Spin resonance of a two-dimensional electron system confined in a GaN/AlGaN heterostructure grown by molecular beam epitaxy was resistively detected over a wide range of magnetic field and microwave frequency. Although the spin-orbit interaction is strong in this type of heterostructure at zero magnetic field, surprisingly the width of the detected spin resonance line was very narrow—down to 6.5 mT at 13.3 T. The spin depolarization time extracted from the resonance linewidth was estimated to be 2 ns. The electron g-factor was measured with high accuracy, resembling a value close to the free-electron value and its dependence on the magnetic field was studied.

<p dir="ltr">The Global Navigation Satellite System (GNSS) is heavily relied on for the navigation of Earth satellites. For satellites in cislunar space and beyond, GNSS is not readily available. As a result, other sources such as NASA's Deep Space Network (DSN) must be relied on for navigation. However, DSN is overburdened and can only support a small number of satellites at a time. Furthermore, communication with external sources can become interrupted or deprived in these environments. Given NASA's current efforts towards cislunar space operations and the expected increase in cislunar satellite traffic, there will be a need for more autonomous navigation options in cislunar space and beyond.</p><p dir="ltr">In this thesis, a navigation system capable of accurate and computationally efficient orbit determination in these communication-deprived environments is proposed and investigated. The emphasis on computational efficiency is in support of cubesats which are constrained in size, cost, and mass; this makes navigation even more challenging when resources such as GNSS signals or ground station tracking become unavailable.</p><p dir="ltr">The proposed navigation system, which is called GRAVNAV in this thesis, involves a two-satellite formation orbiting a planet. The primary satellite hosts an Extended Kalman Filter (EKF) and is capable of measuring the relative position of the secondary satellite; accurate attitude estimates are also available to the primary satellite. The relative position measurements allow the EKF to estimate the absolute position and velocity of both satellites. In this thesis, the proposed navigation system is investigated in the two-body and three-body problems.</p><p dir="ltr">The two-body analysis illuminates the effect of the gravity model error on orbit determination performance. High-fidelity gravity models can be computationally expensive for cubesats; however, celestial bodies such as the Earth and Moon have non-uniform and highly-irregular gravity fields that require complex models to describe the motion of satellites orbiting in their gravity field. Initial results show that when a second-order zonal harmonic gravity model is used, the orbit determination accuracy is poor at low altitudes due to large gravity model errors while high-altitude orbits yield good accuracy due to small gravity model errors. To remedy the poor performance for low-altitude orbits, a Gravity Model Error Compensation (GMEC) technique is proposed and investigated. Along with a special tuning model developed specifically for GRAVNAV, this technique is demonstrated to work well for various geocentric and lunar orbits.</p><p><br></p><p dir="ltr">In addition to the gravity model error, other variables affecting the state estimation accuracy are also explored in the two-body analysis. These variables include the six Keplerian orbital elements, measurement accuracy, intersatellite range, and satellite formation shape. The GRAVNAV analysis shows that a smaller intersatellite range results in increased state estimation error. Despite the intersatellite range bounds, semimajor axis, measurement model, and measurement errors being identical for both orbits, the satellite formation shape also has a strong influence on orbit determination accuracy. Formations that place both satellites in different orbits significantly outperform those that place both satellites in the same orbit.</p><p dir="ltr">The three-body analysis primarily focuses on characterizing the unique behavior of GRAVNAV in Near Rectilinear Halo Orbits (NRHOs). Like the two-body analysis, the effect of the satellite formation shape is also characterized and shown to have a similar impact on the orbit determination performance. Unlike the two-body problem, however, different orbits possess different stability properties which are shown to significantly affect orbit determination performance. The more stable NRHOs yield better GRAVNAV performance and are also less sensitive to factors that negatively impact performance such as measurement error, process noise, and decreased intersatellite range.</p><p dir="ltr">Overall, the analyses in this thesis show that GRAVNAV yields accurate and computationally efficient orbit determination when GMEC is used. This, along with the independence of GRAVNAV from GNSS signals and ground-station tracking, shows that GRAVNAV has good potential for navigation in cislunar space and beyond.</p>

The term spatiotemporal chaos refers to physical phenomena that exhibit irregular oscillations in both space and time. Examples of such phenomena range from cardiac dynamics to fluid turbulence, where the motion is described by nonlinear partial differential equations. It is well known from the studies of low dimensional chaotic systems that the state space, the space of solutions to the governing dynamical equations, is shaped by the invariant sets such as equilibria, periodic orbits, and invariant tori. State space of partial differential equations is infinite dimensional, nevertheless, recent computational advancements allow us to find their invariant solutions (exact coherent structures) numerically. In this thesis, we try to elucidate the chaotic dynamics of nonlinear partial differential equations by studying their exact coherent structures and invariant manifolds. Specifically, we investigate the Kuramoto-Sivashinsky equation, which describes the velocity of a flame front, and the Navier-Stokes equation for an incompressible fluid in a circular pipe. We argue with examples that this approach can lead to a theory of turbulence with predictive power.

