Rozprawy doktorskie na temat „Strong Electron Correlation Effect”

In this work, we investigate effects of strong correlation to quantum transport from atomic first principles. In order to accomplish this task, we use a well established state-of-the-art formalism of quantum transport where density functional theory (DFT) is carried out within the Keldysh non-equilibrium Green's functions (NEGF). To deal with certain strong correlation phenomenon, we integrated an local density approximation plus Hubbard U (LDA+U) exchange-correlation potential into the existing NEGF-DFT formalism. The LDA+U potential correctly accounts for the electronic structure of correlated material. We will present the theory and numerical implementation associated with the NEGF-DFT-(LDA+U) in detail. Extensive tests on the well known correlated material FeO crystal have been carried out and results compared with previous literature as well as to experimental data.
We then apply our NEGF-DFT-(LDA+U) technique to investigate transport physics of spin resolved tunnelling in Fe/MgO/Fe magnetic tunnel junctions (MTJ). We found that interfacial oxygen atoms are enough to localise the 3d electrons of infacial Fe atoms due to strong correlation. This surprising result substantially changes quantum transport properties of the MTJ, in particular it reduces magnetic resistance ratio by about 33%. This strongly correlated physics is absent if the conventional local spin density approximation (LSDA) is used in the NEGF-DFT analysis. Results of LSDA and LDA+ U exchange-correlation potential will be compared. Furthermore, through investigating contributions to scattering states by various atomic orbitals, we clearly identify the reason why LDA+U changes quantum transport in both quantitative and qualitative ways. Finally, we believe this strongly correlated physics should be general in other MTJs involving different oxides.

The 5d transition metal oxides have drawn substantial interest for predictions of being suitable candidates for hosting exotic electronic and magnetic states, including unconventional superconductors, magnetic skyrmions, topological insulators, and Weyl semimetals. In addition to the electron-electron correlation notable in high-temperature 3d transition metal superconductors, the 5d oxides contain a large spin-orbit interaction term in their ground state, which is largely responsible for the intricate phase diagram of these materials. Iridates, or compounds containing 5d iridium bonded with oxygen, are of particular interest for their spin-orbit split Jeff = 1/2 state, which is partially filled without the presence of any additional electron correlation. However, the comparable energetics between a small, finite electron correlation energy and the spin-orbit interaction make the band structure of iridates amenable to small perturbations of the crystalline lattice and ideal for exploring the interplay between these two interactions. While altering the spin-orbit interaction strength of iridium is tenably not feasible, the electron correlation energy can be tuned using a variety of experimental techniques. In this dissertation, the electronic and magnetic properties of iridates at various electron correlation energies are studied by altering the epitaxial lattice strain, dimensionality, and the radius size of the A-site cation. These parameters tune the effective electronic bandwidth of the system, which is inversely proportional to the effective electron correlation energy. The lattice strain and the cationic radius size achieve this by altering the Ir-O-Ir bond angle between nearest neighbor Ir ions. In the case of dimensionality tuning, the effective bandwidth is controlled via the coordination number of each Ir ion. In the first study, a metal-to-insulator transition is observed in thin films of the semi-metallic SrIrO3 as in-plane compressive lattice strain is increased. This observation is consistent with the expectation of compressive lattice strain increasing the effective correlation energy; however, optical spectroscopy spectra reveal the increase is not sufficient for opening an insulating Mott gap. In the second part, the effective correlation energy is adjusted using a dimensional confinement of the layered iridate Sr2IrO4. Here, the coordination number of each Ir ion is reduced using an a-axis oriented superlattice of one-dimensional IrO2 quantum stripes, where several emergent features are revealed in its insulating Jeff = 1/2 state. In the final study, the effective correlation is tuned in a series of mixed-phase pyrochlore iridate thin films, where the Ir atoms take a corner-shared tetrahedral configuration. Here, a transition between conducting to insulating magnetic domain walls is revealed as the correlation energy is increased via A-site chemical doping. Each of these studies sheds light on the pronounced role the effective correlation energy plays in determining the local subset of phases predicted for iridates and related systems featuring strong spin-orbit interactions.

