Thesis (Ph. D.)--University of Wisconsin, 1967. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Oregon, 2005. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 79-80). Also available for download via the World Wide Web; free to University of Oregon users.

Thesis (Ph. D. in Physics)--Vanderbilt University, Dec. 2006. / Title from title screen. Includes bibliographical references.

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RAMOS, Igor Rochaid Oliveira. Estudo de um sistema clássico de dipolos magnéticos carregados em estruturas de bicamadas. 2010. 75 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2010. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-10-31T20:38:15Z No. of bitstreams: 1 2010_dis_iroramos.pdf: 3512055 bytes, checksum: ba5eb29d22412048c4f8a2972b874c7e (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-10-31T21:08:54Z (GMT) No. of bitstreams: 1 2010_dis_iroramos.pdf: 3512055 bytes, checksum: ba5eb29d22412048c4f8a2972b874c7e (MD5) / Made available in DSpace on 2014-10-31T21:08:54Z (GMT). No. of bitstreams: 1 2010_dis_iroramos.pdf: 3512055 bytes, checksum: ba5eb29d22412048c4f8a2972b874c7e (MD5) Previous issue date: 2010 / We study the structural and dynamical properties of a two-dimensional (2D) classical bi-layer crystal of charged magnetic dipolar particles in a setup where the dipoles are oriented perpendicular to the layers and equal density of charged dipolar particles in each layer. The energy of the system is due to the charge - charge interaction (Coulomb interaction) and the dipole - dipole interaction. Due to the long-range nature of the interactions, we use the Ewald summation method to obtain an expression for the energy involving rapidly convergent sums. By comparing the energies of a number of possible crystal geometries, we determine the phase diagram of the system as a function of the parameter η (which is related to the separation between the layers of charged magnetic dipoles and the particle density) and the relative intensity of the magnetic and electrical interactions. By changing the relative intensity of the dipole - dipole interaction with respect to electrical one, we are able to find six diferent stable crystalline structures as a function of η. An interesting feature of the present model system is the possibility to tune between the matched and staggered arrangements by varying the magnetic interaction between the dipoles, e.g. through an external magnetic field. The phase boundaries of the crystalline structures consist of both continuous and discontinuous transitions. In order to investigate the stability of the minimum energy arrangements we also calculate the phonon spectra of the system within the harmonic approximation. In this case, we resort again on the Ewald technique to obtain the rapidly convergent sums. The analysis of the phonon spectra reveals interesting features which are useful in the study of melting. / Estudamos as estruturas e as propriedades dinâmicas de um cristal clássico bidimensional (2D) em bicamadas de partículas dipolares magnéticas carregadas em um arranjo no qual os dipolos são perpendiculares às camadas e com mesma densidade de partículas em cada camada. A energia do sistema é devido à interação carga - carga (interação coulom- biana) e a interação dipolo - dipolo. Devido ao fato dessas interações serem de longo alcance, usamos o método da soma de Ewald para obter uma expressão para a energia envolvendo somas que convergem rapidamente. Comparando as energias de possíveis geometrias do cristal, determinamos o diagrama de fase do sistema em função do parâmetro η (que está relacionado com a distância entre as camadas de dipolos magnéticos carregados e a densidade de partículas) e da intensidade relativa das interações elétrica e magnética. Mudando a intensidade relativa da interacão dipolo - dipolo com respeito à interação elétrica, podemos encontrar seis diferentes estruturas cristalinas estáveis em função de η. Uma característica interessante desse sistema é a possibilidade de permanecer em arranjos nos quais as camadas são ou não deslocadas uma em relação a outra, apenas variando a interação magnética entre os dipolos, por exemplo, através de um campo magnético externo. As transições entre as estruturas cristalinas podem ser contínuas e descontínuas. No intuito de investigar a estabilidade das configurações de mínima energia, calculamos o espectro dos fônons do sistema usando a aproximação harmônica. Para isto, recorremos novamente a técnica de Ewald para obter somas que convergem rapidamente. A análise da relação de dispersão (fônons) revela características do sistema que são de grande utilidade no estudo da transição sólido-líquido (fusão).