The application of a theoretical framework for calculating the radial electric field in the DIII-D tokamak edge plasma is discussed. Changes in the radial electric field are correlated with changes in many important edge plasma phenomena, including rotation, the L-H transition, and ELM suppression. A self-consistent model for the radial electric field may therefore suggest a means of controlling other important parameters in the edge plasma. Implementing a methodology for calculating the radial electric field can be difficult due to its complex interrelationships with ion losses, rotation, radial ion fluxes, and momentum transport. The radial electric field enters the calculations for ion orbit loss. This ion orbit loss, in turn, affects the radial ion flux both directly and indirectly through return currents, which have been shown theoretically to torque the edge plasma causing rotation. The edge rotation generates a motional radial electric field, which can influence both the edge pedestal structure and additional ion orbit losses. In conjunction with validating the analytical modified Ohm’s Law model for calculating the radial electric field, modeling efforts presented in this dissertation focus on improving calculations of ion orbit losses and x-loss into the divertor region, as well as the formulation of models for fast beam ion orbit losses and the fraction of lost particles that return to the confined plasma. After rigorous implementation of the ion orbit loss model and related mechanisms into fluid equations, efforts are shifted to calculate effects from rotation on the radial electric field calculation and compared to DIII-D experimental measurements and computationally simulated plasmas. This calculation of the radial electric field will provide a basis for future modeling of a fast, predictive calculation to characterize future tokamaks like ITER.

The general theme of the thesis is the interplay between magnetization dynamics and spin transport. The main presentation is divided into three parts. The first part is devoted to deepening our understanding on magnetic damping of ferromagnetic metals, which is one of the long-standing issues in conventional spintronics that has not been completely understood. For a nonuniformly-magnetized ferromagnetic metal, we find that the damping is nonlocal and is enhanced as compared to that in the uniform case. It is therefore necessary to generalize the conventional Landau-Lifshitz-Gilbert equation to include the additional damping. In a different vein, the decay mechanism of the uniform precession mode has been investigated. We point out the important role of spin-conserving electron-magnon interaction in the relaxation process by quantitatively examining its contribution to the ferromagnetic resonance linewidth. In the second part, a transport theory is developed for magnons which, in addition to conduction electrons, can also carry and propagate spin angular momentum via the magnon current. We demonstrate that the mutual conversion of magnon current and spin current may take place at magnetic interfaces. We also predict a novel magnon-mediated electric drag effect in a metal/magnetic-insulator/metal trilayer structure. This study may pave the way to the new area of insulator-based spintronics. In the third part of thesis, particular attention is paid to the influence the spin orbit coupling on both charge and spin transport. We theoretically investigate magnetotransport anisotropy and the conversion relations of spin and charge currents in various magnetic systems, and apply our results to interpret recent experiments.

International Telemetering Conference Proceedings / October 30-November 02, 1995 / Riviera Hotel, Las Vegas, Nevada / The Extreme Ultraviolet Explorer (EUVE) satellite was designed to operate with the Tracking and Data Relay Satellite System (TDRSS) and Deep Space Network (DSN). NASA, the Jet Propulsion Laboratory and the Center for EUV Astrophysics have been evaluating a commercially available ground station already used for NASA's Low Earth Orbit (LEO) weather satellites. This ground station will be used in a network of unattended, autonomous ground stations for telemetry reception, processing, and routing of data over a commercial, secure data line. Plans call for EUVE to be the initial network user. This network will be designed to support many TDRSS/DSN compatible missions. It will open an era of commercial, low-cost, autonomous ground station networks. The network will be capable of supporting current and future NASA scientific missions, and NASA's LEO and geostationary weather satellites. Additionally, it could support future, commercial communication satellites in low, and possibly medium, Earth orbit. The combination of an autonomous ground station and an autonomous telemetry monitoring system will allow reduction in personnel. The EUVE Science Operations Center has already reduced console work from three shifts to one by use of autonomous telemetry monitoring software.