In the last decade, a large number of studies have been devoted to the peculiarities of correlated physics found in the quasi-two-dimensional square lattice iridium oxides. It was shown that this 5d family of transition metal oxides has strong structural and electronic similarities to the famous 3d family of copper oxides. Moreover, a delicate interplay of on-site spin-orbit coupling, Coulomb repulsion and crystalline electric field interactions is expected to drive various exotic quantum states. Many theoretical proposals were made in the last decade including the prediction of possible superconductivity in square-lattice iridates emerging as a sister system to high-Tc cuprates, which however met only limited experimental confirmation. One can, therefore, raise a general question: To what extent is the low-energy physics of the quasi-two-dimensional square-lattice iridium oxides different from other transition metal oxides including cuprates? In this thesis we investigate some of the effects which are usually neglected in studies on iridates, focusing on quasi-two-dimensional square-lattice iridates such as Sr2IrO4 or Ba2IrO4. In particular, we discuss the role of the electron-phonon coupling in the form of Jahn-Teller interaction, electron-hole asymmetry introduced by the strong correlations and some effects of coupling scheme chosen to calculate multiplet structure for materials with strong on-site spin-orbit coupling. Thus, firstly, we study the role of phonons, which is almost always neglected in Sr2IrO4, and discuss the manifestation of Jahn-Teller effect in the recent data obtained on Sr2IrO4 with the help of resonant inelastic x-ray scattering. When strong spin-orbit coupling removes orbital degeneracy, it would at the same time appear to render the Jahn-Teller mechanism ineffective. We show that, while the Jahn-Teller effect does indeed not affect the antiferromagnetically ordered ground state, it leads to distinctive signatures in the spin-orbit exciton. Second, we focus on charge excitations and determine the motion of a charge (hole or electron) added to the Mott insulating, antiferromagnetic ground-state of square-lattice iridates. We show that correlation effects, calculated within the self-consistent Born approximation, render the hole and electron case very different. An added electron forms a spin-polaron, which closely resembles the well-known cuprates, but the situation of a removed electron is far more complex. Many-body configurations form that can be either singlets and triplets, which strongly affects the hole motion. This not only has important ramifications for the interpretation of angle-resolved photoemission spectroscopy and inverse photoemission spectroscopy experiments of square lattice iridates, but also demonstrates that the correlation physics in electron- and hole-doped iridates is fundamentally different. We then discuss the application of this model to the calculation of scanning tunneling spectroscopy data. We show that using scanning tunneling spectroscopy one can directly probe the quasiparticle excitations in Sr2IrO4: ladder spectrum on the positive bias side and multiplet structure of the polaron on the negative bias side. We discuss in detail the ladder spectrum and show its relevance for Sr2IrO4 which is in general described by more complicated extended t-J -like model. Theoretical calculation reveals that on the negative bias side the internal degree of freedom of the charge excitation introduces strong dispersive hopping channels encaving ladder-like features. Finally, we discuss how the choice of the coupling scheme to calculate multiplet structure can affect the theoretical calculation of angle-resolved photoemission spectroscopy and scanning tunnelling spectroscopy spectral functions.

This thesis is focused on the electronic, spin-dependent and dynamical properties of thin magnetic systems. Photoemission-related techniques are combined with synchrotron radiation to study the spin-dependent properties of these systems in the energy and time domains. In the first part of this thesis, the strength of electron correlation effects in the spin-dependent electronic structure of ferromagnetic bcc Fe(110) and hcp Co(0001) is investigated by means of spin- and angle-resolved photoemission spectroscopy. The experimental results are compared to theoretical calculations within the three-body scattering approximation and within the dynamical mean-field theory, together with one-step model calculations of the photoemission process. From this comparison it is demonstrated that the present state of the art many-body calculations, although improving the description of correlation effects in Fe and Co, give too small mass renormalizations and scattering rates thus demanding more refined many-body theories including nonlocal fluctuations. In the second part, it is shown in detail monitoring by photoelectron spectroscopy how graphene can be grown by chemical vapour deposition on the transition-metal surfaces Ni(111) and Co(0001) and intercalated by a monoatomic layer of Au. For both systems, a linear E(k) dispersion of massless Dirac fermions is observed in the graphene pi-band in the vicinity of the Fermi energy. Spin-resolved photoemission from the graphene pi-band shows that the ferromagnetic polarization of graphene/Ni(111) and graphene/Co(0001) is negligible and that graphene on Ni(111) is after intercalation of Au spin-orbit split by the Rashba effect. In the last part, a time-resolved x-ray magnetic circular dichroic-photoelectron emission microscopy study of a permalloy platelet comprising three cross-tie domain walls is presented. It is shown how a fast picosecond magnetic response in the precessional motion of the magnetization can be induced by means of a laser-excited photoswitch. From a comparision to micromagnetic calculations it is demonstrated that the relatively high precessional frequency observed in the experiments is directly linked to the nature of the vortex/antivortex dynamics and its response to the magnetic perturbation. This includes the time-dependent reversal of the vortex core polarization, a process which is beyond the limit of detection in the present experiments.
Diese Dissertation beschäftigt sich mit den elektronischen, spinabhängigen und dynamischen Eigenschaften dünner magnetischer Systeme. Auf dem Photoeffekt basierende Untersuchungsmethoden werden zusammen mit Synchrotronstrahlung eingesetzt, um die spinabhängigen Eigenschaften dieser Systeme im Energie- und Zeitbereich zu untersuchen. Im ersten Teil dieser Arbeit wird mit spin- und winkelaufgelöster Photoemission die Stärke von Elektronenkorrelationseffekten in der spinabhängigen elektonischen Struktur von ferromagnetischerm bcc Fe(110) und hcp Co(0001) untersucht. Die experimentellen Ergebnisse werden verglichen mit theoreteischen Berechnungen im Rahmen der Näherung der Drei-Körper-Streuung und der dynamischen Molekularfeldtheorie, zusammen mit Berechnungen des Photoemissionsprozesses im Rahmen des Ein-Stufen-Modells. Ausgehend von diesem Vergleich wird gezeigt, dass die gegenwärtig fortgeschrittensten Rechnung, obgleich sie die Beschreibung von Korrelationseffekten in Fe und Co verbessern, zu kleine Massenrenormalisierungen und Streuraten ergeben, was zu der Forderung nach verfeinerten Vielteilchentheorien unter Einbeziehung von nichtlokalen Fluktuationen führt. Im zweiten Teil wird unter Kontrolle durch die Photoelektronenspektroskopie im Detail gezeigt, wie Graphen durch chemische Gasphasenabscheidung auf den Übergangsmetall-Oberflächen Ni(111) und Co(0001) aufgebracht und mit einer Monolage Au interkaliert werden kann. Für beide Systeme wird eine lineare E(k)-Dispersion masseloser Dirac-Fermionen im Graphen-pi-Band in der Nähe der Fermi-Energie beobachtet. Spinaufgelöste Photoemission des Graphen-pi-Bandes zeigt, dass die ferromagnetische Polarisation von Graphen/Ni(111) und Graphen/Co(0001) vernachlässigbar ist und dass Graphen/Ni(111) nach Interkalation mit Au eine Spin-Bahn-Aufspaltung aufgrund des Rashba-Effekts zeigt. Im letzten Teil wird eine zeitaufgelöste Studie des Röntgenzirkulardichroismus mit Photoelektronenmikroskopie präsentiert, die an einer Permalloy-Probe durchgeführt wurde, die drei als Stachelwände ausgebildete Domänenwände enthält. Es wird gezeigt, wie eine schnelle magnetische Antwort auf der Pikosekundenskala in der Präzessionsbewegung der Magnetisierung durch einen laserangesteuerten Photoschalter erzeugt werden kann. Durch Vergleich mit einer mikromagnetischen Rechnung wird gezeigt, dass die relativ hohe Präzessionsfrequenz, die im Experiment beobachtet wird, in unmittelbarer Beziehung steht zu den Eigenschaften der Vortex/Antivortex-Dynamik und ihrer Antwort auf die magnetische Störung. Das schließt die zeitabhängige Umkehr der Vortexkernpolarisation ein, einem Vorgang der jenseits der Nachweisgrenze der gegenwärtigen Experimente liegt.

