Previous experimental studies have reported wave-like transport of heat in a small number of material systems, such as superfluids like helium II and crystal solids like bismuth. This phenomenon was henceforth referred to as 'second sound'. These rare observations of second sound are partly due to the challenge of obtaining accurate theoretical predictions. In this work, we use an ab-initio framework to study phonon hydrodynamics in 3D crystal fluorides and alkali hydrides, including sodium fluoride (NaF), lithium fluoride (LiF), lithium hydride (LiH), and sodium hydride (NaH). Moreover, we predict the existence of phonon hydrodynamics regime in bilayer graphene systems, including AA-bilayer graphene and AB-bilayer graphene. First, we obtain the second and third-order interatomic force constants using first-principles calculations, which are based on density functional theory (DFT). Secondly, we calculate the lattice thermal conductivity and phonon scattering rates by solving the Boltzmann transport equation (BTE). Thirdly, we apply Guyer's condition to show the phonon hydrodynamics regime based on the average Normal, Umklapp, and Boundary scattering rates. Finally, we examine the effect of different pseudopotentials on the thermal, electronic, and mechanical properties as well as the phonon hydrodynamics regime. In addition, we report the effect of isotopes on the lattice thermal conductivity and phonon hydrodynamics regime. Our calculations predict the existence of the second sound in NaF at 15 K and 8.3 mm characteristic length, consistent with previous experimental work. Based on Guyer's condition, the hydrodynamic window was determined in terms of characteristic lengths (~10² - ~10⁸ nm) and temperatures (up to ~80 K) for fluorides and alkali hydrides. On the other hand, second sound in 2D materials has been predicted to exist at much higher temperatures relative to 3D materials. We report the existence of a second sound for AA-bilayer graphene and AB-bilayer graphene above room temperature at a characteristic length of ~100 nm.

The Fröhlich coupling describes the interaction between electrons and infrared-active vibrations at long wavelength in polar semiconductors and insulators, and may result in the formation of polaronic quasiparticles. Polarons are electrons dressed by a phonon cloud, which can strongly affect the electronic properties of the crystal. Despite their ubiquitous role in a broad range of technologies, first-principles investigations of the electron-phonon interaction in polar materials are scarce. In this thesis we develop a general formalism for calculating the electron-phonon matrix element in polar semiconductors and insulators from first principles, which represents a generalization of the Fröhlich model and can be used to compute the polar electron-phonon coupling as a straightforward post-processing operation. We apply this procedure to explore an important material for photovoltaics, the hybrid lead halide perovskite CH₃NH₃PbI₃. In this case we show that the temperature dependence of emission line broadening is dominated by Fröhlich coupling. Our method is formulated in conjunction with an ab initio interpolation technique based on maximally localized Wannier functions, which allows to describe all forms of electron-phonon coupling on the same footing. We demonstrate the validity of this approach on the prototypical examples GaN and SrTiO₃. Focusing on anatase TiO₂, a transition metal oxide of wide technological interest, we establish quantitatively the effect of including the ab initio Fröhlich coupling in the calculation of electron lifetimes. The rest of the thesis is devoted to exploring the quasiparticle properties in doped oxides. In particular, we investigate angle-resolved photoemission spectra from first principles in doped anatase TiO₂ by proposing a novel framework that combines our ab initio matrix elements, including the dynamical screening arising from the added carriers, and the cumulant expansion approach. We compare our results with experimental data, and show that the transition from a polaronic to a Fermi liquid regime with increasing doping concentration originates from nonadiabatic polar electron-phonon coupling. We further validate this mechanism by calculating angle-resolved photoemission spectra in the ferromagnetic semiconductor EuO.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20699号 / 工博第4396号 / 新制||工||1683(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 北村 隆行, 教授 西脇 眞二, 教授 鈴木 基史 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The purpose of the research contained in this thesis is to allow for the prediction of new structures and properties of crystalline structures due to the application of external pressure by using first-principles numerical computations. The body of the thesis is separated into two primary research projects. The properties of cupric oxide (CuO) have been studied at pressures below 70 GPa, and it has been suggested that it may show room-temperature multiferroics at pressure of 20 to 40 GPa. However, at pressures above these ranges, the properties of CuO have yet to be examined thoroughly. The changes in crystal structure of CuO were examined in these high-pressure ranges. It was predicted that the ambient pressure monoclinic structure changes to a rocksalt structure and CsCl structure at high pressure. Changes in the magnetic ordering were also suggested to occur due to superexchange interactions and Jahn-Teller instabilities arising from the d-orbital electrons. Barium chloride (BaCl) has also been observed, which undergoes a similar structural change due to an s – d transition, and whose structural changes can offer further insight into the transitions observed in CuO. Ammonia borane (NH3BH3) is known to have a crystal structure which contains the molecules in staggered conformation at low pressure. The crystalline structure of NH3BH3 was examined at high pressure, which revealed that the staggered configuration transforms to an eclipsed conformation stabilized by homopolar B–Hδ-∙∙∙ δ-H–B dihydrogen bonds. These bonds are shown to be covalent in nature, comparable in bond strength to conventional hydrogen bonds, and may allow for easier molecular hydrogen formation in hydrogen fuel storage.

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Made available in DSpace on 2014-06-11T19:30:19Z (GMT). No. of bitstreams: 0 Previous issue date: 2009-11-03Bitstream added on 2014-06-13T19:39:43Z : No. of bitstreams: 1 pinto_lmc_me_bauru.pdf: 3368770 bytes, checksum: 3eea6b86b88292db626794855037e7c0 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / A utilização das fases intermetálicas ordenadas como eletrocatalisadores em células a combustível já pode ser considerada como uma solução iminente para os problemas que envolvem a eficiência e as questões econoômicas. Para assegurar que as propriedades geométricas e eletrônicas destes materiais sejam realmente as almejadas para atender a todas as exigências na eletrocatálise das reações de oxidação das moléculas é necessário um estudo aprofundado de caracterização das fases intermetálicas, comumente realizado por criteriosas técnicas experimentais. Entretanto, experimentalmente, a caracterização destes materiais não fornece informações precisas que permitam correlacionar as propriedades dos materiais com o seu desempenho frente a uma dada reação eletrostática. Desta forma, uma estratégia metodológica para se obter um conhecimento mais adequado no estudo das fases intermetálicas é a utilização de métodos computacionais, baseados na Teoria do Funcional da Densidade (DFT). A metodologia empregada neste trabalho aborda uma sistemática para a otimização das propriedades geométricas através da minimização da energia total do sistema, bem como uma avaliação da estrutura eletrônica para estes materiais por meio de projeções sobre os orbitais atômicos na densidade de estados e de mapas de densidade de carga. O processo de otimização é feito por cálculos de campo auto-consistente sucessivos que variam o parâmetro de rede até encontrar uma estrutura que possua energia mínima, este processo pode ser realizado de duas formas, manual e automaticamente pelo código computacional, os resultados obtidos mostram que ambas as formas possuem a mesma precisão, levando a valores quase idênticos e que permitem reproduzir bem os cristais para os materiais estudados. A análise comparativa entre os dados cristalográficos da literatura e os resultados... / The use of ordered intermetallic phases as electrocatalysts in fuel cells can now be regarded as an imminent solution for the problems concerning the efficiency of the device and for economic issue. To ensure that the geometric and electronic properties of these materials are actually suitable for the requirements in the electrocatalysis of melecules oxidation reactions need a meticulous characterization of the intermetallic phases, in general done by standard experimental techniques. However, the characterization of these materials performed solely experimentally does not provide accurate information to enable correlation of the properties of the materials with their performance against a given electrocatalytic reaction. Thus, a methodological strategy for obtaining a better knowledge in the study of ordered intermetallic phases is the use of computational methods, based on the Density Functional Theory. The methodology used in in thius research presents a sytematic optimization of the geometric properties by minimizing the total energy of the system and an evaluation of the electronic structure for these materials by means of the density of states projected onto atomic orbital and charge density maps. The optimization process is done by successively self-consistent field calculations that very the lattice parameter to find a structure that has a minimum energy, this process can be accomplished in two ways, manually and automatically by the computer code, the results show that both forms have the same precision, leading to almost identical identical values and allow to reproduce well the crystals of the studied materials. A comparative analysis of the crystallographic data from the literature and the results presented here show very small errors (in the order of 2-3% for most of the materials), which can be attributed exclusively to the various mathematical approaches applied... (Complete abstract click electronic access below)

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22159号 / 工博第4663号 / 新制||工||1727(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 中村 裕之, 教授 邑瀬 邦明 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Thesis advisor: Fazel Tafti / Since the idea of topology was realized in real materials, the hunt is on for new candidates of topological semimetals with novel electromagnetic responses. For example, topological states can be highly conductive due to a topological protection, which can be destroyed in a magnetic field and lead to an extremely high magnetoresistance. In Weyl semimetals, a transverse current that would usually require a magnetic field to emerge, can be generated by intrinsic Berry curvature without a magnetic field -- the celebrated anomalous Hall effect. In this dissertation, both phenomena mentioned above are studied in rare-earth monopnictides and RAlX material family (R=rare-earths, X=Ge/Si), respectively. The monopnictides are ideal for the study of extreme magnetoresistance because of their topological transitions and abundant magnetic phases. In LaAs, we untied the connection between topological states and the extreme magnetoresistance, the origin of which is clarified. In HoBi, we found an unusual onset of extreme magnetoresistance controlled by a magnetic phase dome. On the other hand, RAlX material family is a new class of Weyl semimetals breaking both inversion and time-reversal symmetries. In particular, in PrAlGeₓSi₁₋ₓ (x=0-1), we unveiled the first transition from intrinsic to extrinsic anomalous Hall effect in ferromagnetic Weyl semimetals, and the role of topology is discussed. In CeAlSi, we found that the Fermi level can be tuned as close as 1 meV away from the Weyl nodes; moreover, a novel anomalous Hall response appears only when the Fermi level is tuned to be near the Weyl nodes. Thus, we established a new transport response solely induced by Weyl nodes. / Thesis (PhD) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第17887号 / 工博第3796号 / 新制||工||1581(附属図書館) / 30707 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 酒井 明, 教授 邑瀬 邦明 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第17888号 / 工博第3797号 / 新制||工||1581(附属図書館) / 30708 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 酒井 明, 教授 邑瀬 邦明 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM