We investigate the phenomenon of phase organization in charge density waves. Coppersmith and Littlewood [87] have argued that charge density waves become organized into a "minimally stable" state when subject to a pulsed driving force. They have also proposed that the pulse duration memory effect, observed by Fleming and Schneemeyer [86], is evidence for this self organizing behaviour. / We review the microscopic origins of charge density waves, experimental results, and theoretical models of charge density waves. We also review theories of complex systems, and, in particular, the phase organization theory proposed by Tang et al. [87]. We focus on how the phase organization theory applies to the dynamics of charge density waves. / We investigate phase organization in a model of elastically coupled particles subject to a periodic potential and a pulsed driving force. By numerical simulation of the model, we show that the phase organization behaviour is contingent on the existence of a large number of inequivalent metastable configurations in the model. We also show that this model is equivalent to a purely elastic model of charge density waves interacting with impurities. / We further investigate phase organization in a model of charge density waves that has been proposed by Karttunen et al. [99], in which the dynamical generation of phase slips is naturally accounted for. Based on the results of numerical simulations, we argue that phase slips reduce or eliminate the phase organization behaviour of charge density waves by breaking the elasticity of the system.

This thesis concentrates on Magnetic Dissipation Force Microscopy---instrumentation, experiments and theory of the origin of magnetic dissipation. / A home-built vacuum magnetic force microscope (MFM) was debugged. The electronic noise in the system was reduced to below the thermal cantilever noise and the microscope now operates at its theoretical maximum (thermally limited) sensitivity. Then, a new technique, magnetic dissipation imaging, was developed. It allows the imaging of variations of 10-17 W in dissipation with sub-100 nm resolution. A normal MFM image and a magnetic dissipation image can be acquired simultaneously on the same area of a sample. / A theory was developed which correlates the dissipation with micromagnetic structures in domain walls. We consider the energy dissipation through coherent generation of phonons via magnetostriction induced by domain wall width oscillations. A quantitative agreement of theory with experiments for a 110 nm thick Co/Ni multi-layer and a 4 nm thick Co film samples was obtained. This theory predicts two new phenomena: a minimum drive force needed to cause wall width oscillations and wall width resonances. / With the above mentioned microscope, magnetic domain structure, micromagnetic domain wall structure and the associated dissipation have been studied on several samples, including a 30 nm thick Ni80Fe20 patterned into 20 mum squares and a CoPtCr recording medium. The dissipation results show strong correlations with magnetic domain structure. In the Ni80 Fe20 sample, the dissipation signal shows pronounced maxima correlated with the domain wall positions. We suggest magnetoelastic losses and eddy current losses due to wall jumps are the origins of the dissipation. With an in-situ magnetizing stage, we also studied magnetization reversal processes and dissipation hysteresis in the Ni80Fe20 sample. Besides the nucleation and growth of reverse domains, the formation of a 360º wall was observed. The CoPtCr sample shows different dissipation properties with both larger and smaller than average dissipation value observed in the transition regions.

High energy ball milling of the Fe-Ti system is carried out over a wide range of compositions. Milling Fe$ rm sb{50}Ti sb{50}$ and Fe$ rm sb{40}Ti sb{60}$ produces a composite material with amorphous regions and nanometer-sized crystals. Milling Fe$ rm sb{67}Ti sb{33}$ leads to a single-phase amorphous alloy which absorbs hydrogen, in sharp contrast to the intermetallic compound of the same composition. The nanocrystalline samples on their part exhibit a narrowing of the miscibility gap and a reduction of the pressure of the absorption plateau. The change in absorption properties, which is due to the interaction between the nanocystals and the amorphous phase, is analyzed using a simplified model of the elastic stress. Finally, hydrogen is absorbed much faster by nanocrystalline alloys than by conventional materials.