This thesis completes my work as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Università degli Studi di Milano that has been carried out, starting in November 2014, mostly at the Laboratorio TASC of IOM-CNR in the premises of the Elettra - Sincrotrone Trieste and FERMI@Elettra infrastructures, in the framework of the NFFA and APE-beamline facilites, as well as by accessing international large scale infrastructures and laboratories. The activity has addressed the development of experimental methodologies and novel instrumentation oriented to the study of the dynamical properties of highly correlated materials after high energy excitation. The science programme has been carried out by exploiting ultrafast femtosecond probes from the optical regime (Ti-Sa lasers, fibre laser oscillators) to the extreme UV-soft X rays at FERMI, to the picosecond hard X-rays from the SPring-8 and Diamond synchrotron radiation source. The sample synthesis of correlated oxides and its characterization has been performed within the NFFA facility and APE-group collaboration in Trieste as well as the design and construction of the all new laser High Harmonic Generation beam line NFFA-SPRINT and its end station for time resolved vectorial electron spin polarimetry. This report concentrates on the main scientific concern of my work that has been the relaxation of external perturbations in a correlated electron material both in the time and space domain. I have employed Photoelectron Spectroscopy (PES) mostly in the Hard X-ray regime (HAXPES), pushing the boundaries of its application to achieve a coherent perspective. The material I have mainly focused on is La0.67Sr0.33MnO3 (LSMO), of high interest for spintronics. This system is prototypical, yielding the highest simplicity in the class of transition metal oxides. In the spatial investigation, I have controlled with high precision the PES probing depth and I have observed the evolution of one spectral feature. I have identified it as probe of electronic hybridization and long-range ordering. I have studied LSMO films of 40 nm in three substrate-induced strain states (1% tensile in-plane, relaxed, 1% compressive in-plane) and a 18 nm film of (Ga,Mn)As (GMA), a well-studied diluted magnetic semiconductor. I have found that the electronic properties to be modified at significant distances from the surface, 4 nm for LSMO and 1.2 nm for GMA, while strain had no detectable effects. In the temporal study, I have employed HAXPES in pump-probe mode (TR-HAXPES) to observe the evolution of the electronic structure after intense optical excitation. A detailed dynamical characterization with optical techniques has allowed me to identify the characteristic time of the collapse of long-range magnetic order to be significantly longer than the one of elemental transition metals. I have ascribed this effect to the half-metallic character of LSMO. With TR-HAXPES I have observed that the whole electronic band-structure evolution is bottlenecked by the slow response of the magnetization, proceeding on hundreds of picoseconds timescales. Finally, I have described the techniques and the instrumentation that can be used to push these investigations to shorter spatial and temporal scales. This has been realized in the form of the NFFA-SPRINT laboratory, a facility open to users, which I participated in designing and developing.

