Made available in DSpace on 2014-06-11T19:35:45Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-09-22Bitstream added on 2014-06-13T20:07:27Z : No. of bitstreams: 1 gomes_mc_dr_bauru.pdf: 4074042 bytes, checksum: d6522228622688cc96df0482f8a71eca (MD5) / Fundação de Amparo à Pesquisa do Estado de Mato Grosso (FAPEMAT) / Este trabalho é resultado de um estudo teórico sobre o GaN e o Semicondutor Magnético Ga1-xMnxN, tanto para bulk (sólido) como para as superfícies nanoestruturadas mais estáveis, dada sua importância para o desenvolvimento como material spintrônico. Analisamos deste material suas propriedades estruturais, energéticas e eletrônicas, a partir de cálculos periódicos baseados na teoria do Funcional da Densidade (DFT), como o funcional híbrido B3LYP, e também apresentamos resultados preliminares do estudo das propriedades magnéticas deste material. Realizamos um estudo pormenorizado das estruturas de bandas e da densidade de estados, do Ga1-xMnxN bulk (x~0,02 a 0,18) quanto em superfícies (x~0,0 a 0,17) para os modelos de supercélula de 32 e 96 átomos bulk e modelos de superfícies com 12 e 24 camadas. Os resultados obtidos nos mostram que para concentrações acima de 6% ocorre um acréscimo na distância de ligação Mn-N na direção c, pelo fato do Mn apresentar um raio atômico superior ao Ga e propriedades físicas e químicas distintas. Para os cálculos com superfícies, foi realizada a substituição do Ga por Mn em diferentes posições relativas na superfície, sub-superfície e core, ocorrendo o aumento da energia total conforme os átomos de Mn se movem para os sítios mais internos da superfície e ao considerar a forma de equilíbrio baseada na estabilidade termodinâmica, os valores das energias superficiais das superfícies (1010) e (1120) do GaN wurtzita são as mais estáveis para a concentração de ~8%. Com o aumento da concentração, ocorre nas estruturas de bandas uma diminuição do gap, tanto para o bulk quanto para as superfícies, porém ele se mantém direto no bulk, com exceção para concentração de 18% e, na superfície (1010), enquanto que na superfície (1120) o gap é indireto... / This work is the result os a theoretical study concerning GaN and the Magnetic Semiconductor Ga1-xMnxN, in both the bulk and the most stable nanoestructured surfaces, due to its importance in the development as spintronic material. We analyze the structural, energetic and electronic properties of this material, by means of periodic calculations based on the Functional Density Theory (DFT), at the hybrid functional B3LYP level, and also present the preliminary results of the study of the magnetic properties of this material. We carried out a detailed study of the band structures and the density of states, for both the Ga1-xMnxN bulk (x~002 a 0,18) and its surfaces (x~0,0 a 0,17) using supercell models constitued by 32 and 96 atoms for the bulk and 12 and 24 atomic layers for the surface slab model. Our results show that for Mn concentrations above 6% there is an increase in the Mn-N bond distance in the c direction, due to the fact that the Mn has an atomic radium greater than that of the Ga and different physical and chemical properties. For the surface calculations, we substituted the Ga for the Mn in different positions relative to the external surface, sub-surface and corre, it was observed that the total energy increased as the Mn atoms moved from the surface layer to the interior sites and when we consider the equilibrium shape based in the thermodynamic stability, the most stable surface energies for the (1010) and (1020) planos of wurtzite GaN are found for the ~8% Mn concentration. When the Mn concentration increases, the band gap decreases for the bulk as well as for the surfaces, the gap being direct for the bulk, except for the 18% concentration, and for the (1010) surface, whereas the gap is found indirect for the (1120) surface for the concentrations 6 and 17%. The analysis of the density... (Complete abstract click electronic access below)

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16875号 / 工博第3596号 / 新制||工||1543(附属図書館) / 29550 / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 佐藤 啓文, 教授 田中 一義, 教授 梶 弘典 / 学位規則第4条第1項該当

