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Spin and orbital orderings are amongst the most important phenomena in the solid state chemistry of oxides. Physical property and powder neutron and X-ray diffraction measurements are reported for a range of mostly low dimensional ternary transition metal oxides which display spin or orbital order. Extensive studies of the physical properties and crystal structure of In2VO5 are reported. The structure of this material consists of one dimensional zig-zag chains of orbitally ordered S = 1/2 V4+. Magnetic susceptibility measurements show an unusual crossover from dominant ferromagnetic (θ = 17 K) to antiferromagnetic (θ = -70 K) exchange at 120 K, which is attributed to ferromagnetic dimerisation driven by magnetic frustration. The magnetic moment also increases from 1.81 to 2.2μB at the 120 K crossover. Heat capacity measurements confirm this scenario as the magnetic entropy tends towards 1/2 Rln3 below 120 K before approximating to Rln2 at high temperature. Synchrotron x-ray diffraction and high resolution neutron powder diffraction show no bulk structural changes, but the b axis, along which the VO6 chains run, shows an anomalous expansion below 120 K. At low temperatures, a downturn in the magnetic susceptibility is seen at 2.5 K, signifying a spin freezing transition. Heat capacity and powder neutron diffraction measurements show no evidence for long range magnetic order down to 0.42 K. The low dimensional brannerite materials MV2O6 (M = Mn, Co, Ni) were synthesised by a sol-gel method. Magnetic properties were investigated by magnetisation, powder neutron diffraction and in the case of CoV2O6, heat capacity measurements. The structure of these materials consists of linear chains of edge sharing MO6 octahedra. Monoclinic MnV2O6 is an isotropic antiferromagnet with TN = 20 K and a reduced magnetic coherence length due to 3 % Mn/V antisite disorder. The magnetic structure consists of ferromagnetic edge-sharing chains with k = (0,0,1/2) and a refined Mn moment of 4.77(7) μB. The triclinic materials CoV2O6 and NiV2O6 are also antiferromagnetic with TN = 7 and 14 K respectively and both show metamagnetic type transitions. Unusually, M(H) isotherms recorded below 5 K for CoV2O6 show a plateau at 1/3 of the saturation magnetisation. This feature, together with a long period modulated magnetic structure, is attributed to strong single ion (Ising) type anisotropy and nearest neighbour ferromagnetic exchange. Preliminary high pressure experiments on NiV2O6 have confirmed a previously reported transition to a columbite phase at 6 GPa and 900 °C. The high pressure polymorph is also antiferromagnetic with TN = 2.5 K. The previously uncharacterised perovskite, PbRuO3 has been prepared using high pressure/temperature synthesis techniques (10 GPa, 1000 °C). Synchrotron powder X-ray diffraction measurements show that the room temperature structure is orthorhombic, Pnma. A first order orbital ordering transition occurs at 75 K with an associated metal insulator transition. Below 75 K, the dxz orbitals are preferentially occupied and the structure is orthorhombic Imma. The transition may be driven by an increase in antiferroelectric Pb2+ displacements, whcih reach a peak at ~ 125 K. A further structural transition to a larger monoclinic cell is also identified at 9.7 K. The physical properties and crystal structures of two low dimensional lead manganese oxides have also been investigated. Acentric Pb2MnO4, which has a structure consisting of edge sharing chains, is antiferromagnetic with TN = 18 K. Powder neutron diffraction shows the magnetic structure consists of antiferromagnetic chains with k = (0,0,0) and a refined Mn moment of 2.74(2) μB. The crystal point group allows piezoelectricity and the magnetic point group symmetry allows piezomagnetism. We speculate that coupled magnetic and electric properties may be observed in this material. The layered material, Pb3Mn7O15, with a structure consisting of 1/2 filled Kagomé layers has also been studied. Single crystals were prepared by a flux growth method and polycrystalline material was prepared by the ceramic method. Powder neutron and synchrotron x-ray diffraction studies show that the single crystals are hexagonal and that the polycrystalline material is orthorhombic. Furthermore, heat capacity measurements show that the hexagonal single crystal material undergoes a glassy magnetic transition. In contrast, powder neutron diffraction shows that the orthorhombic polycrystalline material has coherent long range magnetic order. These differences are attributed to an oxygen deficiency in the polycrystalline magnetic order. These differences are attributed to an oxygen deficiency in the polycrystalline material.