In this dissertation, we review the physics associated with surfaces and interfaces in equilibrium and non-equilibrium. Our emphasis will be on interfaces that are driven far away from equilibrium with special interest in the phenomenon of kinetic roughening. Models which describe non-equilibrium interfaces will be introduced and analyzed using techniques such as the Renormalization Group, Monte Carlo simulations, and direct integration of the equation of motion. Different interface relaxation mechanisms will be discussed with a focus on surface diffusion, which is believed to be the dominant effect in Molecular Beam Epitaxy. These interface growth models generate self-affine structures with various correlations satisfying a dynamic scaling law. We compute the scaling exponents and functions. Finally, we study the effect of quenched impurities on the dynamics of a driven interface with a conservation law. The impurity effect leads to anomalous scaling exponents and qualitatively changes the interface dynamics. Our results are summarized in two articles to be published: Refs. (Govind and Guo, 1992; Govind, Guo and Grant, 1992).

We study the kinetics of domain growth in deeply quenched anisotropic ferromagnets in the presence of a small external field H, using three simulation methods in two dimensions. In particular, we concentrate on the time evolution of the inverse perimeter density, squared magnetization, and structure factor, all of which characterize the domain morphology. The inverse perimeter density evolves as $R sp2$(t,H) $ sim$ t$ sp{n}$, where n = 1 for early time and n = 2 for late time. We characterize the crossover behavior of this growth law and demonstrate that the inverse perimeter density behaves like $R sp2$(t,H) = $ alpha$(H)tf($tH sp2$) where f(x) $ to$ 1 as x $ to$ 0 and f(x) $ to$ x as x $ to$ $ infty$. We further demonstrate that the squared magnetization and the structure factor do not scale, indicating that not all lengths in the problem behave in the fashion mentioned above. An analytical formulation of the problem is also studied with a perturbation theory in the limit H $ to$ 0. The first term is calculated and agrees qualitatively with the computer simulations.

The mixed state of type II superconductors is an ideal medium for the study of correlated systems since the density of vortices which penetrate the sample, as well as the driving force, can be tuned such as to measure their effects on correlations. The weak pinning character of the Fe xNi1-xZr2 metal glasses permits vortex phases to be probed by dissipative transport (longitudinal and Hall) measurements. The complete phase diagram in this regime is mapped out as a function of magnetic field, driving current and temperature using results from longitudinal resistance measurements. The longitudinal measurements show a huge peak effect with a driving force induced pinning phase known to arise from a disordering transition. The Hall resistance measurements lead to remarkable new results: a critical angle dependence of the vortex flow direction when entering or leaving the disordered phase is revealed, which suggests the existence of orientational phase transitions.

Although the study of thermal transport in condensed matter has a very long history, it continues to be an active field of work due to its importance in many applications. The research subject reported in this thesis is on theoretical investigations of thermal energy transport in systems whose linear dimension is less than the wavelength of thermal phonons. Such situations occur in mesoscopic and nanoscopic scale dielectric structures which can now be fabricated in a number of laboratories. Due to the small system dimensions, phonons must be treated as waves. Thermal energy transport, therefore, must be treated as phonon wave propagation through the system. / After reviewing the general physics of thermal energy transport in the classical regime, we derive, for dielectric materials, a formula for thermal energy flux in devices involving multi-terminals each connected to a thermal reservoir at local equilibrium. The energy flux is driven by a temperature bias and traverses the system by virtue of phonon wave scattering. A multi-terminal thermal conductance formula is derived in terms of phonon transmission coefficient. Using our theoretical formulation, we investigate thermal transport properties of both two-terminal and four-terminal dielectric devices by solving the quantum scattering problem using a mode matching numerical technique. / For thermal transport in a T-shaped dielectric nanostructure with two-terminals at low temperature, due to quantum interference the transmission coefficient of phonons becomes quite complicated. We found that the value of phonon transmission coefficients at zero energy may be unity or zero depending on a geometrical ratio of the nanostructure. The transmission has an oscillation behavior with quasi-periodicity and irregularity. The thermal conductance is found to increase monotonically with temperature---a result that we conclude to be generally true for any two-terminal device. We confirm the existence of the universal quantum of thermal conductance which exists at the low temperature limit, and such a quantum is robust against all the system parameters. / The physical behavior of four-terminal thermal conductance for mesoscopic dielectric systems with arbitrary shapes of scattering region is also investigated in detail. If we make a two-terminal measurement in the four-terminal device, the two-terminal conductance is a monotonically increasing function of temperature, and is equal to the universal quantum of thermal conductance masked by a geometric factor. If we make a four-terminal measurement, the four-terminal conductance has a non-monotonic dependence. In the low temperature limit, we predict that the four-terminal conductance diverges inversely proportional to temperature. / Finally, we discuss an interesting theoretical problem on the general behavior of thermal conductance for multi-terminal systems when thermal carriers satisfy fractional exclusion statistics. Our analysis allows us to conclude that results for fractional exclusion statistics are quite different from those of the Bose-Einstein statistics.

The discovery of fullerenes and carbon nanotubes has been very significant to the field of nanotechnology by providing an abundance of stable, highly symmetric, non-reactive, and relatively large molecules that can, in principle, be manipulated one at a time. At the present stage, a theoretical effort should be carried out in order to find and understand novel phenomena in molecule-based nanostructures which could serve as a basis for fabricating useful molecular devices. In this thesis we investigate from first-principles the transport properties of molecular devices: fullerene and carbon nanotube systems. / We begin with charge transport in carbon nanotubes with oxygen, and find that the interaction between oxygen molecules and carbon nanotubes significantly modifies the electronic structure near the Fermi level for both zigzag and armchair tubes. The subtle difference of the adsorption sites of oxygen and the distance between oxygen and nanotubes can cause totally different results of their transport properties. / Then we investigate current flow from the point of view of current density distribution in molecular devices, for current density gives local information of nonequilibrium transport, thereby providing useful and vivid insight to transport properties of molecular electronics. It has been found when an intrinsic carbon nanotube is doped with either a boron or a nitrogen atoms through a replacement of a carbon atom, the local physical properties around the impurity atoms (boron or nitrogen) undergo a significant change, resulting in a dramatic change of the local current distribution. It is suggested that there appears a chiral current flow in the B- and N-doped armchair nanotubes near the impurity. As for a gated C 60 molecular device, the current distribution and the total current flow are both obviously affected by the gate voltage, which indicates the importance of the gate voltage in such a molecular device. / Finally, we discuss the contact effects on transport properties of the molecular devices. We study the effects of the contact geometry as well as the electrode material and find that different orientations of C 60 connected to Au(111) leads can cause significant changes in the current-voltage (I-V) characteristics of such C60 molecular devices. On the other hand, the electrode material is crucial to obtain low resistance ohmic contacts. Our first-principles calculations of transport suggest that Ti has higher affinity for carbide formation. So the choice of proper electrode materials will play an important role in the design of nanoscale devices.

A combined ultra-high vacuum scanning tunneling microscope, atomic force microscope, and field ion microscope UHV (STM/AFM/FIM) system was used to study mechanical and electronic interactions at the atomic scale. A surface science system, consisting of an Auger electron spectrometer, UHV evaporators, ion sputter gun and annealing capabilities, was designed and constructed. A new force sensor preparation method was developed suitable for high stability imaging. Using this improved system, we studied W tip- Au(111) sample interactions in the regimes from weak coupling to strong interaction and simultaneously measured current changes from pA to muA. Close correlation between conductance and interaction forces in a STM configuration was observed. In particular, the electrical and mechanical points of contact were determined based on the simultaneously observed mechanical and electrical response. These points of contact as defined by force and current measurements coincide within measurement error. Ab initio calculations of the current and force as a function of distance in the tunneling regime are in quantitative agreement with experimental results. Simultaneous force-distance and current-distance curves are proven to be essential in understanding processes occurring in scanning tunneling and force microscopies. Finally, the observed contact phenomena and energy dissipation are discussed in the context of nanoelectronics and noncontact atomic force microscopy.

We study theoretically a mechanical oscillator coupled to a superconducting single-electron transistor (SSET), focussing on the regime where incoherent Cooper pair tunnelling in the SSET leads to a negative damping instability of the oscillator. In this regime, large oscillator motion modulates tunnelling in the SSET, which in turn affects the oscillator. This interplay leads to interesting strong feedback effects, including a highly non-thermal stationary oscillator state and a significant enhancement of the low-frequency current noise in the SSET. These effects and others are reminiscent of laser physics: the SSET corresponds to population-inverted atoms while the oscillator corresponds to cavity electro-magnetic field modes. We discuss the extent of this analogy and investigate the linewidth of the oscillator's noise spectrum. Our results are relevant to current experiments, and we point out several feasible measurements that could be done to observe strong feedback effects.