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

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

Within condensed matter physics, systems with strong electronic correlations give rise to fascinating phenomena which characteristically require a physical description beyond a one-electron theory, such as high temperature superconductivity, or Mott metal-insulator transitions. In this thesis, a class of strongly correlated electron systems is considered. These systems exhibit fractionally charged excitations with charge +e/2 or -e/2 in two dimensions (2D) and three dimensions (3D), a consequence of both strong correlations and the geometrical frustration of the interactions on the underlying lattices. Such geometrically frustrated systems are typically characterized by a high density of low-lying excitations, leading to various interesting physical effects. This thesis constitutes a study of a model of spinless fermions on the geometrically frustrated kagome lattice. Focus is given in particular to the regime in which nearest-neighbour repulsions V are large in comparison with hopping t between neighbouring sites, the regime in which excitations with fractional charge occur. In the classical limit t = 0, the geometric frustration results in a macroscopically large ground-state degeneracy. This degeneracy is lifted by quantum fluctuations. A low-energy effective Hamiltonian is derived for the spinless fermion model for the case of 1/3 filling in the regime where |t| << V . In this limit, the effective Hamiltonian is given by ring-exchange of order ~ t^3/V^2, lifting the degeneracy. The effective model is shown to be equivalent to a corresponding hard-core bosonic model due to a gauge invariance which removes the fermionic sign problem. The model is furthermore mapped directly to a Quantum Dimer model on the hexagonal lattice. Through the mapping it is determined that the kagome lattice model exhibits plaquette order in the ground state and also that fractional charges within the model are linearly confined. Subsequently a doped version of the effective model is studied, for the case where exactly one spinless fermion is added or subtracted from the system at 1/3 filling. The sign of the newly introduced hopping term is shown to be removable due to a gauge invariance for the case of hole doping. This gauge invariance is a direct result of the bipartite nature of the hole hopping and is confirmed numerically in spectral density calculations. For further understanding of the low-energy physics, a derivation of the model gauge field theory is presented and discussed in relation to the confining quantum electrodynamic in two dimensions. Exact diagonalization calculations illustrate the nature of the fractional charge confinement in terms of the string tension between a bound pair of defects. The calculations employ topological symmetries that exist for the manifold of ground-state configurations. Dynamical calculations of the spectral densities are considered for the full spinless fermion Hamiltonian and compared in the strongly correlated regime with the doped effective Hamiltonian. Calculations for the effective Hamiltonian are then presented for the strongly correlated regime where |t| << V . In the limit g << |t|, the fractional charges are shown to be effectively free in the context of the finite clusters studied. Prominent features of the spectral densities at the Gamma point for the hole and particle contributions are attributed to approximate eigenfunctions of the spinless fermion Hamiltonian in this limit. This is confirmed through an analytical derivation. The case of g ~ t is then considered, as in this case the confinement of the fractional charges is observable in the spectral densities calculated for finite clusters. The bound states for the effectively confined defect pair are qualitatively estimated through the solution of the time-independent Schroedinger equation for a potential which scales linearly with g. The double-peaked feature of spectral density calculations over a range of g values can thus be interpreted as a signature of the confinement of the fractionally charged defect pair. Furthermore, the metal-insulator transition for the effective Hamiltonian is studied for both t > 0 and t < 0. Exact diagonalization calculations are found to be consistent with the predictions of the effective model. Further calculations confirm that the sign of t is rendered inconsequential due to the gauge invariance for g in the regime |t| << V . The charge-order melting metal-insulator transition is studied through density-matrix renormalization group calculations. The opening of the energy gap is found to differ for the two signs of t, reflecting the difference in the band structure at the Fermi level in each case. The qualitative nature of transition in each case is discussed. As a step towards a realization of the model in experiment, density-density correlation functions are introduced and such a calculation is shown for the plaquette phase for the effective model Hamiltonian at 1/3 filling in the absence of defects. Finally, the open problem of statistics of the fractional charges is discussed.

