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Thesis (Ph. D.)--University of Texas at Austin, 2003. / Supervisor: Jeffrey D. Vaaler. Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (doctoral)--Ludwig Maximilians-Universität zu München, 1979.

A new, novel numerical optimization algorithm is developed, tested, and used to solve difficult numerical problems from the field of astrodynamics. First, a brief review of optimization theory is presented and common numerical optimization techniques are discussed. Then, the new method, called the Learning Approach to Sampling Optimization (LA) is presented. Simple, illustrative examples are given to further emphasize the simplicity and accuracy of the LA method. Benchmark functions in lower dimensions are studied and the LA is compared, in terms of performance, to widely used methods. Three classes of problems from astrodynamics are then solved. First, the N - impulse orbit transfer and rendezvous problems are solved by using the LA optimization technique along with derived bounds that make the problem computationally feasible. This marriage between analytical and numerical methods allows an answer to be found for an order of magnitude greater number of impulses than are currently published. Next, the N -impulse work is applied to design periodic close encounters (PCE) in space. The encounters are defined as an open rendezvous, meaning that two spacecraft must be at the same position at the same time, but their velocities are not necessarily equal. The PCE work is extended to include N -impulses and other constraints, and new examples are given. Finally, a trajectory optimization problem is solved using the LA algorithm and comparing performance with other methods based on two models-with varying complexity-of the Cassini-Huygens mission to Saturn. The results show that the LA consistently outperforms commonly used numerical optimization algorithms.

Electrons not only have charges but also have spin. By utilizing the electron spin, the energy consumption of electronic devices can be reduced, their size can be scaled down and the efficiency of `read' and `write' in memory devices can be significantly improved. Hence, the manipulation of electron spin in electronic devices becomes more and more appealing for the advancement of microelectronics. In spin-based devices, the manipulation of ferromagnetic order parameter using electrical currents is a very useful means for current-driven operation. Nowadays, most of magnetic memory devices are based on the so-called spin transfer torque, which stems from the spin angular momentum transfer between a spin-polarized current and the magnetic order parameter. Recently, a novel spin torque effect, exploiting spin-orbit coupling in non-centrosymmetric magnets, has attracted a massive amount of attention. This thesis addresses the nature of spin-orbit coupled transport and torques in non-centrosymmetric magnetic semiconductors. We start with the theoretical study of spin orbit torque in three dimensional ferromagnetic GaMnAs. Using the Kubo formula, we calculate both the current-driven field-like torque and anti-damping-like torque. We compare the numerical results with the analytical expressions in the model case of a magnetic Rashba two-dimensional electron gas. Parametric dependencies of the different torque components and similarities to the analytical results of the Rashba two-dimensional electron gas in the weak disorder limit are described. Subsequently we study spin-orbit torques in two dimensional hexagonal crystals such as graphene, silicene, germanene and stanene. In the presence of staggered potential and exchange field, the valley degeneracy can be lifted and we obtain a valley-dependent Berry curvature, leading to a tunable antidamping torque by controlling the valley degree of freedom. This thesis then addresses the influence of the quantum spin Hall effect on spin orbit torque in nanoribbons with a hexagonal lattice. We find a dramatic modification of the nature of the torque (field like and damping-like component) when crossing the topological phase transition. The relative agnitude of the two torque components can be significantly modifies by changing the magnetization direction. Finally, motivated by recent experimental results, we conclude by investigating the features of spin-orbit torque in magnetic transition metal dichalcogenides. We find the torque is associated with the valley polarization. By changing the magnetization direction, the torque can be changed from a finite value to zero when the valley polarization decreases from a finite value to zero.