Orientadores: Gaston Eduardo Barberis, Pascoal José Giglio Pagliuso
Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
Made available in DSpace on 2018-08-17T22:20:28Z (GMT). No. of bitstreams: 1 CalderonFilho_CesarJose_M.pdf: 21461723 bytes, checksum: 3089cf622e6e81e41b460dc218211be3 (MD5) Previous issue date: 2011
Resumo: Na física da materia condensada, o estudo de sistemas de eletrons fortemente correlacionados é, com certeza, um dos problemas mais interessantes tanto do ponto de vista experimental como teórico, e são estes materiais que tem sido utilizados recentemente em aplicações tecnológicas. Destes compostos, os multiferroicos apresentam um conjunto de propriedades físicas muito rico. Estes materiais apresentam pelo menos duas das seguintes correlações de longo alcance: (anti)ferromagnetismo, ferroelasticidade e ferroeletricidade. Porém, as transições não precisam ser necessariamente correlacionadas, mas quando são, estas ocorrem simultaneamente, e o efeito magnetoelétrico pode ser induzido por campo. Neste trabalho, foram desenvolvidos cálculos numéricos que simulam o acoplamento magnetoelétrico presente nos multiferróicos minimizando a energia através da técnica de Monte Carlo. Foram desenvolvidos dois modelos muito simples. O primeiro modelo acopla uma rede de Ising 2D com spin 1/2 com uma rede de dipolos elétricos tambem 2D; este acoplamento e tal que a mudança de direção de um dado spin reorienta uma dada componente perpendicular do dipolo elétrico vizinho a este mesmo spin. Assim, para este primeiro modelo, as transições de fase das redes elétrica e magnetica ocorrem na mesma temperatura, sendo o hamiltoniano dependente de três parâmetros. Para o segundo modelo, foram utilizadas novamente duas redes, uma rede de Ising 2D com spin 1/2, e uma rede elétrica que se comporta da mesma maneira que uma rede de Ising 2D. Neste caso, o acoplamento entre o spin e o dipolo eletrico ocorre através de um sistema de dois níveis, gerando a possibilidade de temperaturas de transição independentes para as duas redes. Este segundo modelo tambem depende de três parâmetros
Abstract: In condensed matter physics, the study of strongly correlated electron systems is certainly one of the most interesting problems both from the experimental and the theoretical points of view, also these materials recently being used in technological applications. Among these compounds, the multiferroics show a very rich set of physical properties. These materials have at least two of the following long-range correlations: (anti)ferromagnetism, ferroelasticity and ferroelectricity. However, the transitions need not necessarily to be correlated, but when it happens, they occur simultaneously, and the magnetoelectric effect can be induced by field. In this work, numerical calculations have been developed to simulate the magnetoelectric coupling present in the multiferroics minimizing the energy through Monte Carlo technique. Two simple models have been developed. The first model couples a spin 1/2 2D Ising magnetic lattice with to a 2D lattice of classic electric dipoles; this coupling is such that the change in the spin direction reorients a perpendicular component of the electric dipole neighbor of this same spin. Therefore, for this first model, the phase transitions of the magnetic and electric lattices occur at the same temperature, and the Hamiltonian is dependent of three parameters. For the second model, two lattices have been used again, a 2D Ising lattice for the magnetic system and an electric lattice that also behaves as a 2D Ising lattice. In this case, the coupling between the spin and the electric dipole occurs through a two-level system, generating the possibility of the independent transition temperatures for the two systems. This second model also contains three independent parameters
Mestrado
Física da Matéria Condensada
Mestre em Física

Les proprietes du modele de hubbard a trois bandes pour le plan cuo#2 des supraconducteurs a oxyde de cuivre sont etudiees dans des approximations differentes. A partir du formalisme de mori de decouplage d'equations de mouvement, une methode non perturbative pour la description des correlations electroniques est developpee. Cette methode d'equations de mouvement permet d'etudier separement des effets differents de correlations electroniques dans une situation complexe definie par les equations de mouvement de la fonction de green a une particule. Elle fait appel a une interpretation de la dynamique du modele basee sur la phenomenologie observee ou conjecturee du systeme electronique considere. Le decouplage par projection introduit des fonctions de correlation statiques a une et a deux particules dans les expressions des densites spectrales et de la dispersion des quasi-particules. Ces parametres refletent les effets de correlation et sont a determiner numeriquement de facon autocoherente. Les proprietes de la methode sont presentees et comparees de maniere critique avec d'autres methodes a n particules, par exemple la methode de champ moyen des bosons auxiliaires. Dans l'application aux supraconducteurs a oxyde de cuivre decrits par l'hamiltonien de hubbard a trois bandes, les effets d'une forte repulsion electrostatique intraionique ud dans les orbitales d du cuivre et d'une repulsion coulombienne upd entre les orbitales d de cuivre et p d'oxygene proches voisins sont etudies. On trouve des quasi-particules couplees a des singulets de spin de proches voisins cu-o. Upd introduit un mode de propagation electronique couplee a des excitons lies de transfert de charge

