Thesis (Ph. D.)--University of Wisconsin--Madison, 1975. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 130-137).

Ab initio prediction of the electronic properties of solids is traditionally performed using groundstate Density Functional Theory. These methods are unreliable however, for a class of important problems involving weak attractive forces. These problems include (i) the energetics of hydrogen storage and metal interactions in graphene, (ii) cohesion properties of some polymer systems and (iii) possibly, the weak hydrophobic forces in biomolecules. For these cases a more powerful method than groundstate DFT are timedependent DFT (tdDFT) methods related to the Random-Phase Approximation (RPA). All of these methods proceed by looking at the dynamic density-density response function, whose long-ranged properties naturally lead to the weak forces referred to above. In this thesis we have tested these ideas by investigating electronic response and correlation on the predicted properties of spherical atoms. We have developed and tested a variety of approximations to the timedependent response function through approximations of the tdDFT class and a new method involving greater self-consistency in the screening equation, the inhomogenous STLS approach. Through the development of new methods and computer code, we have solved the response equation allowing us to test our approximations on atoms. Calculation of certain dynamic and static properties of a variety of atoms within our approximations generally agree well with known results. In this thesis we have calculated excitation energies of Helium, dipole polarisabilities and C6 van der Waals (vdW) coefficients of a variety of atoms, and groundstate correlation energies Ec of some atoms. The excitation spectra of Helium generated in our new PGG+c approximation are in good agreement with experiment. The dipole polarisabilities are generally in good agreement with known results, with the exception of Magnesium, Beryllium and Sodium. The C6 coefficients are a little poorer with the exception of Helium where they are nearly exact. Correlation energies are generally reasonable in the PGG+c approximation although they are considerably less accurate than the other properties we have calculated for all atoms other than He. The ISTLS correlation energy of Helium is within 5% suggesting that this method may perform well for larger atoms where our present numerical techniques require improvement. These generally positive results suggest that the approximations we have developed may be applied to more complicated systems such as those described above with good results.

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Despite of enormous progress in experimental nanophysics theoretical studies of low-dimensional electron systems still remains a challenging task. Indeed, most of the structures are strongly correlated, so that an effective perturbative treatment is impossible due to the lack of a small parameter. The problem can be partly solved within the dynamical mean-field theory (DMFT) paradigm, nevertheless the correlations in physically relevant high-temperature superconductors are of purely non-local nature. The recently developed dual fermion approximation, combining field-theoretical diagram technique and numerical methods, allows for explicit account of spatial correlations. The approximation was shown to be of fastest convergence compared with standard DMFT extensions, and along with renormalization group is used here to study Fermi condensation on a triangular lattice near van Hove singularities. The still debated phenomenon of Fermi condensation is believed to be a precursor to strongly correlated low-temperature instability and is found in this thesis to be robust even at high temperature, making its experimental verification feasible. Unlike homogeneous ferromagnetic ordering a variety of non-collinear ground state configurations emerge as a result of competition among exchange, anisotropy, and dipole-dipole interaction. These particle-like states, e.g. magnetic soliton, skyrmion, domain wall, form a spatially localized clot of magnetic energy. Consistent study of spin, which essentially is a quantum mechanical entity, led to the emergence of spintronics (spin-based electronics) and magnonics (photonics with spin waves), in the meanwhile topologically protected magnetic solitons and skyrmions might potentially be applied for data processing and information storage in next generation of electronic technology (rapidly advancing solitonics and skyrmionics). An ability to easily create, address, and manipulate such structures is among the prerequisite forming a basis of "-onics" technology. It is shown here that spins on a kagome lattice, interacting via Heisenberg exchange and Dzyaloshinskii-Moriya coupling, allow the formation of topologically protected edge states through which a skyrmion can propagate. Not only can chemical methods be used to design novel functionality, but also geometric structuring. It is demonstrated that for graphene sandwiched between two insulating media external circularly-polarized light serves as an effective magnetic field. The direct practical implication permits to control light polarization and induce spin-waves propagating on the surface of e.g. a topological insulator. The newly discovered Dirac materials, graphene and three-dimensional topological insulators, are not easy to handle. In fact, the quasiparticle band function is gapless preventing them from being used in integrated circuits, nevertheless the problem is shown here to be partially relaxed by placing a vacancy on top of it.

