde; &minus; &nu;, the difference between the "chiral stress-energy anomaly'' (or signed conformal anomaly) and the chiral charge anomaly. This quantity characterizes correlated fractional quantum Hall fluids, and vanishes in integer quantum Hall fluids which are uncorrelated.</p>

The motivation of the present work is our research on a cantilever-based chemical sensor. We are addressing the question of the source of the surface stress on a gold-coated cantilever due to alkanethiol adsorption. By simultaneously measuring layer thickness and cantilever stress during alkanethiol monolayer self-assembly, the aim is to gain some insight on where the surface stress comes from. The technique we used to monitor layer thickness is ellipsometry. The ellipsometer is used to measure, in situ, the formation dynamics of self-assembled alkanethiol monolayers under different conditions. The thickness evolution is interpreted in terms of monolayer phases (lying-down and standing-up). During these experiments, the ellipsometer is proven to have Angstrom resolution. Following our objectives, a combined stress-thickness set-up is designed and built. Preliminary results are presented while further investigations are currently being made.

Magnetic quantum cellular automata (MQCA) have been proposed as an alternate computing architecture. Single domain magnetic particles represent "1" or "0"; their stray field interaction controls the propagation and manipulation of information. An inherent requirement for an MQCA system is to know the conditions under which nanomagnets switch between the purely "up" (1) and the purely "down" (0) state, and to control this reproducibly. / As a first step to study this, arrays of two types of permalloy particles were designed, simulated, fabricated and imaged, and their switching distributions ascertained. Individual particles were "peanut"-shaped, to investigate the effect of a shape anisotropy for an elliptical particle. Particles had long axes of 750 nm and 250 nm, but had identical aspect ratios. / Particles were simulated with a public domain software package, Object Oriented Micromagnetic Framework (OOMMF), fabricated by electron beam lithography with standard lift-off techniques in the fabrication facility in Sherbrooke, Canada, and imaged in vacuum using a custom built magnetic force microscope in constant height mode with an in plane, in-situ magnetic field. Ensemble hysteresis loops were obtained as was the average switching fields for both arrays. / The 750 nm particles were found experimentally to have a two-step switching process. The first switch occurred at 60 +/- 16 Oe and the second at 130 +/- 56 Oe. These results were nominally better than those obtained in a previous study on similarly sized ellipses. / Simulations on the 250 nm particles predicted that particles of that size would have the single domain configuration as their virgin state, and would have a one-step switching process. The switching field of a typical particle was calculated to be 550 +/- 30 Oe. This was confirmed experimentally, where the switching field distribution had its peak at 490 +/- 40 Oe. Thus, theory and experiment are in agreement, within error.

The epitaxial growth of ultrathin films of NaCl was achieved on a Cu(100) substrate at room temperature. The growth mode was observed to be of the Stranski-Krastanov type using non-contact AFM, with well-oriented square islands growing on top of the first monolayer of NaCl. The frictional properties of different monolayers were measured using contact mode AFM. The friction contrast between the different monolayers of NaCl was found to be too small to be measured within the noise. Atomic stick-slip measurements were performed on a NaCl island. The results are discussed in comparison with relevent literature on thin films of alkali halides.

The surface stress induced during the formation of alkanethiol [HS(CH 2)nCH3] self-assembled monolayers (SAMs) on gold from the vapor phase was measured using a differential cantilever-based sensor. This custom-built system is capable of surface stress measurements with a sensitivity of 5 x 10-5 N/m using commercially-available atomic force microscopy cantilevers. A second system combining cantilever-based sensing and ellipsometry was also designed and built, capable of yielding simultaneous in situ surface stress and film thickness measurements. Scanning tunneling microscopy (STM) with molecular resolution was also performed ex situ in order to characterize the structure of the resulting SAMs. The complementary use of these tools has provided an all-around view of the self-assembly process. / These measurements were performed in order to gain insight into the mechanisms involved in the self-assembly process and into the origins of the associated surface stress. Moreover, these studies were used to characterize and optimize the response of cantilever-based sensors based on functionalized SAM technology in terms of reliability, sensitivity, and reproducibility. / The evolution of the surface stress induced during alkanethiol SAM formation reveals features associated with coverage-dependent structural phase transitions. These results show that both the kinetics of SAM formation and the resulting SAM structure are strongly influenced by the surface structure of the underlying gold substrate, by the impingement rate of the alkanethiol molecules onto the gold surface, and by the cleanliness of the gold surface. In particular, it was found that a minimum gold grain size is necessary in order for the SAM to achieve the standing-up phase, for which large compressive surface stresses (~10 N/m) are measured. In addition, these results show that alkanethiol SAMs can become kinetically trapped in metastable intermediate states (lying-down phase) for formation on small-grained gold surfaces and/or at low alkanethiol vapor concentrations. Theoretical modeling of the origins of the induced surface stress reveals that inter-molecular Lennard-Jones interactions and electrostatic repulsion between adsorbed species play minimal roles in the development of the surface stress. Changes in the electronic structure of the underlying gold substrate are more likely to account for the large compressive surface stresses observed during alkanethiol SAM formation.

Electrochemically-induced changes in surface stress at the solid-liquid interface are measured using a differential cantilever-based sensor. The simultaneous, in situ measurements of the current (charge) and interfacial stress changes are performed by employing an AFM cantilever as both the working electrode (in a conventional three-probe electrochemical cell configuration) and as the mechanical transducer (bending of the cantilever). The custom-built instrument achieves a surface stress sensitivity of 1x10-4 N/m and a dynamic range of 5x105. Combining electrochemistry with cantilever-based sensing provides the extra surface characterization capability essential for the interpretation of the origin of the surface stress. / The objective of the present study is to gain a better understanding of the mechanisms responsible for the nanomechanical motion of cantilever sensors during adsorption and absorption processes. The study of these simple model systems will lead to a general understanding of the cantilever-based sensor's response and provide insights into the physical origin of the measured surface stress. / The surface stress generated by the electrochemically-controlled absorption of ions into a thin polypyrrole film is investigated. A compressive change in surface stress of about -2 N/m is measured when the polymer is electrochemically switched between its oxidized and neutral (swollen) state. The volume change of the polymer phase with respect to the gold-coated cantilever is shown to be responsible for the mechanical motion observed. / The potential-induced surface stress and surface energy change on an Au(111)-textured cantilever, in a 0.1 M HClO4 electrolyte, are simultaneously measured. These measurements revealed that for solid electrodes these two thermodynamic parameters are significantly different. In the double layer region, a surface stress change of -0.55 +/-0.06 N/m is measured during ClO4- adsorption whereas the surface energy variation is smaller by one order of magnitude. The origin of the surface stress change at the metal-electrolyte interface is understood by the variation in electron density at the surface which alters the inter-atomic bonds strength between surface atoms, while the specificity of adsorption of ions is found to be mostly responsible for the fine structure of the surface stress profile.