Complex-oxides have seen an enormous amount of attention in the realm of Condensed Matter Physics and Materials Science/Engineering over the last several decades. Their ability to host a wide variety of novel physical properties has even caused them to be exploited commercially as dielectric, metallic and magnetic materials. Indeed, since the discovery of high temperature superconductivity in the “Cuprates” in the late 1980’s there has been an explosion of activity involving complex-oxides. Further, as the experimental techniques and equipment for fabricating thin films and heterostructures of these materials has improved over the last several decades, the search for new and more exotic properties has intensified. These properties stem from the interfaces formed by depositing these materials onto one another. Whether it be interfacial strain induced by the mismatch between the crystal structures, modified exchange interactions, or some combination of these and other interactions, thin films and heterostuctures provide an invaluable tool the modern condensed matter community. Simply put, a “complex-oxide” is any compound that contains Oxygen and at least two other elements; or one atom in two different oxidation states. Transition Metal Oxides (TMO’s) are a subset of complex-oxides which are of particular interest because of their strong competition between their charge, spin and orbit degrees of freedom. As we progress down the periodic table from 3d to 4d to 5d transition metals, the crystal field, electron correlation and spin-orbit energies become more and more comparable. Therefore, TMO thin films and heterostructures are indispensable to the search for novel physical properties. KTaO3 (KTO) is a polar 5d TMO which has been investigated for its high-k dielectric properties. It is a band insulator with a cubic perovskite crystal structure which is isomorphic to SrTiO3 (STO). This is important because non-polar STO is famous for forming a highly mobile, 2-Dimensional Electron Gas (2DEG) at the hetero-interface with polar LaAlO3 (LAO) as a result of the so-called “polar catastrophe”. Here, I use this concept of polarity to ask an important question: “What happens at hetero-interfaces where two different polar complex oxides meet?” From this question we propose that a hetero-interface between two polar complex-oxides with opposite polarity (I-V/III-III) should be impossible because of the strong Coulomb repulsion between the adjacent layers. However, we find that despite this proposed conflict we are able to synthesize KTO thin films on (110) oriented GdScO3 (GSO) substrates and the conflict is avoided through atomic reconfiguration at the hetero-interface. SrRuO3 (SRO) is a 4d TMO, and an itinerant ferromagnet that is used extensively as an electrode material in capacitor and transistor geometries and proof-of-concept devices. However, in the thin film limit the ferromagnetic transition temperature, TC, and conductivity drop significantly and even become insulating and lose their ferromagnetic properties. Therefore, we ask “Are the transport properties of SRO thin films inherently inferior to single crystals, or is there a way to maintain and/or enhance the metallic properties in the thin film limit?” We have fabricated SRO thin films of various thickness on GSO substrates (tensile strain) and find that all of our samples have enhanced metallic properties and even match those of single crystals. Finally, we ask “Can these enhanced metallic properties in SRO thin films allow us to observe evidence of a topological phase without the complexity of off-stoichiometry and/or additional hetero-structural layers?” Recent reports of oxygen deficient EuO films as well as hetero-structures and superlattices of SRO mixed with SrIrO3 or La0.7Sr0.3MnO3 have suggested that a magnetic skyrmion phase may exist in these systems. By measuring the Hall resistivity, we are able to observer a topological Hall effect which is likely a result of a magnetic skyrmion. We find that of the THE exists in a narrow temperature range and the proposed magnetic skyrmions range in size from 20-120 nm. Therefore, the SRO/GSO system can provide a more viable means for investigating magnetic skyrmions and their fundamental interactions.

Etude theorique du rayonnement de plasmas composes de protons et d'electrons et qui sont le siege d'effets quantiques. Le parametre de born mesure l'importance des effets quantiques: inferieur a 1 pour les temperatures elevees , superieur a 1 pour les basses temperatures. Lorsque les effets quantiques interviennent en tant que correction, la methode des sommes de slater permet d'obtenir un potentiel effectif. On calcule le rayonnement classiquement a l'aide de ce potentiel effectif. Cette situation est valable lorsque la temperature est elevee

Nous avons propose une approche susceptible d'expliquer de nombreuses proprietes electroniques des materiaux dont les porteurs de charge sont soumis a de fortes correlations. Parmi les materiaux consideres, citons en particulier les supraconducteurs a haute temperature critique et les oxydes de metaux de transition de type perovskite. L'approche developpee dans cette these est une extension de la theorie de champ moyen dynamique a des systemes electroniques a plus de un degre de liberte orbital. Elle est basee sur une nouvelle procedure de renormalisation du modele de hubbard a deux bandes pour l'extension aux grandes dimensions spatiales de la structure perovskite. Un traitement selectif des differents processus de sauts entre sites de metal de transition (mt) et d'oxygene (o) plus proche voisins, nous a permis de conserver de nombreux aspects dynamiques non triviaux dans cette limite. Le probleme sur reseau est projete sur un modele d'impurete de type anderson. La solution du probleme local a ete obtenue par la theorie des perturbations iterees et par une extension de l'approximation de non-crossing (nca). La technique de nca nous a permis de decrire correctement d'importantes proprietes spectrales des oxydes de metaux de transition, donnant des informations tres interessantes sur la nature des differentes bandes des densites d'etats du metal de transition et de l'oxygene, et sur leur comportement en fonction du dopage. De plus, le pic coherent de quasiparticule, proche du niveau de fermi, a ete obtenu. Ce pic presente d'interessantes variations en fonction du dopage, mais aussi de la temperature. Ces excitations de basse energie proviennent de resonances de type kondo, entre l'etat vide et un etat hybride mt-o dans le cas du dopage en electrons, et entre cet etat mt-o et un etat singulet de type zhang et rice dans le cas du dopage en trous. L'application de notre approche nca a des familles de materiaux reels comme lamo#3 ou ca#1##xsr#xvo#3 a mis en evidence l'importance fondamentale des excitations a transfert de charge dans ces composes. Une description correcte des proprietes electroniques des oxydes de metaux de transition peut donc etre donnee par une approche de champ moyen dynamique mais necessite la prise en compte a la fois des etats de l'oxygene et de ceux du metal de transition.

