It has become increasingly apparent that the future of electronic devices can and will rely on the functionality provided by single or few dopant atoms. The most scalable physical system for quantum technologies, i.e. sensing, communication and computation, are spins in crystal lattices. Diamond is an excellent host crystal offering long room temperature spin coherence times and there has been exceptional experimental work done with the nitrogen vacancy center in diamond demonstrating many forms of spin control. Transition metal dopants have additional advantages, large spin-orbit interaction and internal core levels, that are not present in the nitrogen vacancy center. This work explores the implications of the internal degrees of freedom associated with the core d levels using a tight-binding model and the Koster-Slater technique. The core d levels split into two separate symmetry states in tetrahedral bonding environments and result in two levels with different wavefunction spatial extents. For 4d semiconductors, e.g. GaAs, this is reproduced in the tight-binding model by adding a set of d orbitals on the location of the transition metal impurity and modifying the hopping parameters from impurity to its nearest neighbors. This model does not work in the case of 3d semiconductors, e.g. diamond, where there is no physical reason to drastically alter the hopping from 3d dopant to host and the difference in wavefunction extent is not as pronounced. In the case of iron dopants in gallium arsenide the split symmetry levels in the band gap are responsible for a decrease in tunneling current when measured with a scanning tunneling microscope due to interference between two elastic tunneling paths and comparison between wavefunction measurements and tight-binding calculations provides information regarding material parameters. In the case of transition metal dopants in diamond there is less distinction between the symmetry split d levels. When considering pairs of transition metal dopants an important quantity is the exchange interaction between the two, which is a measure of how fast a gate can be operated between the pair and how well entanglement can be created. The exchange interaction between pairs of transition metal dopants has been calculated in diamond for several directions in the (110) plane, and for select transition metal dopants in gallium arsenide. In tetrahedral semiconductors transition metal dopants provide an internal degree of freedom due to the symmetry split d levels and this included functionality makes them special candidates for single spin based quantum technologies as well as physical systems to learn about fundamental physics.

Tight-binding potential combined with a simulated annealing method is used to study the generalized stacking fault structure and energy of gold. The potential is chosen to fit band structures and total energies from a set of first-principles calculations (Phys. Rev. B54, 4519 (1996)). It is found that the relaxed stacking fault energy (SFE) and anti-SFE are equal to 46 and 102 mJ/m², respectively, and in good agreement with the first principles calculations and experiment. In addition, the potential predicts that the c/a of hcp-like stacking fault structure in Au is slightly smaller than the ideal one. / Singapore-MIT Alliance (SMA)

As devise sizes approach the nanoscale, novel device geometries and materials are considered, and new types of essential physics becomes important and new physical switching mechanism are considered, and as our intuitive understanding of device behavior is stretched accordingly, increasing first-principles simulation is required to understand and predict device behavior. To this end, initially I worked to capture the richness of the confinement and transport physics in quantum-wire devices. I developed an efficient fully three dimensional atomistic quantum transport simulator within a nearest-neighbor atomistic tight-binding framework. However, I soon adapted this work to the study of transport in graphene mono-layer and bilayer nano-ribbons. Motivated by proposals for use of nano-ribbons to create band gaps in otherwise gapless graphene monolayers, I studied the effects of edge disorder in such graphene nano-ribbon FETs. I found that ribbon widths sufficiently narrow to produce useful bandgaps, would also lead to an extreme sensitivity to ribbon-edge roughness and associated performance degradation and device-to-device variability. Going beyond conventional switching but staying with the graphene material system, to model electron-hole condensation in two graphene monolayers separated by a tunnel dielectric potentially beyond room temperature, I developed a self-consistent atomistic tight-binding treatment of the required interlayer exchange interaction within non-local Hartree-Fock mean-field theory. Such condensation, associated many-body enhanced interlayer current flow, and gate-control thereof is the basis for the beyond-CMOS Bilayer-pseudoSpin Field Effect Transistor (BiSFET) proposed by colleagues. I studied the effect of various system parameters and on interlayer charge imbalance on the strength of the condensate state. I also modeled the critical current, the maximum interlayer current that can be supported by the condensate, its detailed dependence on the nature and strength of the required interlayer bare tunneling and on charge imbalance. The results presented here are expected to be used to refine devices models of the BiSFET, and may serve as guides to experiments to observe such a condensate state. / text