Les systèmes à électrons fortement corrélés sont d’intérêt particulier pour le calcul ab initio, cherchant à modéliser ces systèmes à partir des premiers principes. La théorie de la fonctionnelle de la densité associée à une prise en compte des corrélations locales en DFT+U ou en DFT+DMFT, permet de reproduire qualitativement la physique de ces systèmes. Cependant, ces méthodes font intervenir les paramètres d’interaction effective de Hubbard U et de Hund J. Ces derniers peuvent eux-mêmes être calculés de manière ab initio, notamment avec l’approximation de phase aléatoire contrainte (cRPA), ouvrant la voie au développement de schémas de calcul les plus prédictifs possible. Nous utilisons un schéma DFT+U/cRPA, dont le principe consiste à calculer les paramètres U et J en cRPA et la structure électronique en DFT+U de manière auto-cohérente. Nous appliquons ce schéma aux lanthanides allant du cérium au lutétium (en détaillant le cas du cérium dans ses phases gamma et alpha), et aux dioxydes des actinides allant de l'uranium au curium. Nous effectuons d'abord une étude de l'état fondamental en DFT+U en fonction de U, en détaillant l'influence des états métastables. Nous étudions le rôle de la localisation des orbitales corrélées sur l'interaction effective dans un cas particulier. Nous détaillons ensuite les valeurs de U obtenues en cRPA en fonction de celles utilisées pour le calcul DFT+U. Nous nous intéressons plus particulièrement aux effets des processus d’écrantage sur les valeurs de U obtenues. Nous montrons que les limitations du schéma observées (multiplicité des valeurs auto-cohérentes de U obtenues et/ou incompatibilité de ces valeurs avec les spectres de photoémission expérimentaux) sont causées par certains processus d’écrantage spécifiques. Cela suggère d'améliorer la description de ces processus d'écrantage en modifiant le modèle. / Strongly correlated electron systems are particularly interesting for ab initio calculus, which aims to model these systems from first principles. Density functional theory, improved by taking into account local correlations within DFT+U or DFT+DMFT, enables to reproduce qualitatively the physics of these systems. Nonetheless, these methods require the Hubbard and Hund effective interaction parameters U and J. These can be computed from first principles as well, within the constrained random phase approximation (cRPA), paving the way for numerical schemes as predictive as possible. Here, we use a DFT+U/cRPA scheme, which aims to compute self-consistently the U and J parameters with cRPA and the DFT+U electronic structure. We apply this scheme to lanthanides from cerium to lutetium (insisting on cerium and its gamma and alpha phases), and dioxides of actinides from uranium to curium. We study the DFT+U ground state in function of U, giving more details about the influence of metastable states. We study as well the influence of the localization of correlated orbitals on the effective interaction in a particular case. We then detail the values of U obtained with cRPA, in function of those used for the DFT+U calculation. We study more particularly the effects of screening processes on the obtained values of U. We show that the limitations of the scheme (multiplicity of self-consistent values of U and/or their incompatibility with experimental photoemission spectra) are caused by specific screening processes. This suggests to improve the description of these screening processes by modifying the model.

Although band theory is about a century old, it remains relevant today as a tool for the treatment of electrons in solids. The confluence of mathematical ideas like geometry and topology with band theory has proven to be a ripe avenue for research in the past few decades. The importance of Fermi surface geometry, especially in conjunction with electronic correlation, has been well recognized. One particular thread in this direction is probing the occurrence of non-trivial Fermi surface geometry, and its influence on macroscopic properties of materials. A notable example of exotic Fermi surface geometry arises from singular points of the dispersion, and these have been known since 1953. The investigation into these was reignited recently, culminating in the work presented in this thesis. In this dissertation, I investigate two broad categories of singular points in bands. At a singular point, either the dispersion or the Fermi surface fail to be smooth. This may cause distinct signatures in transport and spectroscopic properties when the singular point occurs close to the Fermi level. In the two dimensional setting, I classify using catastrophe theory, the point singularities arising from higher order saddles of the dispersion. These are the more exclusive cousins of the regular van Hove saddle that cause, among other things, a power law divergence in the density of states. The role of lattice symmetries in aiding or preventing the occurrence of these singularities is also carefully explored. In the case of three dimensional bands, I investigate the spectroscopic properties of the nodal point singularity, arising from a linear band crossing. In particular, I determine the distinct signature of nodal points in the analytic, momentum resolved, joint density of states (JDOS) and the numerically calculated resonant inelastic x-ray scattering (RIXS) spectrum, within the fast collision approximation that ignores core hole effects. The results presented here will be the stepping stone towards a careful future calculation, incorporating the potential edge singularity effects through core hole potential. Such a calculation may be directly comparable with ongoing experiments.

Understanding the low-temperature properties of strongly correlated materials requires accurate measurement of the physical properties of these systems. Specific heat and nuclear spin-lattice relaxation are two such properties that allow the investigation of the electronic behavior of the system.  In this thesis, nanocalorimetry is used to measure specific heat, but also as basis for new experimental approach, developed to disentangle the different contributions to specific heat at low temperatures. The technique, that we call Thermal Impedance Spectroscopy (TISP) allows independent measurement of the electronic and nuclear specific heat at low temperatures based on the frequency response of the calorimeter-sample assembly. The method also enables simultaneous measurements of the nuclear spin-lattice relaxation time (T1). The nuclear spin lattice relaxation, as 1/T1T, and electronic specific heat, as C/T, provide information about the same quantity, electronic density of states, in the system. By comparing these properties in strongly correlated systems, we can obtain insights of electronic interactions.  Metallic indium is studied using thermal impedance spectroscopy from 0.3 K to 7 K at 35 T. The magnetic field dependence of nuclear spin-lattice relaxation rate is measured. Indium is a simple metallic system and the expected behavior of the nuclear spin-lattice relaxation is similar to that of the electronic specific heat. The results of the measurement are matched with the expectation from a simple metallic system and Nuclear Magnetic Resonance (NMR) measurements. This demonstrates the effectiveness of the new technique.  The heavy-fermion superconductor CeCoIn5 is studied using thermal impedance spectroscopy and ac-calorimetry. This material is located near a quantum critical point (QCP) bordering antiferromagnetism, as evidenced by doping studies. The nature of its quantum criticality and unconventional superconductivity is still elusive. Contrasting specific heat and nuclear spin-lattice relaxation in this correlated system helps to reveal the character of its quantum criticality.  The quantum criticality in CeCoIn5 is also studied using X-ray Absorption Spectroscopy (XAS) across the superconducting transition and X-ray Magnetic Circular Dichroism (XMCD) at 0.1 K and 6 T. The element-specific probe zooming in on cerium in this material indicates two things, a mixed valence of Ce in the superconducting state and a very small magnetic moment, that implies resonance-bond like antiferromagnetic local ordering in the system.

The search for novel quantum states is a fundamental theme in condensed matter physics. The almost boundless number of possible materials and complexity of the theory of electrons in solids make this both an experimentally and theoretically exciting and challenging research field. Particularly, the concept of quantum criticality resulted in a range of discoveries of novel quantum phases, which can become thermodynamically stable in the vicinity of a second order phase transition at zero temperature due to the existence of quantum critical fluctuations. One of the materials in which a novel quantum phase is believed to form close to a proposed quantum critical point is Sr₃Ru₂O₇. In this quasi-two-dimensional metal, the critical end point of a line of metamagnetic first order phase transitions can be suppressed towards zero temperature, theoretically leading to a quantum critical end point. Before reaching absolute zero, one experimentally observes the formation of an anomalous phase region, which has unusual ‘nematic-like’ transport properties. In this thesis magnetocaloric effect and specific heat measurements are used to systematically study the entropy of Sr₃Ru₂O₇ as a function of both magnetic field and temperature. It is shown that the boundaries of the anomalous phase region are consistent with true thermodynamic equilibrium phase transitions, separating the novel quantum phase from the surrounding ‘normal’ states. The anomalous phase is found to have a higher entropy than the low and high field states as well as a temperature dependence of the specific heat which deviates from standard Fermi liquid predictions. Furthermore, it is shown that the entropy in the surrounding ‘normal’ states increases significantly towards the metamagnetic region. In combination with data from other experiments it is concluded that these changes in entropy are most likely caused by many body effects related to the underlying quantum phase transition.

Orientador: Ricardo Luís Doretto / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-03T02:41:35Z (GMT). No. of bitstreams: 1 Leite_LeonardoDaSilvaGarcia_M.pdf: 1468749 bytes, checksum: 2f4e22a34c4a72b7b68eec6673285298 (MD5) Previous issue date: 2017 / Resumo: Nesse trabalho, calculamos a entropia de emaranhamento de um antiferromagneto de Heisenberg dimerizado em uma rede quadrada. Dois padrões de dimerização distintos são considerados: colunar e alternado. Em ambos os casos, focamos na fase de sólidos de singletos (VBS) que é descrita pela representação dos operadores de ligação. Nesse formalismo, o hamiltoniano de spin original é mapeado em um modelo efetivo de bósons interagentes com excitações de tripleto. O hamiltoniano efetivo é estudado na aproximação harmônica e o espectro das excitações elementares e o diagrama de fase dos dois modelos dimerizados são determinados. Consideramos um subsistema unidimensional (cadeia) de comprimento $L$ dentro de uma rede quadrada com condições periódicas de contorno e calculamos a entropia de emaranhamento. Seguimos um procedimento analítico baseado na teoria de ondas de spin modificadas que havia sido desenvolvido originalmente para calcular a entropia de emaranhamento em fases magneticamente ordenadas. Em particular, esse procedimento nos permite considerar subsistemas unidimensionais compostos por até 200 sítios. Combinamos esse procedimento com o formalismo dos operadores de ligação na aproximação harmônica e mostramos que, para os dois modelos de Heisenberg dimerizados, a entropia de emaranhamento da fase VBS obedece uma lei de área. Tanto para a dimerização colunar quanto para a alternada, mostramos que a entropia de emaranhamento aumenta à medida que o sistema se aproxima da transição de fase quântica entre as fases Néel-VBS / Abstract: Nesse trabalho, calculamos a entropia de emaranhamento de um antiferromagneto de Heisenberg dimerizado em uma rede quadrada. Dois padrões de dimerização distintos são considerados: colunar e alternado. Em ambos os casos, focamos na fase de sólidos de singletos (VBS) que é descrita pela representação dos operadores de ligação. Nesse formalismo, o hamiltoniano de spin original é mapeado em um modelo efetivo de bósons interagentes com excitações de tripleto. O hamiltoniano efetivo é estudado na aproximação harmônica e o espectro das excitações elementares e o diagrama de fase dos dois modelos dimerizados são determinados. Consideramos um subsistema unidimensional (cadeia) de comprimento $L$ dentro de uma rede quadrada com condições periódicas de contorno e calculamos a entropia de emaranhamento. Seguimos um procedimento analítico baseado na teoria de ondas de spin modificadas que havia sido desenvolvido originalmente para calcular a entropia de emaranhamento em fases magneticamente ordenadas. Em particular, esse procedimento nos permite considerar subsistemas unidimensionais compostos por até 200 sítios. Combinamos esse procedimento com o formalismo dos operadores de ligação na aproximação harmônica e mostramos que, para os dois modelos de Heisenberg dimerizados, a entropia de emaranhamento da fase VBS obedece uma lei de área. Tanto para a dimerização colunar quanto para a alternada, mostramos que a entropia de emaranhamento aumenta à medida que o sistema se aproxima da transição de fase quântica entre as fases Néel-VBS / Mestrado / Física / Mestre em Física / 1547615/2015 / CAPES

Thermal conductivity and thermopower measurements in strongly correlated electron systems at low temperatures