Etude du comportement en fatigue plastique de l'acier inoxydable austenitique 316l fatigue sous vide a 20, 300 et 600c dans une gamme de deformations plastiques imposees. Analyse par microscopie electronique en transmission des sous-structures a20 et 600c, les microstructures sont constituees de bandes de glissement persistantes, de murs-canaux des labyrinthes et des structures en echelle evoluant vers la structure cellulaire. A300c, une structure de contraste en cotes de velours est observee. Cette structure est fonction de la deformation plastique et provoque un durcissement cyclique secondaire. Presentation de modeles de formation et d'evolution de microstructures de fatigue

Cette these est faite de deux parties independantes. La premiere partie traite les corrections quantiques a l'effet hall anormal. On peut montrer que les termes de l'anomalie coulombienne se compensent mutuellement en considerant la diffusion asymetrique des electrons de conduction par les moments magnetiques d'une couche metallique desordonnee. La contribution venant de la localisation faible est coupee par la diffusion asymetrique qui brise la coherence de phase. Les resultats sont en bon accord avec une experience recente sur des couches minces de fer desordonnees et ferromagnetiques. Dans la seconde partie, on etudie la formation de moments magnetiques localises dans des systemes comme des semiconducteurs dopes en phase metallique. Le calcul est base sur le modele de hubbard avec desordre hors-diagonal. En examinant le modele a une seule impurete faiblement rattachee au reseau, on trouve un moment magnetique localise, et cela meme dans l'approximation de hartree-fock pour la repulsion locale u. On associe a ce moment magnetique un effet kondo dont la nature et la dependance par rapport au desordre sont discutees. Les resultats sont compares a une experience recente mesurant le pouvoir thermoelectrique du si:p. La correspondance est satisfaisante

Mise au point d'un programme de calcul des elements de matrice de l'operateur de retard relativiste pour etudier l'influence du terme complet de breit exprime en jauge de lorentz, sur l'energie des niveaux atomiques. Lois d'echelle demontrant l'importance numerique des termes relativistes d'ordre eleve pour les ions a 2 electrons. Demonstration de l'influence de la jauge sur les valeurs moyennes du terme complet de breit, lorsqu'elles sont prises sur des fonctions autocoherentes du modele de dirac-fock. Par la methode mcdf, mise en evidence des correlations magnetiques et d'une contribution relativiste dans les correlations electrostatiques; valeurs de ces correlations pour le niveau fondamental et des niveaux 1s2p. Etude des effets du terme de breit sur la structure fine 1s2p dans le domaine relativiste: le terme magnetique induit une inversion des niveaux de j=0 et du niveau profond de j=1 par rapport aux previsions non relativistes

