Echange et Corrélation dans la Structure Electronique des Solides, du Silicium à l'Oxyde Cuivreux: Approximation GW et au-delà

Electronic structure of crystals is a polymorphic name that covers a wide range of properties of electrons in periodic solids. For instance, it can refer to the probability to find an electron at space point r, in other words, to the electronic density. It can refer to the energy needed to extract an electron of the material, the ionization energy, or alternatively, the energetic gain when an electron is added to the system, the electron affinity. It can also stand for the response of the electrons of the solid to an external perturbation (a photon or a fast electron). All these properties characterize the electronic structure of the solid. They describe indeed different aspects. Some of them are ground-state properties, others correspond to excited states. Some of them conserve the number of particles, others do not. As a consequence, the properties that are generically called “electronic structure” are measured with distinct experimental setups, e.g. direct and inverse photoemission, optical absorption, electron energy-loss... And analogously, the theoretical description and prediction of these properties require distinct frameworks. This thesis work will handle the issue of the electronic structure of the cuprous oxide, Cu2O. Therefore, different theoretical methods will be used and results will be compared to a wide range of experimental techniques. The present manuscript contains different parts, which may seem at firstsight independent. It is therefore instructive to draw here quickly, what has been the historical development of this work, from the original project tothe final achievements. Three years ago, I started this thesis work with the purpose to describetheoretically the electronic structure of copper oxides, begining with the simplest one Cu2O, going to the antiferromagnetic CuO, and possibly to the CuO2 planes of high Tc superconducting cuprates. The cuprous oxide, Cu2O, with its closed electronic d shell was thought as a starting point to initiate the study of the Cu–O bonding in the different oxides. Cu2O is alsointeresting because spectacular excitonic series have been measured in its optical absorption or reflectivity spectra during the sixties [1–3]. However, it has been quickly clear that the usual electronic structuremethods, as density-functional theory (DFT) or even the state-of-the-art GW approximation of the many-body perturbation theory (MBPT), were unable to give a proper description of Cu2O. Comparison with existing photoemission or optical measurements were surprisingly bad. This unexpected failure changed the aims of my thesis, which turned into the analysis andthe cure of the shortcomings of the theoretical methods applied to Cu2O. I have been motivated to develop a number of theoretical and technical toolsthat, as I hope, will also be useful in future studies of other materials. The failure of the state-of-the-art GW approximation could come from two distinct reasons: either the current implementation of the method that worked well with simpler materials uses further assumptions, that are not anymore valid with the complex oxide Cu2O; or the GW approximationitself is not enough to account for the electronic structure of Cu2O. In order to check which hypothesis was the right one, I had first to identify, analyse and avoid all further approximations used in a standard GW, as it has most often been performed for 20 years. This part required some code and method development in order to remove the technical approximations that were used to make calculations easier, in our code or in most existing codes. This is what part II of this manuscript is concerned with. Ofcourse, the implementation of new pieces of code had to be checked on simple textbook examples, before being applied to copper oxide. That is why part II provides many results on bulk silicon and solid argon. This methodological part aims at removing the single plasmon-pole approximation that models the dynamical behavior of the screened Coulomb interaction W in GW. It has also the purpose of going beyond the usual perturbative evaluation of GW and to perform real self-consistent calculations within GW or within simpler approximations. The application of these developments to cuprous oxide are postponed to part IV, where all results concerning Cu2O are gathered. Alternatively, if the failure of the GW approximation were really a breakdown of the first-order perturbation theory in W (the coupling constantof the perturbation procedure), one would have to include further correcting terms to improve the results: these terms are commonly called “vertex corrections”. Due to their complexity, there is no unique method in literature to approximate them. Having in mind the purpose of applying vertex corrections to cuprous oxide, I had, first of all, the general task to define a proper scheme to do that, and to provide meaningful approximations to it. The basic idea was to start from the earlier developmentsmade in our group concerning the comparison between time-dependent DFT (TDDFT) and MBPT [4–6] in order to simplify the otherwise untractabletask of calculating vertex corrections. Deep insight in the respective role of the two theories was required and the study finally ended with theoreticalachievements that went farther than the initial project. Part III exposes first the advances made in the understanding of the link between TDDFT and MBPT with a new simple equation that derives the crucial, but unfortunately unknown, kernel of TDDFT from the central quantity of MBPT, the self-energy. Second, part III shows how the same kind of ideas can be used in the other direction (namely, use TDDFT in order to progress within MBPT), to make the calculation of vertex corrections easier. These developments were applied in practice to simple materials: once again bulk silicon and solid argon. In fact, even though the derived vertex correction are “simpler”, the calculations remain orders of magnitude more complicated than the usual GW ones. It is still out of reach to apply these developments to cuprous oxide at present. Nevertheless, this derivation and study of vertex corrections allowed me to draw the general conclusion that strong cancellations between vertex corrections occur.The last part of the present work (part IV) presents all my results concerning Cu2O, from ground-state DFT studies to new self-consistent GW results, and also from theoretical data to experimental measurements. This part starts with a standard electronic structure study, first DFT geometricalstructure, Kohn-Sham band structure and characterization of the orbitals, then standard perturbative GW evaluation of the quasiparticle bandstructure. As said earlier, this study unexpectedly fails, in particular, for the band gap and the optical threshold. That is why the methods developpedin the previous parts are indeed needed in the study of Cu2O. Moreover, existing valence band photoemission experiments did not allow one to detect some of the states found in our and previous bandstructure calculations. However, the experiments were performed on polycrystalline samples and therefore yielded spectra resulting from an integration over the whole Brillouin zone. This was the motivation to apply for beamtime at the synchrotron Elettra in Trieste, Italy together with my collaborators (theoreticians and experimentalists). The purpose was to obtain precise angle-resolved photoemission spectra of the valence states of Cu2O.This means k-point resolved information. After a one-week experimental shift at Elettra, an important part of the present work was to compare our state-of-the-art measurements to the theoretical data, taking into account the experimental aspects, as photoemission cross-sections, evaluation of the causes of experimental uncertainties... Indeed, it turned out that a careful comparison of state-of-the-art experimental and theoretical approaches could remove all existing or seeming contradictions, within the remaining uncertainty of the respective approaches. This is a significant part of the present study of the electronic structure of Cu2O. This part ends with the calculation of energy-loss and optical absorption spectra of cuprous oxide, the latter being now also in good agreement with experiment. The theories and methods used throughout this text are extensively described in the first part of the manuscript. It seemed important to me to provide an accurate account for the theoretical background, because most of the present work deals with improvements of existing methods, or going beyond some piece of the theory. In order to explain the achievements, one first needs to have clearly in mind the existing grounds. Let me then open this text with the part concerning the theoretical background my work is based on.La structure électronique des cristaux est un nom polymorphe qui couvre une large gamme de propriétés des électrons dans les solides périodiques. Par exemple, il peut se référer à la probabilité de trouver un électron au point r de l'espace, en d'autres termes, à la densité électronique. Il peut faire référence à l'énergie nécessaire pour extraire un électron du matériau, l'énergie d'ionisation, ou encore, le gain énergétique lorsqu'un électron est ajouté au système, l'affinité électronique. Il peut aussi se lire la réponse des électrons du solide à une perturbation externe (un photon ou un électron rapide). Toutes ces propriétés caractérisent la structure électronique du solide. Ils décrivent les aspects bien différents. Certains d'entre eux sont propriétés de l'état, d'autres correspondent à des états excités. Certains d'entre eux de conserver le nombre de particules, d'autres pas. En conséquence, les propriétés qui sont généralement appelés «structure électronique» sont mesurées avec différentes configurations expérimentales, par exemple photoémission directe et inverse, l'absorption optique, de l'énergie d'électrons de perte ... Et de manière analogue, la description théorique et la prévision de ces propriétés requièrent cadres distincts. Ce travail de thèse se chargera de la question de la structure électronique de l'oxyde cuivreux, Cu2O. Par conséquent, différentes méthodes théoriques seront utilisées et les résultats seront comparés à un large éventail de techniques expérimentales