Thesis advisor: Vidya Madhavan / We present Scanning Tunneling Microscopy (STM) studies on several systems in which spin-orbit coupling leads to new and interesting physics, and where tuning by doping and/or strain can significantly modify the electronic properties, either inducing a phase transition or by sharply influencing the electronic structure locally. In the perovskite Iridate insulator Sr3Ir2O7, we investigate the parent compound, determining the band gap and its evolution in response to point defects which we identify as apical oxygen vacancies. We investigate the effects of doping the parent compound with La (in place of Sr) and Ru (in place of Ir). In both cases a metal-insulator transition (MIT) results: at x ~ 38% with Ru, and x ~ 5% with La. In the La-doped samples we find nanoscale phase separation at dopings just below the MIT, with metallic spectra associated with clusters of La atoms. Further, we find resonances near the Fermi energy associated with individual La atoms, suggesting an uneven distribution of dopants among the layers of the parent compound. Bi2Se3 is a topological insulator which hosts linearly dispersing Dirac surface states. Doping with In (in place of Bismuth) brings about topological phase transition, achieving a trivial insulator at x ~ 4%. We use high-magnetic field Landau level spectroscopy to study the surface state’s properties approaching the phase transition and find, by a careful analysis of the peak positions find behavior consistent with strong surface-state Zeeman effects: g~50. This interpretation implies, however, a relabeling of the Landau levels previously observed in pristine Bi2Se3, which we justify through ab initio calculations. The overall picture is of a g-factor which steadily decreases as In is added up to the topological phase transition. Finally, we examine the effects of strain on the surface states of (001) thin films of the topological crystalline insulator SnTe. When these films are grown on closely-related substrates—in this case PbSe(001)—a rich pattern of surface strain emerges. We use phase-sensitive analysis of atomic-resolution STM topographs to measure the strain locally, and spatially-resolved quasiparticle interference imaging to compare the Dirac point positions in regions with different types of strain, quantifying for the first time the effect of anisotropic strain on the surface states of a topological crystalline insulator. / Thesis (PhD) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Purpose The purpose of this research was to evaluate the outcomes, specifically diplopia and enophthalmos, as well as the complications of surgical repair of orbital fractures using the transconjunctival surgical approach. Methods A cross-sectional descriptive study was conducted. Thirty patients who underwent surgical repair of an orbital fracture were included in this case series. All patients were operated using the transconjunctival surgical approach and in all cases the fracture was repaired with 0.4 mm nylon foil sheeting (Supramid). The preoperative and postoperative clinical findings of visual acuity, diplopia and enophthalmos were analyzed, and postoperative complications were noted. Results Twenty-five of 30 patients (83%) presented with diplopia preoperatively and two patients (7%) had persistent diplopia postoperatively. Enophthalmos of greater than 2mm was present in 16 of 30 patients (53%) preoperatively and five patients (17%) had persistent enophthalmos postoperatively. All patients with persistent postoperative diplopia and or enophthalmos underwent late surgical repair (mean 10.6 months). Ten patients (33%) in this series were repaired early, within three weeks of trauma, and had no diplopia or enophthalmos postoperatively. There were no cases of lower lid retraction or ectropion. There was one complication related to the nylon foil. Conclusion The transconjunctival surgical approach used to repair orbital fractures was associated with good functional outcomes and few complications. Early surgical repair of orbital blowout fractures and the use of nylon foil sheeting are supported by this case series.

During the first three-and-a-half years of the Tropical Rainfall Measuring Mission (TRMM), the TRMM satellite operated at a nominal altitude of 350 km. To reduce drag, save maneuvering fuel, and prolong the mission lifetime, the orbit was boosted to 403 km in August 2001. The change in orbit altitude produced small changes in a wide range of observing parameters, including field-of-view size and viewing angles. Due to natural climatic variability, it is not possible to evaluate possible changes in precipitation retrievals from the satellite data alone. We estimate changes in TRMM Microwave Imager (TMI) and the Precipitation Radar (PR) precipitation retrievals due to the orbit boost by comparing them with surface rain gauges on ocean buoys operated by the NOAA Pacific Marine Environment Laboratory (PMEL). For each rain gauge, we compute the bias between the satellite and the gauge for pre- and post-boost time periods. For the TMI, the satellite is biased ~12% low relative to the gauges during the pre-boost period and ~1.5% low during the post-boost period. The mean change in bias relative to the gauges is approximately 0.4 mm day^-1. The PR is biased significantly low relative to the gauges during both boost periods. The change in bias is rain rate dependent, with larger changes in areas with higher mean precipitation rates.

In this dissertation we treat some problems about possible density of orbits for non-hypercyclic operators and we enlarge the class of known non-orbit-transitive operators. One of the questions related to hypercyclic operators that we answer is whether the density (in the set of positive real numbers) of the norms of the elements in the orbit for each nonzero vector in the Hilbert space is sufficient to imply that at least one vector has orbit dense in the Hilbert space. We show that the density of the norms is not a sufficient condition to imply hypercyclicity by constructing a weighted bilateral shift that, on one hand, satisfies the orbit-density property (in the sense defined above), but, on the other hand, fails to be hypercyclic. The second major topic that we study refers to classes of operators that are not hypertransitive (or orbit-transitive) and is related to the invariant subspace problem on Hilbert space. It was shown by Jung, Ko and Pearcy in 2005 that every compact perturbation of a normal operator is not hypertransitive. We extend this result, after introducing the related notion of weak hypertransitivity, by giving a sufficient condition for an operator to belong to the class of non-weakly-hypertransitive operators. Next, we study certain 2-normal operators and their compact perturbations. In particular, we consider operators with a slow growth rate for the essential norms of their powers. Using a new idea, of accumulation of growth for each given power on a set of different orthonormal vectors, we establish that the studied operators are not hypertransitive.

Thesis (Aeronautical and Astronautical Engineer)--Naval Postgraduate School, December 2002. / Thesis advisor(s): I. Michael Ross, Dennis Byrnes. Includes bibliographical references (p. 119-120). Also available online.