The crystal growth conditions of compounds of the series Srn+1IrnO3n+1 (n=1, 2 and ∞) are investigated. It was found that the ratio of IrO2:SrCO3 in the starting mixture is the most important variable in determining the phase formed. Good quality samples of Sr3Ir2O7 were found to have a sharp change in gradient at the Néel temperature of 287.5 K and no secondary T* transition between 230 K and 260 K. All crystals of Sr3Ir2O7 grown were found to be heavily oxygen deficient by EPMA regardless of the crystal growth conditions used with an average stoichiometry of Sr2:87Ir2O6:27. Adding more electrons via replacing strontium with lanthanum causes (Sr(1-x)Lax)3Ir2O7 to become metallic by x=0.072, which also fully quenches the long-range antiferromagnetic order. Heat capacity and resistivity measurements show that metallic (Sr(1-x)Lax)3Ir2O7 is a weakly correlated Fermi-liquid metal. Given that there are only subtle changes to the structure upon lanthanum doping, the metal-insulator transition is a result of electron doping rather than structural distortions. No structural phase transitions were found up to a temperature of 800°C and no additional evidence was found to support the Bbcb space group model of the structure of Sr3Ir2O7. Using crystals five times better in quality than those reported in the literature, SrIrO3 was found to be a Fermi-liquid metal, rather than a non-Fermi liquid metal as previously reported, and no superconductivity was found down to temperatures of 20 mK. A known Pt(III) compound, CaPt2O4, was found to be a weakly correlated metal down to 2 K and a novel Pt(III) based compound, K2CaPt3-δ O6 (δ ≈ 0.4), was discovered. K2CaPt3δ-O6 has a structure consisting of monolayers of edge-sharing PtO6 octahedra separated by layers of ordered K+ and Ca2+ ions in a 2:1 ratio. The structure of K2CaPt3-δO6 was found to be flexible to doping with copper, causing the magnetic properties to change from temperature independent to paramagnetic.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18772号 / 理博第4030号 / 新制||理||1581(附属図書館) / 31723 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 川上 則雄, 教授 田中 耕一郎, 教授 松田 祐司 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

In the first part of this work, we describe a theory of the ground states and charge gaps in the fractional quantum Hall states of graphene. The theory relies on knowledge of these properties for filling fractions smaller than one. Then, by the application of two mapping rules, one is able to obtain these properties for fractional quantum Hall states at arbitrary fillings, by conceiving the quantum Hall ferromagnets as vacua on which correlated electrons or correlated holes are added. The predicted charge gaps and phase transitions between different fractional quantum Hall states are in good agreement with recent experiments. In the second part, we investigate the low energy theory for the neutral Landau level of bilayer graphene. We closely analyze the way different terms in the Hamiltonian transform under the action of particle-hole conjugation symmetries, and identify several terms that are relevant in explaining the lack of such symmetry in experiments. Combining an accurate parametrization of the electronic structure of bilayer graphene with a systematic account of the impact of screening we are able to explain the absence of particle-hole symmetry reported in recent experiments. We also study the energetics of fractional quantum Hall states with coherence between n=0 and n=1 cyclotron quantum numbers, and obtain a general formula to map the two-point correlation function from their well-known counterparts made from only n=0 quantum numbers. Bilayer graphene has the potential for realizing these states which have no analogue in other two-dimensional electron systems such as Gallium Arsenide. We apply this formula to describe the properties of the n=0/n=1 coherent Laughlin state which displays nematic correlations. / text

Neutron scattering measurements were used to investigate the magnetic and crystal structure and magnetic excitations of three compounds characterized as low-dimensional quantum magnets. The materials are frustrated systems with low spin quantum number. The first was a powder sample of AgNiO₂. The Ni ions form a triangular lattice antiferromagnet in which, according to the published crystal structure, both the orbital order and magnetic couplings are frustrated. However, it is shown here that there was a small distortion of the crystal structure at 365 K, which is proposed to result from charge disproportionation and this relieves the orbital frustration. The magnetic structure was investigated and, below 20 K, the triangular lattice of electron-rich Ni sites was observed to order into antiferromagnetic stripes. Investigations of the magnetic excitations showed that the main dispersions were within the triangular plane, indicating a strong two-dimensionality. The dispersion was larger along the stripes than between the stripes of collinear spins. The second material investigated was CoNb₂O₆, a quasi Ising-like ferromagnet. It was studied with a magnetic field applied transverse to the Ising direction. The magnetic field introduced quantum fluctuations which drove a phase transition at a field comparable to the main exchange interaction. The phase diagram of the magnetic order was mapped outs and a transition from an ordered phase to a paramagnetic phase was identified at high field. This low-temperature high-field phase transition was further investigated by inelastic neutron scattering measurements to observe the change in the energy gap and magnetic excitation spectrum on either side of the transition. The spectrum had two components in the ordered phase and had sharp magnon modes in the paramagnetic phase. The third material was the spin-half layered antiferromagnet CuSb₂O₆. It has a square lattice of Cu²⁺ ions in which the main interaction is across only one diagonal of the square. The magnetic structure was studied by neutron scattering with a field applied along the direction of the zero-field ordered moment. A spin-flop was observed at low field and there was evidence for a high-field transition. The magnetic excitation spectrum was unusual in that it had an intense resonance at 13 meV at the magnetic Brillouin zone boundary.