Quantum Monte Carlo (QMC) methods can yield highly accurate energies for correlated quantum systems. QMC calculations based on many-body wave functions are considerably more accurate than density functional theory methods, and their accuracy rivals that of the most sophisticated quantum chemistry methods. This thesis is concerned with the development of improved wave function forms and their use in performing highly-accurate quantum Monte Carlo calculations. All-electron variational and diffusion Monte Carlo (VMC and DMC) calculations are performed for the first-row atoms and singly-positive ions. Over 98% of the correlation energy is retrieved at the VMC level and over 99% at the DMC level for all the atoms and ions. Their first ionization potentials are calculated within chemical accuracy. Scalar relativistic corrections to the energies, mass-polarization terms, and one- and two-electron expectation values are also evaluated. A form for the electron and intracule densities is presented and fits to this form are performed. Typical Jastrow factors used in quantum Monte Carlo calculations comprise electron-electron, electron-nucleus and electron-electron-nucleus terms. A general Jastrow factor capable of correlating an arbitrary of number of electrons and nuclei, and including anisotropy is outlined. Terms that depend on the relative orientation of electrons are also introduced and applied. This Jastrow factor is applied to electron gases, atoms and molecules and is found to give significant improvement at both VMC and DMC levels. Similar generalizations to backflow transformations will allow useful additional variational freedom in the wave function. In particular, the use of different backflow functions for different orbitals is expected to be important in systems where the orbitals are qualitatively different. The modifications to the code necessary to accommodate orbital-dependent backflow functions are described and some systems in which they are expected to be important are suggested.

京都大学 / 0048 / 新制・課程博士 / 博士(人間・環境学) / 甲第21849号 / 人博第878号 / 新制||人||210(附属図書館) / 2018||人博||878(吉田南総合図書館) / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 田部 勢津久, 教授 内本 喜晴, 教授 加藤 立久, 教授 吉田 寿雄 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DGAM

This thesis covers the structural, electronic, magnetic, and vibrational properties of graphene and silicene. In Chapter I, we will start with an introduction to graphene and silicene. In Chapter II, we will briefly discuss about the methodology (i. e. density functional theory)In Chapter III, we will introduce band gap opening in graphene either by introducing defects/doping or by creating superlattices with h-BN substrate. In Chapter IV, we will focus on the structural and electronic properties of K and Ge-intercalated graphene on SiC(0001). In addition, the enhancement of the superconducting transition temperature in Li-decorated graphene supported by h-BN substrate will be discussed. In Chapter V, we will discuss the vibrational properties of free-standing silicene. In addition, superlattices of silicene with h-BN as well as the phase transition in silicene by applying an external electric field will be discussed. The electronic and magnetic properties transition metal decorated silicene will be discussed, in particular the realization of the quantum anomalous Hall effect will be addressed. Furthermore, the structural, electronic, and magnetic properties of Mn decorated silicene supported by h-BN substrate will be discussed. The conclusion is included in Chapters VI. Finally, we will end with references and a list of publications for this thesis.