Tesis de Doctorado para optar al grado de Doctor en Ciencias con mención en Física. / Los aisladores topológicos a grandes rasgos son aisladores en el bulto y presentan estados de borde metálicos que están protegidos por alguna simetría del sistema, mediciones ARPES han logrado detectar estos estados, mientras que en experimentos de transporte, estos estados son empañados por una contribución debido a los defectos e impurezas del bulto. El objetivo de esta tesis es proponer un modelo teórico, que sea capaz de entregar una mayor robustez a los estados de superficies de los aisladores topológicos. Para este fin, usaremos una geometría de heteroestructuras o super-redes, ya que dada la gran experiencia experimental en el crecimiento de este tipo de sistema, su realización experimental es factible. Debido el vertiginoso avance del área de los aisladores topológicos, comenzaremos con una breve reseña histórica de el surgimiento de este tipo de materiales (capítulo 1), para luego explicar la física subyacente de los aisladores topológicos en uno de los modelos de aislador topológico mas sencillos, el modelo SSH (capítulo 2) y un modelo mas interesante, el modelo BHZ (capítulo 4), este último fue comprobado experimentalmente. Luego, pasaremos a implementar la estrategia de super-redes para generar nuevos estados de borde topológicamente protegidos, que tienen como base el modelo SSH (capítulo 3) y el modelo BHZ (capítulo 5). En esta tesis, mediante el método tight-binding y los modelos sencillos anteriormente mencionados, logramos diseñar exitosamente nuevos estados de borde topológicos, los que son mucho más resistentes al desorden atómico que sus estructuras base (SSH o BHZ), incluso cuando este desorden destruye la simetría que permite la existencia del orden topológico.

It is known that a highly ordered medium allows certain wave functions to move unhindered throughout and in this manner achieve delocalization. It is also known that if one introduces disorder into a medium, wave packets will not be able to move as freely and will instead be trapped or localized. In this thesis, I have simulated a medium in which the amount of disorder can be modified and using this I have shown that the shape of the localization can be altered.

In the first part of this thesis, we demonstrate theory and computations for finite-energy line defect solutions in an improvement of Ericksen-Leslie liquid crystal theory. Planar director fields are considered in two and three space dimensions, and we demonstrate straight as well as loop disclination solutions. The possibility of static balance of forces in the presence of a disclination and in the absence of ow and body forces is discussed. The work exploits an implicit conceptual connection between the Weingarten-Volterra characterization of possible jumps in certain potential fields and the Stokes-Helmholtz resolution of vector fields. The theoretical basis of our work is compared and contrasted with the theory of Volterra disclinations in elasticity. Physical reasoning precluding a gauge-invariant structure for the model is also presented. In part II of the thesis, the time-harmonic Schrodinger equation with periodic potential is considered. We derive the asymptotic form of the scattering wave function in the periodic space and investigate the possibility of its application as a DtN non-reflecting boundary condition. Moreover, we study the perfectly matched layer method for this problem and show that it is a reliable method, which converges rapidly to the exact solution, as the thickness of the absorbing layer increases. Moreover, we use the tight-binding method to numerically solve the Schrodinger equation for Graphene sheets, symmetry-adapted Carbon nanotubes and DNA molecules to demonstrate their electronic behavior in the presence of local defects. The results for Y-junction Carbon nanotubes depict very interesting properties and confirms the predictions for their application as new transistors.

Numerical simulations have been performed to study the single-charge-induced ON current fluctuations (random telegraphic noise) in conventional (MOSFET) and non-conventional (silicon nanowire) nanoscale field-effect transistors. A semi-classical three-dimensional particle-based Monte Carlo device simulator (MCDS 3-D) has been integrated and used in this work. Quantum mechanical space-quantization effects have been accounted for via a parameter-free effective potential scheme that has been proved quite successful in describing charge set back from the interface and quantization of the energy (bandgap widening) within the channel region of the device. The effective potential is based on a perturbation theory around thermodynamic equilibrium and leads to a quantum field formalism in which the size of the electron depends upon its energy. To treat full Coulomb (electron-ion and electron-electron) interactions properly, the simulator implements two different real-space molecular dynamics (MD) schemes: the particle-particle-particle-mesh (P3M) method and the corrected Coulomb approach. For better accuracy, particularly in case of nanowire FETs, bandstructure parameters (bandgap, effective masses, and density of states) have been computed via a 20-band nearest-neighbor sp3d5s* tight-binding scheme. Also, since the presence of single impurities in the channel region represents a rare event in the carrier transport process, necessary event-biasing algorithms have been implemented in the simulator that, while enhancing the statistics, results in a faster convergence in the chan-nel current. The study confirms that, due to the presence of single channel charges, both the electrostatics (carrier density) and dynamics (mobility) are modified and, therefore, simultaneously play important roles in determining the magnitude of the current fluctuations. The relative impact (percentage change in the ON current) depends on an intricate interplay of device size, geometry, crystal direction, gate bias, temperature, and energetics and spatial location of the trap.

Made available in DSpace on 2014-06-11T19:30:20Z (GMT). No. of bitstreams: 0 Previous issue date: 2010-07-07Bitstream added on 2014-06-13T18:47:33Z : No. of bitstreams: 1 ribeiro_av_me_bauru.pdf: 3385358 bytes, checksum: 8e2e43e5facbedc0c7e25a21e63fe6ac (MD5) / São calculadas as estruturas eletrônicas de arranjos atômicos periódicos unidimensionais, bidimensionais e tridimensionais, através do método de combinação linear de orbitais atômicos (método tight binding). Esses orbitais correspondem aos átomos isolados das espécies químicas que compõem o arranjo atômico sob investigação. Combinações lineares deles, com coeficientes apropriados, aproximam a forma das funções de onda eletrônicas do arranjo atômico. Nos casos em que a sobreposição dos orbitais é desprezada, a contribuição de cada orbital atômico para função de Bloch é mostrada nas representações gráficas das estruturas de bandas calculadas. Após uma brve apresentação do método tight binding, são calculadas as estruturas de bandas de cadeias lineares de átomos de Carbono que têm um ou dois átomos por célula unitária. Essas cadeias são chamadas de cumuleno e poliino, respectivamente. Dentre os arranjos atômicos bidimensionais de interesse, é calculada a estrutura de bandas do grafeno. Essas energias são comparadas com resultados disponíveis na literatura. Para este material é realizada uma breve discussão sobre as bandas 'pi' provenientes de orbitais 'p IND. z' e sobre como a sobreposição dos orbitais atômicos afeta a forma das bandas. O método também é aplicado na modelagem de cristais tridimensionais. São calculadas as estruturas de bandas doo diamante, Germânio (com estrutura de diamente), Arseneto de Gálio (com estrutura zincblend) e Nitreto de Gálio (com estrutura de wurtzita). Os resultados obtidos são comparados com aqueles reportados por outros autores que usaram métodos ab initio / The eletronic structures of periodic arrangements of atoms in one, two and three dimensions are calculated by a linear combinations of atomic orbitals (tight binding method). Those orbitals correspond to the isolated atoms of the chemical species composing the atomic arrangement under investigation. Suitable linear combinations of such states approximate the shape of the eletronic wave functions of the atomic arrangement. When the overlapping of the atomic orbitals is disregarded, the contribution of each orbital to the Bloch state is displayed in the graphs of the band structures. After a brief description of the tight binding method, the band structures of linear chains of Carbon atoms are calculated. The cases of one and two atoms per unit cell are considered. They correspond to cumulene and polyyne, respectively. Among the two-dimensional atomic arrangements of interest, we focus the calculation of the band structure of graphene. The calculated bands are compared with available results. Some attention is devoted to the 'pi' bands associated to the 'p IND. z' orbitals is presented. The effects of the overlapping of the atomic orbitals are discussed. The method is also applied to model three-dimensional crystels. The band structures of diamong, germanium (with diamond structure), Gallium Arsenide (with zincblende structure) and Gallium Nitride (with wurtzite structure) are obtained. The results are compared with those reported by other authors who applied ab initio methods

The focus of this report is the derivation of the Bogoliubov-de Gennes equations for superconductors from a tight-binding model, restricting ourselves to the case of s-wave superconductors.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20381号 / 工博第4318号 / 新制||工||1669(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 白石 誠司, 准教授 浅野 卓 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM