<p>Extensive ⁷⁵As nuclear magnetic resonance (NMR) studies were conducted on a variety of 122 iron-based superconductors. NMR frequency swept spectra and the spin-lattice relaxation rate (<i>T</i>₁^-1) were measured in CaFe₂As₂ as a function of temperature. The temperature dependence of the internal hyperfine field was extracted from the spectra, and <i>T</i>₁^-1 exhibits an anomalous peak attributed to the glassy freezing of domain walls associated with filamentary superconductivity. The field dependence of <i>T</i>₁^-1 and subsequent bulk resistivity and magnetization measurements also show signatures of filamentary superconductivity nucleated at antiphase domain walls. Systematic doping-dependent NMR studies were also carried out on Ni- and Co-doped BaFe₂As₂. In the Ni-doped variant, local magnetic inhomogeneities were observed via field swept NMR spectral analyses, and the doping dependence of the N&eacute;el temperature T_N was confirmed by fits to (<i>T</i>₁T)^-1(T). Spectral wipeout and stretched exponential relaxation behavior in the Co-doped variant reveal inhomogeneous behavior and the emergence of a cluster spin glass state. The NMR measurements bring into question the details of the phase transition from coexisting antiferromagnetism and superconductivity to pure superconductivity.

<p> This dissertation focuses on the problem of increasing the lifetime of wireless sensors. This problem is addressed from two different angles: energy harvesting and data compression. Energy harvesting enables a sensor to extract energy from its environment and use it to power itself or recharge its batteries. Data compression, on the other hand, allows a sensor to save energy by reducing the radio transmission bandwidth. </p><p> This dissertation proposes a fractal-based photodiode fabricated on standard CMOS process as an energy harvesting device with increased efficiency. Experiments show that, the fractal based photodiodes are 6% more efficient compared to the conventional square shaped photodiode. The fractal shape photodiode has more perimeter-to-area ratio which increases the lateral response, improving its efficiency. </p><p> With increased efficiency, more current is generated but the open-circuit voltage still remains low (0.3<i>V</i>&ndash;0.45<i>V</i> depending on illumination condition). These voltages have to be boosted up to higher values if they are going to be used to power up any sensory circuit or recharge a battery. We propose a switched-inductor DC-DC converter to boost the low voltage of the photodiodes to higher voltages. The proposed circuit uses two on-chip switches and two off-chip Components: an inductor and a capacitor. Experiments show a voltage up to 2.81V can be generated from a single photodiode of 1mm² area. The voltage booster circuit achieved a conversion efficiency of 59%. </p><p> Data compression was also explored in an effort to reduce energy consumption during radio transmission. An analog-to-digital converter (ADC), which can jointly perform the tasks of digital conversion and entropy encoding, has also been proposed in this dissertation. The joint data conversion/compression help savings in area and power resources, making it suitable for on-sensor compression. The proposed converter combines a cyclic converter architecture and Golomb-Rice entropy encoder. The converter hardware design is based on current-mode circuits and it was fabricated on a 0.5 &mu;m CMOS process and tested. Experiment results show a lossless compression ratio of 1.52 and a near-lossless compression of 5.2 can be achieved for 32 &times; 32 pixel image.</p>

<p> This thesis describes experimental studies of magnetization dynamics in both spin valves (SVs) and magnetic tunnel junctions (MTJs) subject to spin-polarized currents. A spin-polarized electrical current can transfer its angular momentum to a ferromagnet through a spin-transfer torque (STT), resulting in intriguing magnetization dynamics such as the reversal of the magnetization direction, precession and relaxation. </p><p> The ferromagnetic systems investigated were nanopillars, tens to hundreds of nanometers in cross section and a few nanometers in thickness, which were further integrated into SV or MTJ structures. </p><p> The magnetization switching and relaxation studies were performed on all-perpendicularly magnetized SVs. The switching probabilities were investigated for different pulse conditions at room temperature, where thermal fluctuations can play an important role. The pulse duration was varied over 10 orders of magnitude, from the fundamental timescales of magnetization precessional dynamics, 50 ps, to 1 s. Three switching regimes were found at different timescales. In the short-time regime, the switching probability was mainly determined by the angular momentum transfer between the current and the magnetization. In the long-time regime, the switching becomes thermal activation over an effective energy barrier modified by the STT. In the crossover regime, both spin-transfer and thermal effects are important. </p><p> The magnetization relaxation was studied by a two-pulse correlation method, where the relaxation time is measured by the interval between the two pulses. The thermal effects were shown to be important even at nanosecond time scales. </p><p> The switching and precession of magnetization were also studied in structures where a perpendicular spin polarizing layer is employed with an in-plane magnetized MTJ. When subject to pulses, the initial STT from the polarizer to the free layer is perpendicular to the free layer plane. For a large enough STT, this tilts the free layer magnetization out of the plane to create a large demagnetization field, typically at tens or hundreds of millitesla. This demagnetization field then becomes the dominant magnetic field acting on the free layer, leading to the precession of its magnetization. This magnetization precession was observed through real-time device resistance measurements, where precessions with hundreds of picoseconds are found from single current pulse stimuli. </p>

