Total energy calculations based on density functional theory are generally a good approach to obtain the properties of solids. The local density approximation (LDA) is widely used for calculating the ground state properties of electronic systems; for excited states the errors are in general unknown. The important aspects of LDA pertain to the modeling of the exchange-correlation interaction. If the exchange-correlation potential is approximately the same for the ground and excited states, one expects good results from the LDA calculations for excited states. In this thesis, we utilize the total energy technique for numerical computations of the electronic structure of iron in several magnetic phases and crystalline structures. 1. Body-centered-cubic iron in the ferromagnetic and several antiferromagnetic configurations. We use the total energy results to obtain the parameters in a model Heisenberg Hamiltonian. These include the interaction parameters up to 6-th nearest neighbors. Based on this model Hamiltonian we calculate properties such as the critical (Curie) temperature and spin stiffness constant. We assume that the total exchangecorrelation energy functional is the same in the ferromagnetic ground state and the antiferromagnetic excited states. Our model parameters are based directly on ab initio calculations of the electronic structure. Our calculation yields good results compared with experimental values and earlier work. Some other physical quantities, related to the phase transition, and spin waves are also discussed. 2. Face-centered-tetragonal iron. If iron is grown on a proper substrate ( e.g., Cu(100) ), the crystal structure of the thin film displays a face-centered-tetragonal distortion due to the lattice constant misfit between the film and substrate. Therefore, we performed calculations for fct iron in its ferromagnetic, antiferromagnetic, and nonmagnetic phases for a wide range of values of the lattice parameters. In the ferromagnetic calculations, we found two minima in the total energy: one is close to.the bcc structure and the other ( with a lower energy ) is close to fcc. In the antiferromagnetic and nonmagnetic calculations, we found in each case that there is only one minimum near the fcc structure, providing us clear evidence that the antiferromagnetic and nonmagnetic states are (meta)stable near the fcc region and unstable in bcc region. The antiferromagnetic and nonmagnetic states are almost degenerate near the fcc minimum, but the antiferromagnetic phase has the lowest total energy in the whole fct region. Magnetic moments are also calculated for a variety of fct structures. Near the fcc minimum we found that two ferromagnetic phases co-exist, one with a low spin and one with a high spin. These results are consistent with experimental facts and other earlier calculations. Some structural properties, such as the elastic constants and the bulk modulus, are also studied and compared with experimental data and some earlier calculations. / Graduation date: 1992

The desire for low cost electronics has led to a huge increase in research focused on organic materials. These materials are appealing due to their unique electrical and material-processing properties and are rapidly being adopted in old and new electronic applications. To create practical devices requires a further understanding of the charge transport properties of the unique anisotropic molecular crystal structures. This work looks at how doping with the transition-metal element manganese can alter the electronic structure of the organic material pentacene. It has been found that using manganese as a dopant provides novel physical characteristics previously not encountered in organic field effect transistors based on pentacene. These organic thin films were characterized using X-ray absorption spectroscopy and the results compared to computational density functional theory analysis.

The desire for low cost electronics has led to a huge increase in research focused on organic materials. These materials are appealing due to their unique electrical and material-processing properties and are rapidly being adopted in old and new electronic applications. To create practical devices requires a further understanding of the charge transport properties of the unique anisotropic molecular crystal structures. This work looks at how doping with the transition-metal element manganese can alter the electronic structure of the organic material pentacene. It has been found that using manganese as a dopant provides novel physical characteristics previously not encountered in organic field effect transistors based on pentacene. These organic thin films were characterized using X-ray absorption spectroscopy and the results compared to computational density functional theory analysis.

Thesis (M.Sc.), Memorial University of Newfoundland, 2000. / Bibliography: leaves 162-182.

Thesis (Ph. D.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 103-104). Also available on the World Wide Web.

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This thesis reports an \textit{ab initio} study of the Ge(111)-5$\times$5 reconstruction, which forms on top of a Si(111)-7$\times$7 substrate. Detailed descriptions of the structural and electronic properties of this surface, obtained from density-functional calculations, are presented and analyzed. A study of In clusters on this surface is performed, and compared with recent experimental work on this system. The effect of surface strain as well as the issue of the Si-Ge interface is addressed. Also, a preliminary investigation of some dynamical aspects of an In atom on the 5$\times$5 surface is presented. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2007-10-26 14:14:48.513 / NSERC HPCVL WestGrid

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Since the discovery of conductance quantization within a nanostnucture, investigations have sought out causes to conductance fluctuations beyond the established plateaus. The focus of this work is to show the fundamental effects upon conductance due to longitudinal potentials and double quantum boxes when confined by hardwall boundaries. A theoretical model based upon a tight-binding recursive tureen's function methodology was modified to incorporate potential barrier variations. A qualitative evaluation, as well as, explanation of the model's results and limitations is discussed. / Department of Physics and Astronomy