Dissertations / Theses on the topic 'Applied Physic'

The water vapor concentration plays an important role for many atmospheric processes. The mean concentration is key to understand water vapor's effect on the climate as a greenhouse gas. The fluctuations about the mean are important to understand heat fluxes between Earth's surface and the boundary layer. These fluctuations are linked to turbulence that is present in the boundary layer. Turbulent conditions are simulated in Michigan Tech’s multiphase, turbulent reaction chamber, the Π chamber. Measurements for temperature and water vapor concentration were recorded under forced Rayleigh- Bénard convection at several turbulent intensities. These were used to calculate the saturation ratio, often referred to as the relative humidity. The fluctuations in the water vapor concentration were found to be the more important than the temperature for the variability of the saturation ratio. The fluctuations in the saturation ratio result in some cloud droplets experiencing a higher supersaturation than other cloud droplets, causing those "lucky" droplets to grow at a faster rate than other droplets. This difference in growth rates could contribute to a broadening of the size distribution of cloud droplets, resulting in the enhancement of collision-coalescence. These fluctuations become more pronounced with more intense turbulence.

The home built Magneto-Optical Kerr Effect (MOKE) microscope was implemented to probe the magnetic domain patterns of thin film samples. A CCD camera is introduced to the existing MOKE rotation measurement setup. The images captured by the camera are used to analyze the change in the magnetic domain patterns by the Kerr effect. Using 6.5× zoom lens and 1.33× extension tube attached to the CCD camera, the size of the sample down to a 1.02mm by 0.75 mm area can be viewed in the image capture. Magnetic hysteresis loop is first measured to investigate the magnetic switching behavior and measure the coercivity. Domain images that exhibited the most significant change were mostly captured between magnetic remanence and near coercivity. The images of samarium cobalt thin film at or near magnetic coercivity showed the changes in shape and light intensity of the magnetic domain light patterns due to the Kerr rotation.

Magnetic Force Microscopy (MFM) is a viable method of analyzing magnetic characteristics of ferromagnetic nanostructure materials. The nanosphere template produces a variety of magnetic domains for the endeavor of advancing technological applications such as nanomagnetic logic and magnetoplasmonic nanoparticles. These nanomagnetic applications demand very specific magnetic characteristics, so the profiling of different magnetic domains using magnetic force microscopy is essential. During this work, we studied the magnetic domains that came from permalloy material on nanosphere templates. We investigated the most optimal MFM scan parameters that could produce a viable and trustworthy magnetic domain image. After analyzing two types of samples with different nanosphere template arrangements scanned at two scan angles, 0° and 90°, relative to the cantilever, it was concluded that both scan angles produced optimal images. The optimal lift scan height, relative to the sample surface, was determined to be 65 nm away, where the magnetic domain is most accurately observed. When the tip magnetization was reversed, the MFM images show a corresponding flipping of the magnetic domain characteristics, but maintained the domain size and pattern. Further research is needed to determine the cause of magnetic domain ripples that appear in images.

Magnetic nanostructures have widespread applications in many areas of physics and engineering, and nanosphere lithography has recently emerged as promising tool for the fabrication of such nanostructures. The goal of this research is to explore the magnetic properties of a thin film of ferromagnetic material deposited onto a hexagonally close-packed monolayer array of polystyrene nanospheres, and how they differ from the magnetic properties of a typical flat thin film. The first portion of this research focuses on determining the optimum conditions for depositing a monolayer of nanospheres onto chemically pretreated silicon substrates (via drop-coating) and the subsequent characterization of the deposited nanosphere layer with scanning electron microscopy. Single layers of permalloy (Ni80Fe20) are then deposited on top of the nanosphere array via DC magnetron sputtering, resulting in a thin film array of magnetic nanocaps. The coercivities of the thin films are measured using a home-built magneto-optical Kerr effect (MOKE) system in longitudinal arrangement. MOKE measurements show that for a single layer of permalloy (Py), the coercivity of a thin film deposited onto an array of nanospheres increases compared to that of a flat thin film. In addition, the coercivity increases as the nanosphere size decreases for the same deposited layer. It is postulated that magnetic exchange decoupling between neighboring nanocaps suppresses the propagation of magnetic domain walls, and this pinning of the domain walls is thought to be the primary source of the increase in coercivity.