<p> Human perception of the visual world is limited through the homogeneity of design and the standardization of materials. After constructing a lighting apparatus made of steel and thousands of transparent thread, a small amount of light will be directed onto the apparatus and reflected and refracted multiple times, spreading light over a large area. However, visual perception of the light reflecting and refracting through the apparatus will change according to an observer's location in relation to the apparatus. Ultimately, the goal of this thesis is to engage one's perception of the visual world using properties of transparent materials to maximize the diffusion of light.</p>

<p> Digital imaging is growing rapidly making Complimentary Metal-Oxide-Semi conductor (CMOS) image sensor-based cameras indispensable in many modern life devices like cell phones, surveillance devices, personal computers, and tablets. For various purposes wireless portable image systems are widely deployed in many indoor and outdoor places such as hospitals, urban areas, streets, highways, forests, mountains, and towers. However, the increased demand on high-resolution image sensors and improved processing features is expected to increase the power consumption of the CMOS sensor-based camera systems. Increased power consumption translates into a reduced battery life-time. The increased power consumption might not be a problem if there is access to a nearby charging station. On the other hand, the problem arises if the image sensor is located in widely spread areas, unfavorable to human intervention, and difficult to reach. Given the limitation of energy sources available for wireless CMOS image sensor, an energy harvesting technique presents a viable solution to extend the sensor life-time. Energy can be harvested from the sun light or the artificial light surrounding the sensor itself. </p><p> In this thesis, we propose a current-mode CMOS hybrid image sensor capable of energy harvesting and image capture. The proposed sensor is based on a hybrid pixel that can be programmed to perform the task of an image sensor and the task of a solar cell to harvest energy. The basic idea is to design a pixel that can be configured to exploit its internal photodiode to perform two functions: image sensing and energy harvesting. As a proof of concept a 40 &times; 40 array of hybrid pixels has been designed and fabricated in a standard 0.5 <i>&micro;m</i> CMOS process. Measurement results show that up to 39 <i>&micro;W</i> of power can be harvested from the array under 130 Klux condition with an energy efficiency of 220 nJ /pixel /frame. The proposed image sensor is a current-mode image sensor which has several advantages over the voltage-mode. The most important advantages of using current-mode technique are: reduced power consumption of the chip, ease of arithmetic operations implementation, simplification of the circuit design and hence reduced layout complexity.</p>

The Harris (1962) model, which is consistent with local thermodynamic equilibrium (LTE) does not represent the earth's magnetotail, which has a magnetic field component normal to the equatorial plane. However, inclusion of a normal magnetic $B\sb{z}$ component can be made without violating the slow-flow MHD approximation, under the condition ${B\sb{z}\over B\sb{x}}\ll 1$. A procedure is developed for constructing a family of two-dimensional asymptotic equilibrium solutions that are ordered with respect to the magnetic disturbance index $K\sb{p}$ and based on fitting to a three-dimensional magnetospheric model magnetotail. The low quality of fits for magnetically active configurations is indicative of their consistency with the assumed pressure function. This, in turn, implies that high magnetic activity levels of the real magnetosphere are ruled by different thermodynamic conditions than those associated with LTE.

I show that the difference in 'spin up' and 'spin down' angular intensities of electrons emitted from inner core levels of atoms in a magnetic crystal is a spin hologram. Its inverse 'pictures' the component of the spins in the neighborhood of the emitter in the direction that the detected electrons are spin analyzed. Measuring the spin difference in three orthogonal directions determines a vector hologram which gives an image of the atomic spin vectors. I develop the theory and illustrate the feasibility and potential of this novel technique by means of model calculations. Furthermore, I give a detailed treatment of the elastic scattering processes that are responsible for the spin holograms, and study the effect of multiple scattering and self-interference which could cause complications.