Orientadores: Ricardo Rodrigues Urbano, Carlos Rettori / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-02T02:37:27Z (GMT). No. of bitstreams: 1 Lesseux_GuilhermeGorgen_D.pdf: 15008598 bytes, checksum: bc12c78a89f519f63d096bf87e9bde84 (MD5) Previous issue date: 2017 / Resumo: Este trabalho teve por objetivo a investigação de sistemas com elétrons fortemente correlacionados via ressonância magnética nuclear (NMR) e eletrônica (ESR). Os seguintes sistemas foram investigados: i) o isolante Kondo SmB6 dopado com impurezas de Er3+ (via ESR), ii) compostos supercondutores à base de FeAs, da família BaFe 2As2 (via NMR) e, iii) o composto férmion pesado CeRhIn5 (via NMR em altos campos magnéticos). O estudo no composto de SmB6 via ESR (9.5 GHz) dos íons de Er3+ revelou um conjunto de quatro transições em baixa temperatura com uma anisotropia que não corresponde à esperada para transições entre níveis de campo cristalino cúbico. Mostramos que o efeito Jahn-Teller (JT) dinâmico associado a vibrações anarmônicas dos íons de Er3+ nos interstícios dos octaedros de B na rede de SmB6 explica a anisotropia das transições finas em baixa temperatura e concorda com o comportamento térmico da intensidade destas linhas de ressonância. Como resultado deste trabalho, ficou então proposta uma nova interpretação dos resultados sob a luz de um efeito de rattling anarmônico dos íons de Er3+ na matriz. Nenhum efeito de isolante topológico tipo Kondo foi evidenciado nos experimentos de ESR. Para os compostos de BaFe2As2 puro e com pouca substituição química (~ 0.5%) de Mn, Co e Cu realizamos um estudo detalhado da evolução das transições de alta temperatura, estrutural e magnética, que ocorrem no diagrama de fase dessa família de supercondutores. Combinando experimentos de NMR para o 75As em altos campos magnéticos, difração de raios-X de alta resolução e calor especíco mostramos que a fase ortorrômbica é estabilizada por flutuações magnéticas via acoplamento magneto elástico e que o ajuste do ângulo entre as ligações de As-Fe-As é o parâmetro mais relevante para a supressão das temperaturas de transição estrutural e magnética. Logo, este ajuste estrutural leva a um rearranjo da ocupação dos orbitais 3d do Fe aumentando a ocupação nos orbitais planares (3dxy), condição fundamental para que a fase supercondutora se forme nestes materiais. Já para o composto de CeRhIn5 investigamos a transição quântica induzida por campo magnético que ocorre em torno de 30 T (~ 1 K) neste material via NMR do 115In em ultra altos campos magnéticos. Observamos uma alteração no knight-shift associado ao In(1), que ocupa os planos de CeIn3. Apesar de nenhum efeito evidente na forma de linha, há um knight-shift líquido de 2.3% através da transição em Bc ~ 30 T. Isto demonstra uma mudança efetiva na densidade de estados no nível de Fermi consistente com a reconstrução da superfície de Fermi em torno de 30 T previamente reportada por medidas de oscilações quânticas. O fato de não se observar alteração de forma de linha espectral em 30 T nos permitiu concluir que a estrutura magnética incomensurável do CeRhIn5 não é drasticamente alterada através da transição o que corrobora com o cenário de um ponto crítico itinerante em torno de 50 T para CeRhIn5. Esta tese demonstra a relevância da técnica de Ressonância Magnética (NMR e ESR) na investigação das propriedades físicas de sistemas com elétrons fortemente correlacionados / Abstract: The present work aimed the investigation of strongly correlated electron systems via nuclear (NMR) and electronic (ESR) magnetic resonance. The following systems were investigated: i) Er3+ doped SmB6 Kondo insulator (via ESR), ii) FeAs-based superconducting compounds, from the BaFe2As2 family (via NMR) and, iii) CeRhIn5 heavy fermion compound (via ultra-high magnetic field NMR). The study on the SmB6 via Er3+ ESR (9.5 GHz) revealed a set of four resonance transitions at low temperature which show an anisotropy that does not correspond to the expected for transitions of pure cubic crystal field levels. We have shown that the dynamic Jahn-Teller (JT) effect associated to anharmonic rattling vibrations of Er3+ ions at the interstitial of the B-octahedron in the SmB6 lattice explains the anisotropy of the narrow lines at low temperature and agrees with the thermal behavior of the intensity of these resonance lines. As a result of this work, we proposed a new interpretation of the results under the light of a anharmonic rattling of the Er3+ ions in the SmB6 lattice. No topological insulator effect was evidenced by our ESR experiments. For the BaFe2As2 undoped and slightly substituted (~ 0.5%) of Mn, Co and Cu compounds we have performed a detailed study of the evolution of the high temperature transitions, structural and magnetic, which occur in the phase diagram of this superconductor family. Combining 75As high field NMR, high resolution X-ray diffraction and specific heat experiments we have shown that the orthorhombic phase is stabilized by magnetic fluctuations via magneto-elastic coupling and that the tuning of the angle of the As-Fe-As bounds is the most relevant parameter to the suppression of the structural and magnetic transition temperatures. Thus, this structural tuning leads to a rearrangement of the occupancy of the Fe-3d orbitals increasing the occupancy of the planar orbital (3dxy), which is a fundamental condition to the formation of the superconducting phase in these materials. Finally, for the CeRhIn5 compound we have investigated the magnetic field-induced quantum transition which occurs around ~ 30 T (~ 1 K) in this material via 115In ultra-high magnetic field NMR. We observed a knight-shift alteration for the In(1), which is sited in the CeIn3 planes. There is a net knight-shift of 2.3% across the transition at Bc ~ 30 T. Although lineshape effects have not been detected, it demonstrates an effective change in the density of states at the Fermi level consistent with a Fermi surface reconstruction around 30 T previously reported by quantum oscillation measurements. We did not observe a change in the spectral lineshape across 30 T and it leads us to the conclusion that the CeRhIn5 incommensurate magnetic structure is not drastically altered across the transition which is consistent with the scenario of an itinerant quantum critical point at 50 T for CeRhIn5. This thesis demonstrates the relevance of the magnetic resonance (NMR and ESR) techniques in the investigation of physical properties of strongly correlated electron systems / Doutorado / Física / Doutor em Ciências / 140837/2013-2 / CNPQ

High-quality platelet single crystals of Tl₂Ba₂CuO₆±δ (Tl-2201) have been grown using a novel time-varying encapsulation scheme, minimizing the thallium oxide loss that has plagued other attempts and reducing cation substitution. This encapsulation scheme allows the melt to be decanted from the crystals, a step previously impossible, and the remaining cation substitution is homogenized via a high-temperature anneal. Oxygen annealing schemes were developed to produce sharp superconducting transitions from 5 to 85 K without damaging the crystals. The crystals' high homogeneity and high degree of crystalline perfection are further evidenced by narrow rocking curves; the crystals are comparable to YSZ-grown YBa₂Cu₃O₆₊δ by both metrics. Electron probe microanalysis (EPMA) ascertained the crystals' composition to be Tl₁.₉₂₀₍₂₎Ba₁.₉₆₍₂₎Cu₁.₀₈₀₍₂₎O₆₊δ; X-ray diffraction found the composition of a Tc = 75 K crystal to be Tl₁.₉₁₄₍₁₄₎Ba₂Cu₁.₀₈₆₍₁₄₎O₆.₀₇₍₅₎, in excellent agreement. X-ray refinement of the crystal structure found the crystals orthorhombic at most dopings, and their structure to be in general agreement with previous powder data. Cation-substituted Tl-2201 can be orthorhombic, orthorhombic crystals can be prepared, and these superconduct, all new results. X-ray diffraction also found evidence of an as yet unidentified commensurate superlattice modulation. The Tl-2201 crystals' electronic structure were studied by X-ray absorption and emission spectroscopies (XAS/XES). The Zhang-Rice singlet band gains less intensity on overdoping than expected, suggesting a breakdown of the Zhang-Rice singlet approximation, and one thallium oxide band does not disperse as expected. The spectra correspond very closely with LDA band structure calculations, and do not exhibit the upper Hubbard bands arising from strong correlations seen in other cuprates. The spectra are noteworthy for their unprecedented (in the high-Tc cuprates) simplicity. The startling degree to which the electronic structure can be explained bodes well for future research in the cuprates. The overdoped cuprates, and Tl-2201 in particular, may offer a unique opportunity for understanding in an otherwise highly confusing family of materials.