We provide estimates of both cross section and rate for the stimulated attachment of a second positron into the (1s^{2 1}S^e) state of the H¯ ⁺ ion using Ohmura and Ohmura’s (1960 Phys. Rev. 118 154) effective range theory, Reiss’s strong field approximation (1980 Phys. Rev. A 22, 1786), and the principle of detailed balancing. Our motivation for producing H¯⁺ ion include its potential to be used as an intermediate state in bringing antihydrogen to ultra-cold (sub-mK) temperatures required for a variety of studies, which include both spectroscopy and the probing of the gravitational interaction of the anti-atom. We show that both cross section and rate are increased with the use of a resonant laser field.

Efficiency of solar cells is degraded by deposition of mineral dust as well as other particles, and experiments reveal that losses can be significant (up to ~85%) depending on various factors. However, little is known about the role of light scattering and absorption in reducing optical transmission to the solar cell semiconductor. This dissertation first develops a fundamental model of optical losses due to particle-on-substrate scattering for light propagating into the forward direction. We use discrete dipole approximation with surface interaction (DDA-SI), which is a numerical solution of light scattering for an arbitrarily shaped particle-on-substrate. Using DDA-SI, we studied transmission losses due to hemispheric backward scattering (HBS) and absorption. A parameter called the fraction of power lost, defined as the ratio of HBS efficiency plus absorption efficiency to extinction efficiency, was found appropriate to describe optical losses into the forward direction. We found that fine particles lead to higher losses (per optical depth or layer optical thickness) than coarser ones. Losses into the forward direction are maximized when the ratio of skin depth to particles diameter approaches unity.

In addition, we conducted a resuspension-deposition experiment with two types of mineral dust, optically absorbing and non-absorbing dust. The dust samples were suspended and deposited onto glass slides, acting as surrogates for solar cells. Dust-deposited glass slides with increasing amounts of mass per area were spectroscopically characterized using a spectrophotometer with an integrating sphere (SIS) detector system. The SIS device allowed us to measure forward-hemisphere scattering, HBS, and direct beam transmission. Transmission into the forward direction was found to decrease as function of optical depth, depending on the absorptivity of the dust. Multiple-scattering radiative transfer theory, specifically the two-stream model as well as Monte Carlo stochastic calculations, were used to describe transmission as function of optical depth for both absorbing and nonabsorbing dust, yielding good agreement with experimental results within ~5%. Two-stream model and Monte Carlo techniques yield a multiple-scattering transmission calculation that depends on the single-scattering parameters of albedo and asymmetry parameter.

This study has the potential to help with solar energy forecasting, aiding smart power grids in predicting and adapting to variations in solar cell energy output due to aerosol deposition. In addition, this study can help optimize cleaning procedures and schedules to save water in desert and semi-arid regions by describing transmission losses as function of dust type.

We have investigated the Faraday effect of bismuth-doped rare-earth iron-garnets with varying doping levels of gallium from z = 1.0 to 1.35. We used lutetium to control the film's in-plane magnetic properties and found that gallium doping levels above the compensation point caused a loss of anisotropy control, a canted out-of-plane magnetization in the film, and an extremely weak but linear coercivity above 10 micro-Tesla fields. Using these results we focused on in-plane films to create 8 layer stacks of 500 um thick films to achieve a minimum detectable field of 50 pT at 1 kHz. Unlike previous Magneto-Optic (MO) studies that typically used thin films of approximately 1um thickness, we used approximately 400um thick films to allow experimentation with the final, robust, ideal form the MO sensor would take. We measured what most other MO studies with garnets neglected: the magnetic anisotropy axis or structure within the film. Knowledge of this structure is essential in improving the sensitivity of a stacked MO probe. Studying thick films proved to be key to understanding the magnetic anisotropy and domain properties that can degrade or enhance the sensitivity of the Faraday rotation in bismuth doped rare-earth iron-garnets to an applied magnetic field and to pointing the direction of future research to develop the conditions for rugged magnetometer sensors.