In this thesis, several aspects of correlation effects in band insulators that were hitherto not addressed, both in equilibrium and non-equilibrium, are studied using the Ionic Hubbard model (IHM) at half-filling and Dynamical mean-field theory (DMFT). The IHM is an extension of the Hubbard model on a bipartite lattice with a staggered on-site “ionic" potential Δ added in. The Hubbard model by itself is a tight-binding model of electrons hopping between (orbitals localized on) lattice sites, with amplitudes t and t’ for first neighbor and second neighbor hopping respectively, together with a coulomb repulsion energy cost of U for double occupancy (i.e., with both up and down spin electrons) at any lattice site. The studies in this thesis address strong correlation effects in this model using DMFT techniques which map the lattice problem to a self consistently embedded quantum impurity problem. The impurity problem itself is solved using state of the art continuous time quantum monte-carlo (CTQMC) techniques (Ch.s 3 and 4) as well as the somewhat simpler Iterated Perturbation Theory (IPT) (Ch.s 3, 5 and 6). Chapter 3 of the thesis is devoted to a detailed analysis of the properties of the half-filled t-U-Δ IHM (i.e., with t’=0) on a Bethe lattice of infinite connectivity. It is shown conclusively here that for a finite Δ and at zero temperature (T=0), the IHM has a first order transition from the Band Insulator (BI) phase to an antiferromagnetic insulating (AFI) phase at a threshold U = UAF (which increases with Δ) such that the BI to AFI transition preempts the transition from the BI to the correlation induced paramagnetic metallic (PM) phase. Evidence is also presented for a quantum transition to a sliver of half-metal (HM) phase just after the AF order turns on, followed by a return to the AFI phase on further increasing U. With increasing T, the AF order is lost via a first-order transition for weak to intermediate U/t. In the strongly correlated regime, where the effective low-energy Hamiltonian is the Heisenberg model, the transition from the AFI to the paramagnetic Mott insulating phase (adiabatically connected to the BI phase) is of second order. As T increases, the range of U over which the AF order is stable shrinks, collapsing eventually to a line of tricritical points that separates the surfaces of first- and second-order phase transitions in the T-U-Δ space. In Chapter 4 of the thesis the t-t’-U-Δ IHM on a square lattice is studied using DMFT+CTQMC. It is shown here that the presence of t’ frustrates the AF order for weak to intermediate correlations, resulting in the stabilization of the correlation induced PM phase over a broad region of the parameter space, although the AFI is always the stable phase for large enough U. In the intermediate coupling regimes, two other interesting phases show up as a result of the competition among U, Δ and t’ : a Ferrimagnetic metallic (FM) phase which has non-zero values of the uniform as well as staggered magnetization; and even more interesting, an anti-ferromagnetic half-metal (AFHM) phase, where only the staggered magnetization is non-vanishing, and in which one spin-species has gapless excitations while the other is gapped. A rich phase diagram involving these phases is obtained and compared with the one obtained using the simpler Hartree-Fock approximation. The same model is treated in Chapter 5 using DMFT+IPT, to obtain spectral functions not easy to obtain using the CTQMC techniques. The results are consistent with and reinforce the conclusions of Chapter 4. Questions as to how closed quantum systems approach equilibrium after a quench are of great current interest, especially so because of experiments in cold atom systems. Chapter 6 of the thesis is devoted to a study of the consequences of an ionic potential quench in the t-U-Δ IHM on a Bethe lattice using non-equilibrium DMFT+IPT techniques. Two types of ionic potential quench are studied: (1) From a nonzero Ionic potential Δi to a uniform state with Δf = 0, and (2) Ionic potential Δi = 0 to a non-zero Δf quench, at different values of U. The evolutions of the staggered density and the double occupancy towards their final equilibrium values are found to be oscillatory with envelope functions that have power law tails despite the presence of interactions, which behavior is intriguing and worthy of further exploration. The exponents of the power law increase with increasing U, consistent with interactions promoting faster relaxation. In addition to the above original contributions the thesis also contains an introductory chapter (Ch. 1), a methods chapter (Ch. 2), and several appendices that present technical details.

Strongly correlated systems on geometrically frustrated lattices can stabilize a large number of interesting phases that includes a wide array of novel Mott insulators in both bosonic and electronic systems. Charge fluctuations in a Mott insulator are suppressed due to strong mutual interaction among the particles. The presence of frustration is of particular importance as the physics it offers is often rich, unexpectedly complicated, and continues to raise many open questions. The thesis elucidates some of these issues on a kagome lattice where strong interactions among the particles in the Mott phase impose non-trivial local constraints depending on the filling fraction on the lattice. These Mott insulators, in addition to featuring unusual magnetic and/or charge ordering, can also harbor topologically ordered states of quantum matter, e.g., resonating valence bond liquids realized in certain quantum dimer models on non-bipartite lattices. The dimer models can be regarded as low-energy effective theories for different types of bosonic models in the strong-coupling limit. Exploring this connection is a central theme of this thesis with the aim of realizing novel strongly correlated ground states. Past studies of these models have revealed the existence of various ordered and disordered phases with distinct signatures. Among these low-energy phases, the presence of a stable topological liquid at a particular point, known as Rokhsar-Kivelson point, in the phase diagram is notable. The classical versions of the dimer model are also known to have garnered a vast interest in various fields ranging from problems of pure mathematical origin to ones in physical chemistry as well as statistical physics. Pioneered by Kasteleyn, several analytical works came forward to exactly calculate the partition function of the problem from which other physical observables can be derived. Classical numerical methods are extensively applied to these models to verify the analytical predictions. We introduce a new classical algorithm here to compute the correlation functions of a classical dimer model on a square (bipartite) and a triangular (non-bipartite) lattice based on a tensor network construction. The method, called tensor network renormalization group, turns out to be a powerful tool for simulating short-ranged gapped systems as inferred from our results benchmarked against the classical Monte-Carlo technique and compared with past analytical studies. One should note that the quantum dimer model at the Rokhsar-Kivelson point can also be described as an infinite temperature canonical ensemble of classical dimers because of the particular structure of the ground state which is an equal weight superposition in the configuration manifold. The geometry of the lattice plays a pivotal role in deciding the nature of the phases that arise in the dimer models. Many physical properties of the dimer liquid phase can be extracted in the simple classical setting which certainly allows for a deep understanding of the classical models to be developed. The liquid phase is gapped on non-bipartite lattices and gapless on bipartite lattices, which is reflected in the decay of correlation functions with spatial distances. In general on non-bipartite lattices, the topological nature of the dimer liquid is characterized by a Z2 topological order which survives even when the model is perturbed away from the Rokhsar-Kivelson point. Stability of this liquid phase not only depends on the lattice geometries but notably on dimer concentrations also. In this context, we focus on a particular variant of the dimer model on a triangular lattice which is known as the quantum fully packed loop model. The model is composed of nonintersecting closed loops made of dimers and governed by the same Hamiltonian as the quantum dimer model. The loop model provides an effective low-energy description of a strongly correlated bosonic system at 1/3 filling on the kagome lattice. The corresponding Bose-Hubbard Hamiltonian consists of nearest-neighbor hopping and all possible repulsive interactions within a hexagonal plaquette. Conspicuous features of the zero-temperature phase diagram for this model include (i) presence of a stable Z2 liquid even without any Rokhsar-Kivelson potential term (in distinction to the standard quantum dimer model), and (ii) an unconventional phase transition from the liquid phase to a novel crystalline phase that has nematic order (dubbed lattice nematic). For a deeper understanding of the physics, a mapping to an Ising gauge theory is presented. The gauge theoretic description provides a useful way to predict the nature of the quantum phase transition to lie in the O(3) universality class. Finally a fermionic model at the same 1/3 filling is considered in which the ground state exhibits a number of exotic local orderings resulting from the spin-charge interplay of electrons. The Hamiltonian comprises nearest-neighbor hopping, strong on-site Coulomb interaction, and repulsive interaction terms only between nearest-neighbors. In the strong correlation limit, this fermionic problem maps to a two-color fully packed loop model – a model in which the loop segments carry an additional quantum number as color on a honeycomb lattice. The effective theory is governed by coherent three-particle ring exchanges and nearest-neighbor antiferromagnetic spin exchanges. The competition between these two leads to a phase diagram composed of a novel plaquette ordered state (known as the plaquette phase) that undergoes phase transition to a new kind of charge ordered state which we call a short loop phase. From our numerical analysis, we conclude that the plaquette phase features an unusual antiferromagnetic order with gapless spin excitations while the charge-ordered state is subjugated by spin fluctuations of localized electrons arranged in small hexagonal loops on the kagome lattice.

In the last decade, a large number of studies have been devoted to the peculiarities of correlated physics found in the quasi-two-dimensional square lattice iridium oxides. It was shown that this 5d family of transition metal oxides has strong structural and electronic similarities to the famous 3d family of copper oxides. Moreover, a delicate interplay of on-site spin-orbit coupling, Coulomb repulsion and crystalline electric field interactions is expected to drive various exotic quantum states. Many theoretical proposals were made in the last decade including the prediction of possible superconductivity in square-lattice iridates emerging as a sister system to high-Tc cuprates, which however met only limited experimental confirmation. One can, therefore, raise a general question: To what extent is the low-energy physics of the quasi-two-dimensional square-lattice iridium oxides different from other transition metal oxides including cuprates? In this thesis we investigate some of the effects which are usually neglected in studies on iridates, focusing on quasi-two-dimensional square-lattice iridates such as Sr2IrO4 or Ba2IrO4. In particular, we discuss the role of the electron-phonon coupling in the form of Jahn-Teller interaction, electron-hole asymmetry introduced by the strong correlations and some effects of coupling scheme chosen to calculate multiplet structure for materials with strong on-site spin-orbit coupling. Thus, firstly, we study the role of phonons, which is almost always neglected in Sr2IrO4, and discuss the manifestation of Jahn-Teller effect in the recent data obtained on Sr2IrO4 with the help of resonant inelastic x-ray scattering. When strong spin-orbit coupling removes orbital degeneracy, it would at the same time appear to render the Jahn-Teller mechanism ineffective. We show that, while the Jahn-Teller effect does indeed not affect the antiferromagnetically ordered ground state, it leads to distinctive signatures in the spin-orbit exciton. Second, we focus on charge excitations and determine the motion of a charge (hole or electron) added to the Mott insulating, antiferromagnetic ground-state of square-lattice iridates. We show that correlation effects, calculated within the self-consistent Born approximation, render the hole and electron case very different. An added electron forms a spin-polaron, which closely resembles the well-known cuprates, but the situation of a removed electron is far more complex. Many-body configurations form that can be either singlets and triplets, which strongly affects the hole motion. This not only has important ramifications for the interpretation of angle-resolved photoemission spectroscopy and inverse photoemission spectroscopy experiments of square lattice iridates, but also demonstrates that the correlation physics in electron- and hole-doped iridates is fundamentally different. We then discuss the application of this model to the calculation of scanning tunneling spectroscopy data. We show that using scanning tunneling spectroscopy one can directly probe the quasiparticle excitations in Sr2IrO4: ladder spectrum on the positive bias side and multiplet structure of the polaron on the negative bias side. We discuss in detail the ladder spectrum and show its relevance for Sr2IrO4 which is in general described by more complicated extended t-J -like model. Theoretical calculation reveals that on the negative bias side the internal degree of freedom of the charge excitation introduces strong dispersive hopping channels encaving ladder-like features. Finally, we discuss how the choice of the coupling scheme to calculate multiplet structure can affect the theoretical calculation of angle-resolved photoemission spectroscopy and scanning tunnelling spectroscopy spectral functions.

A dissertation submitted in fulfilment of the requirements for the degree of Master of Science in Physiotherapy in the Faculty of Medicine, University of the Witwatersrand, 1980
Studies were carried out on muscle specimens from patients with various neuromuscular disorders, including malignant hyperthermia carriers , in order to determine the value of muscle tension tests in the diagnosis of malignant hyperthermia. I t was found that when the results of muscle tension studies were considered together with those of other tests, this technique facilitate d the detection of malignant hyperthermia carriers . When used alone, however, the study of muscle tension could not be regarded as infallibly predictive .