A system for mass-selective capture of ions by a Paul trap was designed and tested. / An Paul trap and associated electronics have been assembled and tested, selectively trapping the ionic clusters C$ sp{+} sb2$ and C$ sp{+} sb3$ from a pulsed laser ion source. The ions were injected at 90 eV, trapped, extracted and detected, several parameters related to these processes were investigated. Simple models of the injection mechanism employed and for evaluating the pseudo-potential well depth of the ion trap are proposed. The overall efficiency of the ion injection system has been evaluated and compared to other methods of ion capture by an RFQ trap from an external source.

<p> Polymer brushes consist of multiple monomers connected together with one of the polymer chain's ends attached to a surface. Polymer brushes have shown great promise for a wide variety of applications including drug delivery dendrimer systems and as tunable brushes that can change their shape and physical properties in response to changes in their environment. Regularly branched polymer brushes which are structured as a function of their chemical indices are investigated here using the self-consistent mean field theory for electrically neutral polymers. The brushes were described using weighting functions, <i> f(n)</i>, were <i>n</i> was the fewest number of monomers from a specified location to a free end. Brushes with weighting functions of the form <i>f(n)=n^b, f(n)=e^bn</i>, as well as <i>f(n)=d^an</i> when <i>d</i> 2 and &alpha; > 2 were found to match the parabolic free chain end profile expected, while it was determined that polymer brushes described using <i>f(n)=n^b</i> must be very small in order to remain in equilibrium. However, brushes described by <i>f(n)</i>=2<i>G^(N-n)_N</i> and <i>f(n)</i>2<i>ⁿ</i> were found to be unstable for real, positive values of the potential of the system.</p>

<p> InAs/GaSb superlattices are a material system well suited to growth via molecular beam epitaxy. The ability to tune the band gap over the entire mid and long wave infrared spectrum gives a large number of applications for devices made from InAs/GaSb superlattice material. The growth of high quality InAs/GaSb superlattice material requires a careful study of the parameters used during epitaxial growth. This work investigates the growth of tunnel junctions for InAs/GaSb based superlattice light emitting diodes, the presence of defects in GaSb homoepitaxial layers, and variations in the growth rate of InAs/GaSb superlattice samples. Tunnel junctions in cascaded structures must provide adequate barriers to prevent carriers from leaking from one emission region to the next without first recombining radiatively, while at the same time remain low in tunneling resistance for current recycling. A variety of tunnel junction designs are compared in otherwise identical four stage InAs/GaSb superlattice light emitting diodes, which past studies have found hole confinement to be problematic. GaSb was used on the p-side of the junction, while various materials were used on the n-side. Al0.20In0.80As0.73Sb0.27 tunnel junctions function best due to the combination of favorable band alignment and ease of growth. Pyramidal defects have been observed in layers of GaSb grown by molecular beam epitaxy on GaSb substrates. These defects are typically 3-8 nanometers high, 1-3 microns in diameter, and shaped like pyramids. Their occurrence in the growth of GaSb buffer layers can propagate into subsequent layers. Defects are nucleated during the early stages of growth after the thermal desorption of native oxide from the GaSb substrate. These defects grow into pyramids due to a repulsive Ehrlich-Schwoebel potential on atomic step edges leading to an upward adatom current. The defects reduce in density with growth of GaSb. The insertion of a thin AlAsSb layer into the early stages of the GaSb buffer increases the rate of elimination of the defects, resulting in a smooth surface within 500nm. The acceleration of defect reduction is due to the temporary interruption of step-flow growth induced by the AlAsSb layer. This leads to a reduced isolation of the pyramids from the GaSb epitaxial layer, and allows the pyramidal defects to smooth out. Investigations into varying the superlattice growth rate have not been reported widely in the literature. Due to the frequent use of soaks, growth interrupts, and other interface structuring steps the superlattice growth rate and the interface layer sequence are linked. In order to properly study the effects of growth rate variations and interface design changes it is necessary to account for the effect on growth rate due to the interfaces. To this end it is useful to think of the effective growth rate of the superlattice, which is the total layer thickness divided by the total time, per superlattice period. Varying the effective growth rate of superlattice photoluminescence samples shows a peak in output at ~0.5 monolayers per second. Investigations into the structural properties of the superlattices show no decrease in structural uniformity for effective growth rates up to ~1.4 monolayers per second.</p>