Geometric integrators yield high-fidelity numerical results by retaining conservation laws in the time advance. A particularly powerful class of geometric integrators is symplectic integrators, which are widely used in orbital mechanics and accelerator physics. An important application presently lacking symplectic integrators is the guiding center motion of magnetized particles represented by non-canonical coordinates. Because guiding center trajectories are foundational to many simulations of magnetically confined plasmas, geometric guiding center algorithms have high potential for impact. The motivation is compounded by the need to simulate long-pulse fusion devices, including ITER, and opportunities in high performance computing, including the use of petascale resources and beyond.

This dissertation uses a systematic procedure for constructing geometric integrators — known as variational integration — to deliver new algorithms for guiding center trajectories and other plasma-relevant dynamical systems. These variational integrators are non-trivial because the Lagrangians of interest are degenerate - the Euler-Lagrange equations are first-order differential equations and the Legendre transform is not invertible. The first contribution of this dissertation is that variational integrators for degenerate Lagrangian systems are typically multistep methods. Multistep methods admit parasitic mode instabilities that can ruin the numerical results. These instabilities motivate the second major contribution: degenerate variational integrators. By replicating the degeneracy of the continuous system, degenerate variational integrators avoid parasitic mode instabilities. The new methods are therefore robust geometric integrators for degenerate Lagrangian systems.

These developments in variational integration theory culminate in one-step degenerate variational integrators for non-canonical magnetic field line flow and guiding center dynamics. The guiding center integrator assumes coordinates such that one component of the magnetic field is zero; it is shown how to construct such coordinates for nested magnetic surface configurations. Additionally, collisional drag effects are incorporated in the variational guiding center algorithm for the first time, allowing simulation of energetic particle thermalization. Advantages relative to existing canonical-symplectic and non-geometric algorithms are numerically demonstrated. All algorithms have been implemented as part of a modern, parallel, ODE-solving library, suitable for use in high-performance simulations.

Thin films of chlorine (Cl) and copper (Cu) doped zinc selenide (Cl:ZnSe and Cu:ZnSe) were fabricated by pulsed laser deposition (PLD) with the goal of enabling a multilayered semiconductor structure for a mid-infrared (mid-IR) electrically excited laser. Doping of ZnSe is achieved by varying the mass ratio of zinc chloride (ZnCl₂) or copper selenide (Cu₂Se) to ZnSe precursors in starting pressed powder targets. Appropriate adjustment of the fraction of dopant precursor in the mixtures allows for the control of the dopant concentration, N_D–N _A for N_D >> N_A (or N_A-N_D for N _A >> N_D) in the thin films, where N_D is the donor concentration and N _A is the acceptor concentration. PLD is used to ablate the Cl:ZnSe or Cu:ZnSe targets, to produce thin films on gallium arsenide (GaAs) substrates. Impedance spectroscopy allows current-voltage and capacitance-voltage (C-V) characterization. Specifically Mott-Schottky measurements determine N_D-N_A (or N _A-N_D) of the fabricated thin film samples with comparisons to the nominal dopant concentration of the targets. The Mott-Schottky, 1/C² vs. V, measurements for determining N_D-N_A were calibrated against well-characterized silicon wafers with known values of N _D. The goal of this project was to demonstrate a reliable method for controlling the dopant concentration in PLD-deposited Cl:ZnSe and Cu:ZnSe thin films. The results obtained allows for the fabrication of Cl:ZnSe and Cu:ZnSe thin films with known N_D-N _A for use in a mid-IR electrically-excited laser devices under development in our research group.

The Langmuir-Blodgett deposition process is investigated for creating polystyrene nanosphere monolayers on a hydrophilic silicon substrate. The monolayers are fabricated over areas ~1 cm² and sputter coated with 100Å of permalloy. The quality of the monolayers is analyzed with optical microscope image processing, and 2D Fourier transforms of electron microscope images. The magnetic switching behavior of the sputtered samples is measured using an alternating gradient magnetometer, and compared to completely flat permalloy. The magnetic hysteresis measurements are done at different angle between the easy and hard axis of the flat permalloy films. The measurements show different hysteresis shapes for nanosphere patterned permalloy and flat permalloy, with the difference becoming greater nearer the hard axis of the flat permalloy samples. The ambiguity of an easy or hard axis on a curved surface is likely to contribute to the difference in magnetic switching behavior between the two sample types.