A unique feature of the 5d-iridates is that the spin-orbit interaction (SOI) and Coulomb interactions U are of comparable strength and therefore compete vigorously. The relative strength of these interactions stabilizes new exotic ground states that provide a fertile ground for studying new physics. SOI is proportional to Z^4 (Z is the atomic number), and it is now recognized that strong SOI can drive novel narrow-gap insulating states in heavy transition metal oxides such as iridates. Indeed, strong SOI necessarily introduces strong lattice degrees of freedom that become critical to new physics in the iridates. This dissertation thoroughly examines a wide array of newly observed novel phenomena induced by adjusting the relative strengths of U and SOI interactions via slight chemical doping and application of hydrostatic pressure in the layered iridates, particularly, BaIrO3 and Sr2IrO4.

Analytical solutions to complex trajectory design problems are scarce, since only a few specific cases allow for closed-form solutions. The main purpose of this dissertation is to design simple algorithms for trajectory design using continuous thrust, with a focus on low-thrust applications. By “simple” here we seek to achieve algorithms that either admit an analytical solution, or require minimal input by the user and minimal computation time. The three main contributions of this dissertation are: designing Lyapunov-based closed-loop guidance laws for orbit transfers, finding semi-analytical solutions using a constant magnitude thrust, and perturbation theory for approximate solutions to low-thrust problems. The technical aspect that these problems share in common is that they all use, fully or partially, a thrusting model in which the energy of the system is kept constant. Many orbit transfer problems are shown to be solved with this thrusting protocol. / text

<p>Highly correlated quantum chemical methods have been appliedto study the photophysical properties of substituted benzenes.With the inclusion of spin-orbit coupling, the phosphorescencesof these molecules have been calculated usingMulti-CongurationalSelf- Consistent Field (MCSCF) quadraticresponse theory. The Herzberg-Teller approximation has beenadopted to evaluate the vibronic contributions tophosphorescence.</p><p>The performance of hybrid density functional theory (DFT) atthe B3LYP level is examined in comparison to the MP2, CCSD andCCSD(T) methods for the geometry and permanent dipole moment ofp-aminobenzoic acid. The time-dependent DFT/B3LYP method isapplied to calculate the two-photon absorption of a series ofZinc-porphyrin derivatives in combination with a two-statemodel. The transitions between excited singlet and tripletstates of Zinc and Platinum based organometallic compounds havebeen computed using DFT quadratic response theory. The resultsare used to simulate the non-linear propagation of laser pulsesthrough these materials utilizing a dynamical wave propagationmethod.</p>

In this thesis we investigate the existence and stability of periodic solutions of Hamiltonian systems with a discrete symmetry. The global existence of periodic motions can be proven using the classical techniques of the calculus of variations; our particular interest is in how the stability type of the solutions thus obtained can be determined analytically using solely the variational problem and the symmetries of the system -- we make no use of numerical or perturbation techniques. Instead, we use a method introduced in [41] in the context of a special case of the three-body problem. Using techniques from symplectic geometry, and specifically the Maslov index for curves of Lagrangian subspaces along the minimizing trajectories, we verify conditions which preclude the existence of eigenvalues of the monodromy matrix on the unit circle. We study the applicability of this method in two specific cases. Firstly, we consider another special case from celestial mechanics: the hip-hop solutions of the 2N-body problem. This is a family of Z_2-symmetric, periodic orbits which arise as collision-free minimizers of the Lagrangian action on a space of symmetric loops [14, 53]. Following a symplectic reduction, it is shown that the hip-hop solutions are brake orbits which are generically hyperbolic on the reduced energy-momentum surface. Secondly we consider a class of natural Hamiltonian systems of two degrees of freedom with a homogeneous potential function. The associated action functional is unbounded above and below on the function space of symmetric curves, but saddle points can be located by minimization subject to a certain natural constraint of a type first considered by Nehari [37, 38]. Using the direct method of the calculus of variations, we prove the existence of symmetric solutions of both prescribed period and prescribed energy. In the latter case, we employ a variational principle of van Groesen [55] based upon a modification of the Jacobi functional, which has not been widely used in the literature. We then demonstrate that the (constrained) minimizers are again hyperbolic brake orbits; this is the first time the method has been applied to solutions which are not globally minimizing. / Thesis (Ph.D, Mathematics & Statistics) -- Queen's University, 2013-09-25 10:47:53.257