abstract: Since its inception about three decades ago, silicon on insulator (SOI) technology has come a long way to be included in the microelectronics roadmap. Earlier, scientists and engineers focused on ways to increase the microprocessor clock frequency and speed. Today, with smart phones and tablets gaining popularity, power consumption has become a major factor. In this thesis, self-heating effects in a 25nm fully depleted (FD) SOI device are studied by implementing a 2-D particle based device simulator coupled self-consistently with the energy balance equations for both acoustic and optical phonons. Semi-analytical expressions for acoustic and optical phonon scattering rates (all modes) are derived and evaluated using quadratic dispersion relationships. Moreover, probability distribution functions for the final polar angle after scattering is also computed and the rejection technique is implemented for its selection. Since the temperature profile varies throughout the device, temperature dependent scattering tables are used for the electron transport kernel. The phonon energy balance equations are also modified to account for inelasticity in acoustic phonon scattering for all branches. Results obtained from this simulation help in understanding self-heating and the effects it has on the device characteristics. The temperature profiles in the device show a decreasing trend which can be attributed to the inelastic interaction between the electrons and the acoustic phonons. This is further proven by comparing the temperature plots with the simulation results that assume the elastic and equipartition approximation for acoustic and the Einstein model for optical phonons. Thus, acoustic phonon inelasticity and the quadratic phonon dispersion relationships play a crucial role in studying self-heating effects. / Dissertation/Thesis / M.S. Electrical Engineering 2013

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / Nesta tese, estudamos as propriedades estruturais e dinÃmicas, bem como, a fusÃo de sistemas coloidais. Inicialmente, abordamos o problema de determinar as estruturas de mÃnima energia e o espectro de fÃnons de um sistema de dipolos magnÃticos carregados, organizados em uma estrutura de bicamadas e orientados perpendicularmente ao plano das camadas. Este sistema pode ser sintonizado atravÃs de seis diferentes fases cristalinas, atravÃs da variaÃÃo de parÃmetros tais como a separaÃÃo entre as camadas e/ou a carga e/ou o momento de dipolo das partÃculas. A presenÃa de carga elÃtrica nas partÃculas dipolares à responsÃvel pela nucleaÃÃo de cinco fases onde as camadas nÃo estÃo alinhadas verticalmente e uma fase desordenada, que nÃo sÃo encontradas no sistema em bicamadas de dipolos magnÃticos previamente apresentado na literatura. Estas fases extras sÃo uma consequÃncia da competiÃÃo entre a repulsÃo coulombiana e a interaÃÃo atrativa entre os dipolos em diferentes camadas. As estruturas de mÃnima energia sÃo sumarizadas em um diagrama de fases associado à separaÃÃo entre camadas e a importÃncia relativa entre as interaÃÃes elÃtrica e magnÃtica. Determinamos, ainda, a ordem das transiÃÃes estruturais entre as vÃrias configuraÃÃes de mÃnima energia. O espectro de fÃnons do sistema foi calculado usando a aproximaÃÃo harmÃnica. Um comportamento nÃo-monotÃnico do espectro de fÃnons à encontrado como funÃÃo da interaÃÃo efetiva entre as partÃculas. A estabilidade termodinÃmica das diferentes fases à determinada. Em seguida, estudamos o sistema de bicamadas de dipolos magnÃticos carregados para temperaturas diferentes de zero, investigando a fusÃo do sistema atravÃs do critÃrio de Lindemann modificado, em funÃÃo dos parÃmetros: (i) a distÃncia entre as camadas η e (ii) a intensidade relativa da interaÃÃo magnÃtica com respeito à interaÃÃo elÃtrica λ. Para λ suficientemente grande, uma das fases (a fase hexagonal com alinhamento vertical) exibe um comportamento reentrante na temperatura de fusÃo em funÃÃo de η. Uma vez que a carga e o momento de dipolo magnÃtico das partÃculas coloidais pode ser alterado, por exemplo, pela variaÃÃo do pH da soluÃÃo na qual estÃo imersos e por um campo magnÃtico externo, respectivamente, este sistema pode ser em princÃpio verificado experimentalmente. Por Ãltimo, um sistema bidimensional (2D) coloidal binÃrio consistindo de dipolos interagentes à investigado. Dentro da aproximaÃÃo harmÃnica, calculamos o espectro de fÃnons do sistema em funÃÃo da composiÃÃo, da razÃo entre os momentos de dipolo e da razÃo entre as massas das partÃculas pequenas e grandes. AtravÃs de uma anÃlise sistemÃtica dos espectros de fÃnons, determinamos a regiÃo de estabilidade das diferentes estruturas das ligas coloidais. As lacunas no espectro de frequÃncia dos fÃnons, as frequÃncias Ãticas no limite de longos comprimentos de onda e a velocidade do som sÃo tambÃm discutidos. Usando o critÃrio de Lindemann modificado e dentro da aproximaÃÃo harmÃnica, estimamos a temperatura de fusÃo da sub -rede gerada pelas partÃculas grandes. / This thesis presents the study of the structural and dynamical properties, as well as, melting of colloidal systems. Initially, we study the structure and phonon spectrum of a system of charged magnetic dipoles, organized in a bilayer structure and oriented perpendicular to the plane of the layers. This system can be tuned through six different crystalline phases by changing parameters such as the interlayer separation and/or the charge and/or dipole moment of the particles. The presence of the electric charge on the dipole particles is responsible for the nucleation of five staggered phases and a disordered phase which are not found in the magnetic dipole bilayer system previously presented in the literature. These extra phases are a consequence of the competition between the repulsive Coulomb and the attractive dipole interlayer interaction. The minimum energy structures are summarized in a phase diagram associated to the separation between the layers and to the relative importance between the magnetic and electric interactions. We determine the order of the structural phase transitions. The phonon spectrum of the system was calculated within the harmonic approximation. A non-monotonic behavior of the phonon spectrum is found as a function of the effective strength of the inter-particle interaction. The thermodynamic stability of the different phases is determined. Then, we study the bilayer system of charged magnetic dipoles for nonzero temperatures, investigating the melting behavior of the system through the modified Lindemann criterion, as a function of the parameters: (i) the distance between the layers η and (ii) the relative intensity of the magnetic interaction with respect to the electric interaction λ. For large enough λ, one of the phases (the matching hexagonal phase) exhibits a re-entrant melting behavior as a function of η. Since the charges and the magnetic dipole moment of the colloidal particles can be altered, for example, by changing the pH of the solution in which they are immersed or an external magnetic field, respectively, this system can be in principle verified experimentally. Last, a two-dimensional (2D) binary colloidal system consisting of interacting dipoles is investigated. Within the harmonic approximation, we obtained the phonon spectrum of the system as a function of the composition, dipole moment ratio and mass ratio between the small and big particles. Through a systematic analysis of the phonon spectra, we are able to determine the stability region of the different lattice structures of colloidal alloys. The gaps in the phonon frequency spectrum, the optical frequencies in the long-wavelength limit and the sound velocity are discussed as well. Using the modified Lindemann criterion and within the harmonic approximation, we estimated the melting temperature of the sub-lattice generated by the big particles.

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A detailed understanding of fundamental material properties can be obtained through the study of atomic vibrations, performed experimentally with neutron scattering techniques and coupled with the two powerful new computational methodologies I have developed. The first approach involves phonon-based simulations of the pair distribution function - a histogram of localised atomic positions generated experimentally from total scattering data. This is used to reveal ordering behaviour, to validate interatomic models and localised structure, and to give insights into how far dynamic behaviour can be studied using total scattering techniques. Most importantly, the long-standing controversy over dynamic disorder in β-cristobalite is resolved using this technique. Inelastic neutron spectroscopy (INS) allows \emph{direct} study of vibrational modes through their interaction with the neutron beam, and is the experimental basis for the second strand of the new methodology. I have developed new simulation and refinement tools based on the next generation of spectrometers currently being commissioned at the ISIS pulsed neutron source. This allows a detailed powder spectroscopy study of cristobalite and vitreous silica demonstrating that the Bose peak and so-called 'fast sound' features can be derived from standard lattice dynamics in both the crystal and the amorphous counterpart, and allowing discussion of their origins. Given the controversy in the literature, this is a key result. The new methodology also encompasses refinement of interatomic models against powder INS data, with aluminium providing a successful test-case. A more complex example is seen in calcite, with experimental data collected during the commissioning of the new MERLIN spectrometer. Simulated one-phonon coherent INS spectra for the single crystal and powder (the latter including approximations to multi-phonon and multiple scatter) are fully convolved with experimental resolution functions. These are used in the analysis of the experimental data, yielding previously unpublished dispersion curves and soft mode information, as well as allowing the effectiveness of powder refinement of more complex materials to be assessed. Finally, I present further applications with technologically important materials - relaxor ferroelectrics and high temperature pnictide superconductors. The conclusions draw together the different strands of the work, discussing the importance of these new advances together with future developments and scientific applications.