Different metals and metal complexes have been used as catalysts in many industries such as commodity petrochemicals, fine and specialty chemicals, polymers, environmental services, agrochemicals and pharmaceuticals. Although these catalysts allow for increased reaction rates and selectivity, they can also be toxic, expensive and of limited supply (cf. Pt group metals). This has led researchers to the intensive study of first row metal catalysts, with nickel standing out as the most widely studied to date. As found for other first row metal catalysts, nickel’s easy access to oxidation states 0-3 allows for a number of different one- and two-electron mechanisms and novel transformations. In Chapter 2 we use a phosphine-free, tridentate N,N,N ligand to generate an active catalyst for the C-N cross-coupling reaction of aryl halides with amines. The catalyst demonstrated excellent turnover numbers (up to 484) for the amination reactions that are proposed to proceed through a Ni(I)-Ni(III) cycle. In Chapter 3 we investigate the Ni coordination chemistry of a biomimetic SNS thiolate ligand. Protonation of the Ni bis(thiolate) complex, Ni(-SNS)2, removes one SNS ligand, affording crystals of a thiolate-bridged dimer dication, {[Ni(--SNS)]2}2+ that exhibits unique anionic tridentate ligand dynamics. Dissolving these crystals, even in weakly-coordinating solvents such as dichloromethane, gives a mixture of ‘naked’ Ni2+ and paramagnetic, trinuclear {[Ni(--SNS)2]2Ni}2+. Although this equilibrium lies far to the right (no diamagnetic dication visible in NMR), addition of ancillary ligands proceeds smoothly to provide several mono- and dinuclear Ni thiolate products, [Ni-SNS)L]n – potential bifunctional catalysts for further studies. In Chapter 4 we demonstrate using chemical and electrochemical techniques that one-electron reduction of Ni(-SNS)2 triggers quantitative imine C-C bond coupling, forming [Ni(S2N2)]- with a redox-active ligand. Spectroelectrochemical studies indicated reversible oxidation and reduction steps give three stable redox states, ([Ni(S2N2)]0/-/2-), that were characterized by NMR, EPR and UV-Vis spectroscopy, X-ray diffraction and computational chemistry. While the Ni(0) dianion (and not the Ni(I) anion) reacted reversibly with phenol and carbon dioxide, results from Chapter 5 showed that reactions with strong electron-acceptor fluoroalkenes proceeded more cleanly with the Ni(I) anion. The latter reactions afforded a mixture of fluoroalkenyl and fluoroalkyl products resulting from C-F bond activation and electron transfer/H atom abstraction, respectively. In Chapter 6 we discuss our results in the context of the current state of the art and suggest some avenues for future development.

This thesis presents the experimental studies of the magnetic and hyperfine interaction properties of four novel Fe-based superconductors (ThFeAsN, CsEuFe4As4, Rb1-δEuFe4As4, and EuFe0.97Ni0.03As2) and one new non-superconducting compound (EuFeAs2). It is supplemented by ab-initio calculations of the electronic structure and magnetism of the three superconductors. The experimental studies are based on the results of x-ray diffraction, magnetic susceptibility, magnetization, and 57Fe and 151Eu Mössbauer spectroscopy measurements. The superconductor ThFeAsN crystallizes in the tetragonal space group P4/nmm with the lattice parameters a = 4.0356(1) Å and c = 8.5286(1) Å. It is shown that there is no magnetic order of the Fe magnetic moments down to 2.0 K. We suggest that nonappearance of Fe magnetism in ThFeAsN may be because of an internal uniaxial chemical pressure whose presence is manifested by the unusually small c/a ratio. We provide theoretical evidence for a mixture of ionic and covalent chemical bonding and metallic characteristics. We present a detailed analysis of the calculated energy band structure of ThFeAsN. A quadrupole doublet well describes the shape of the Mössbauer spectra with a small quadrupole splitting that increases with lowering temperature. Good agreement is found between the calculated and extrapolated 0 K quadrupole splitting values. A fair agreement is noted between the experimental Debye temperature 332(2) K and 370 K of the calculated one. We show that the superconductor CsEuFe4As4 crystallizes in the tetragonal space group P4/mmm with the lattice parameters a = 3.8956(1) Å and c = 13.6628(5) Å. We show that the Fe atoms carry no magnetic moment down to 2.1 K and that the ferromagnetic order is related to the Eu magnetic moments. We establish that the Curie temperature Tc = 15.97(8) K found from the temperature dependence of the hyperfine magnetic field at 151Eu nuclei is well-matched with the temperature dependence of the transferred hyperfine magnetic field at 57Fe nuclei that is produced by the ferromagnetically ordered Eu sublattice. The magnetic moments of the Eu atoms are shown to be perpendicular to the crystallographic c-axis. The T 3⁄2 power-law perfectly describes the temperature dependence of the principal component of the electric field gradient tensor, both at Fe and Eu sites. The calculated and the measured parameters of the hyperfine-interaction are in excellent agreement with each other. We determine that the Debye temperature of CsEuFe4As4 is 295(3) K. Ab-initio calculations suggest a mixture of ionic, covalent, and metallic bonding between the constituent atoms in the CsEuFe4As4 superconductor. We confirm that the strongly localized Eu f states are the origin of the magnetic moment of CsEuFe4As4, in agreement with the experimental results. We show that an almost zero magnetic moment carried by the Fe atoms is caused by the spin-up and spin-down states' apparent symmetry. We show that the Fermi surfaces have hole-like and electron-like pockets located at the center and corners of the Brillouin zone, respectively. The superconductor Rb1-δEuFe4As4 crystallizes in the tetragonal space group P4/mmm with the lattice parameters a = 3.8849(1) Å and c = 13.3370(3) Å. We show that the Fe atoms carry no magnetic moment down to 2.1 K and that the ferromagnetic order is associated solely with the Eu magnetic moments. The Curie temperature Tc = 16.54(8) K is found from the temperature dependence of both the hyperfine magnetic field at 151Eu nuclei and the transferred hyperfine magnetic field at 57Fe nuclei induced by the ferromagnetically ordered Eu sublattice. We find that the Eu magnetic moments lie in the ab plane. It is observed that the temperature dependence of the principal component of the electric field gradient tensor, at both Fe and Eu sites, is well described by a T3⁄2 power-law relation. There is good agreement between the calculated and measured parameters of the hyperfine-interaction. We determine that the Debye temperature of Rb1-δEuFe4As4 is 391(8) K. Ab-initio calculations indicate the presence of a mixture of ionic, covalent, and metallic bonding between the constituent atoms in the RbEuFe4As4 superconductor. We show that the magnetic moment of RbEuFe4As4 is mainly a result of the strongly localized Eu f states. It is shown that an almost zero magnetic moment carried by the Fe atoms originates from an apparent symmetry of the spin-up and spin-down states. We show that the electrical and chemical properties of RbEuFe4As4 are closely associated with the presence of the Fe 3d states in the Fermi energy region. The Fermi surfaces display hole-like and electron-like pockets, respectively, at the center and corners of the Brillouin zone. We find that in both the EuFeAs2 compound and 14 K superconductor EuFe0.97Ni0.03As2 the antiferromagnetic ordering of the Fe sublattice is of a spin-density-wave type with the Néel temperatures and Fe saturation magnetic moments of 106.2(1.9) K, 0.78(1) μB and 56.6(2.2) K, 0.47(1) μB, respectively. We show that the Néel temperatures and the saturation hyperfine magnetic fields in the two compounds with the antiferromagnetically ordered Eu sublattice are 44.4(5) K, 294.2(7) kOe and 43.5(1) K, 290.5(1) kOe respectively. The 3% substitution of Fe by Ni in EuFeAs2, aside from producing superconductivity in EuFe0.97Ni0.03As2, radically reduces the strength of magnetism of the Fe sublattice and has nearly no impact on the magnetism of the Eu sublattice. The appearance of antiferromagnetically ordered Fe and Eu sublattices in EuFe0.97Ni0.03As2 verifies that superconductivity and magnetism coexist in this compond. The growth of the magnitude of the main component of the electric field gradient tensor, at both Fe and Eu sites, with reducing temperature, is well described by a T3⁄2 power-law relation. We determine the Debye temperatures of EuFeAs2, EuFe0.97Ni0.03As2, and the FeAs2 impurity phase to be 355(18), 428(14), and 594(25) K, respectively. In summary, for all of the studied compounds, there is no magnetic ordering associated with iron sub-lattices in the ThFeAsN, CsEuFe4As4, and Rb1-δEuFe4As4 compounds. The iron sublattice is magnetically ordered in the EuFeAs2 and the EuFe0.97Ni0.03As2 superconductor. There is a coexistence of magnetism and superconductivity associated with europium in the CsEuFe4As4, Rb1-δEuFe4As4, EuFe0.97Ni0.03As2 compounds. There is a good agreement between the calculated and the measured hyperfine and magnetic parameters for most studied compounds.

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Through the exploitation of data-sparsity ---a catch all term for savings gained from a variety of approximations--- it is possible to reduce the computational cost of accurate electronic structure calculations to linear. Meaning, that the total time to solution for the calculation grows at the same rate as the number of particles that are correlated. Multiple techniques for exploiting data-sparsity are discussed, with a focus on those that can be systematically improved by tightening numerical parameters such that as the parameter approaches zero the approximation becomes exact. These techniques are first applied to Hartree-Fock theory and then we attempt to design a linear scaling massively parallel electron correlation strategy based on second order perturbation theory. / Ph. D.