Neste trabalho investigamos os efeitos do magnetismo orbital sobre o gás de elétrons tridimensional e sobre íons de camadas abertas em matrizes metálicas. Derivamos uma expressão analítica fechada para a energia de troca do gás de elétrons tridimensional na presença de fortes campos magnéticos, incluindo contribuições do segundo nível de Landau e polarização de spin arbitrária. Esse cálculo generaliza e corrige resultados anteriores disponíveis na literatura. Em seguida, realizamos um cálculo numérico da energia de troca do gás de elétrons tridimensional na presença de campos magnéticos, permitindo a ocupação de um número ilimitado de níveis de Landau, possibilitando assim a obtenção da energia de troca para quaisquer valores de campo magnético e densidade. Em uma abordagem independente, usamos as aproximações de Thomas-Fermi e Thomas-Fermi-Dirac para construir modelos simples para a função dielétrica do gás de elétrons tridimensional no regime de campos magnéticos muito fortes (apenas o primeiro nível de Landau ocupado). Finalmente, estabelecemos vínculos entre os tratamentos fenomenológicos e de primeiros princípios do magnetismo orbital em íons de camadas abertas em matrizes metálicas. Esses vínculos forneceram um embasamento teórico para o uso dos termos de polarização orbital em cálculos Kohn-Sham e levaram à obtenção de expressões aproximadas para os funcionais de troca-correlação da teoria do funcional da densidade de corrente. / In this work, we investigate the effects of orbital magnetism in the three-dimensional electron gas and in open-shell ions in a solid. We derive a closed analytical expression for the exchange energy of the three-dimensional electron gas in strong magnetic fields including the contribution of the second Landau level and arbitrary spin polarization. This calculation generalizes and corrects earlier results available in the literature. Next, we perform a numerical calculation of the exchange energy of the three-dimensional electron gas in a magnetic field, allowing several Landau levels to be occupied, to obtain the exchange energy for arbitrary values of magnetic field and density. In an independent approach, we use the Thomas-Fermi and Thomas-Fermi-Dirac approximations to construct simple model dielectric functions for the three-dimensional electron gas in the strong magnetic field regime (where only the lowest Landau level is occupied). Finally, we establish links between the phenomenological and the first-principles treatment of orbital magnetism in open-shell ions in solids. These links provide a theoretical foundation for the use of orbital polarization terms in Kohn-Sham calculations and allow to obtain approximations to the exchange-correlation functionals of current-density functional theory.

"Orbital flight of CubeSats in extremely Low Earth Orbit, defined here as an altitude between 150 – 250 km, has the potential to enable a wide range of missions in support of atmospheric measurements, national security, and natural resource monitoring. In this work, a mission study is presented to demonstrate the feasibility of using commercially available sensor and electric thruster technology to extend the orbital lifetime of a 3U CubeSat flying at an altitude of 210 km. The CubeSat consists of a 3U configuration and assumes the use of commercially available sensors, GPS, and electric power systems. The thruster is a de-rated version of a commercially available electrospray thruster operating at 2 W, 0.175 mN thrust, and an Isp of 500 s. The mission consists of two phases. In Phase I the CubeSat is deployed from the International Space Station orbit (414 km) and uses the thruster to de-orbit to the target altitude of 210 km. Phase II then begins during which the propulsion system is used to extend the mission lifetime until propellant is fully expended. A control algorithm based on maintaining a target orbital energy is presented in which simulated GPS updates are corrupted with measurement noise to simulate state data which would be available to the spacecraft computer. An Extended Kalman Filter is used to generate estimates of the orbital dynamic state between the 1 Hz GPS updates, allowing thruster control commands at a frequency of 10 Hz. For Phase I, operating at full thrust, the spacecraft requires 25.21 days to descend from 414 to 210 km, corresponding to a ΔV = 96.25 m/s and a propellant consumption of 77.8 g. Phase II, the primary mission phase, lasts for 57.83 days, corresponding to a ΔV = 119.15 m/s during which the remaining 94.2 g of propellant are consumed. "

Neste trabalho investigamos os efeitos do magnetismo orbital sobre o gás de elétrons tridimensional e sobre íons de camadas abertas em matrizes metálicas. Derivamos uma expressão analítica fechada para a energia de troca do gás de elétrons tridimensional na presença de fortes campos magnéticos, incluindo contribuições do segundo nível de Landau e polarização de spin arbitrária. Esse cálculo generaliza e corrige resultados anteriores disponíveis na literatura. Em seguida, realizamos um cálculo numérico da energia de troca do gás de elétrons tridimensional na presença de campos magnéticos, permitindo a ocupação de um número ilimitado de níveis de Landau, possibilitando assim a obtenção da energia de troca para quaisquer valores de campo magnético e densidade. Em uma abordagem independente, usamos as aproximações de Thomas-Fermi e Thomas-Fermi-Dirac para construir modelos simples para a função dielétrica do gás de elétrons tridimensional no regime de campos magnéticos muito fortes (apenas o primeiro nível de Landau ocupado). Finalmente, estabelecemos vínculos entre os tratamentos fenomenológicos e de primeiros princípios do magnetismo orbital em íons de camadas abertas em matrizes metálicas. Esses vínculos forneceram um embasamento teórico para o uso dos termos de polarização orbital em cálculos Kohn-Sham e levaram à obtenção de expressões aproximadas para os funcionais de troca-correlação da teoria do funcional da densidade de corrente. / In this work, we investigate the effects of orbital magnetism in the three-dimensional electron gas and in open-shell ions in a solid. We derive a closed analytical expression for the exchange energy of the three-dimensional electron gas in strong magnetic fields including the contribution of the second Landau level and arbitrary spin polarization. This calculation generalizes and corrects earlier results available in the literature. Next, we perform a numerical calculation of the exchange energy of the three-dimensional electron gas in a magnetic field, allowing several Landau levels to be occupied, to obtain the exchange energy for arbitrary values of magnetic field and density. In an independent approach, we use the Thomas-Fermi and Thomas-Fermi-Dirac approximations to construct simple model dielectric functions for the three-dimensional electron gas in the strong magnetic field regime (where only the lowest Landau level is occupied). Finally, we establish links between the phenomenological and the first-principles treatment of orbital magnetism in open-shell ions in solids. These links provide a theoretical foundation for the use of orbital polarization terms in Kohn-Sham calculations and allow to obtain approximations to the exchange-correlation functionals of current-density functional theory.

A simple theoretical method for calculating electronic band structure of wurtzite materials based on the linear combination of orbital model is presented. To abtain better description of the conduction band structures, second-nearest-neighbor s and p state interaction are included. We suggest that the zinc-blende InN has a direct band gap of ~2 eV and an indirect band gap of ~0.7 eV located at L-points. Due to band folding effect, the wurtzite InN thus has a direct band gap of ~0.7 eV located at £F3-point.

Many oceanographers have conducted field experiments on internal waves in the South China Sea using SAR imagery, ADCP and CTD. The results arising from these field studies are mostly in terms of wave amplitudes and flow velocities. Despite schematic diagram depicting the orbits of water particle motion has been accepted for more than decades, evidence has not been available from field observations or laboratory experiments. Laboratory experiments on water-particle motion were conducted on the propagation of elevation and depression ISW in a stratified two-layer fresh/brine fluid system in a steel-framed wave tank of 12 m long with cross section of 0.7 m high by 0.5 m wide. Numerical modeling was also performed using in put data identical to laboratory experiments. Based on our results of the numerical and laboratory experiments, the velocity field displays significant vortex while an internal solitary wave (ISW) propagates on a flat bottom. The strong vortex appears in the region of wave crest or trough. The track of fluid particle velocity in the upper layer is asymmetric and is moving in the opposite direction to that in the lower layer. The maximum horizontal velocity occurs at the crest of an elevation ISW and at the trough of a depression ISW. However, no horizontal flow is found on the interface of the still water level, and no vertical velocity at the wave peak. The vertical and horizontal velocities are antecedent with the water depth. For an elevation ISW, the maximum horizontal velocity appears in the lower layer, and vice versa for a depression ISW. The direction of the horizontal velocity in the upper layer is opposite to that in the lower layer. This study presents the results of numerical calculations and laboratory observations of the particles originally resting on a specific level and their movements within an ISW. The finding generated from this research would benefit others on the verification of field results or analytical theory for fluid particle motions associated with ISW.

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references (leaves 183-185). Available also in a digital version from Dissertation Abstracts.

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