Polarization anisotropy in the optical properties of silicon ellipsoids

A new real space quantum mechanical approach with local field effects included is applied to the calculation of the optical properties of silicon nanocrystals. Silicon ellipsoids are studied and the role of surface polarization is discussed in details. In particular, surface polarization is shown to be responsible for a strong optical anisotropy in silicon ellipsoids, much more pronounced with respect to the case in which only quantum confinement effects are considered. The static dielectric constant and the absorption spectra are calculated, showing that the perpendicular and parallel components have a very different dependence on the ellipsoid aspect ratio. Then, a comparison with the classical dielectric model is performed, showing that the model only works for large and regular structures, but it fails for thin elongated ellipsoids.Comment: 5 pages, 4 figures, International Conference on NANO-Structures Self-Assemblin

Assessment of the interplay between blood and skin vascular abnormalities in adult purpura fulminans

RATIONALE: Purpura fulminans in adults is a rare but devastating disease. Its pathophysiology is not well known. OBJECTIVES: To understand the pathophysiology of skin lesions in purpura fulminans, the interplay between circulating blood and vascular alterations was assessed. METHODS: Prospective multicenter study in four intensive care units. Patients with severe sepsis without skin lesions were recruited as control subjects. MEASUREMENTS AND MAIN RESULTS: Twenty patients with severe sepsis and purpura fulminans were recruited for blood sampling, and skin biopsy was performed in deceased patients. High severity of disease and mortality rates (80%) was observed. Skin biopsies in purpura fulminans lesions revealed thrombosis and extensive vascular damage: vascular congestion and dilation, endothelial necrosis, alteration of markers of endothelial integrity (CD31) and of the protein C pathway receptors (endothelial protein C receptor, thrombomodulin). Elevated plasminogen activating inhibitor-1 mRNA was also observed. Comparison with control patients showed that these lesions were specific to purpura fulminans. By contrast, no difference was observed for blood hemostasis parameters, including soluble thrombomodulin, activated protein C, and disseminated intravascular coagulation markers. Bacterial presence at the vascular wall was observed specifically in areas of vascular damage in eight of nine patients tested (including patients with Streptococcus pneumoniae, Neisseria meningitidis, Escherichia coli, and Pseudomonas aeruginosa infection). CONCLUSIONS: Thrombi and extensive vascular damage with multifaceted prothrombotic local imbalance are characteristics of purpura fulminans. A "vascular wall infection" hypothesis, responsible for endothelial damage and subsequent skin lesions, can be put forward

Colored-noise thermostats \`a la carte

Recently, we have shown how a colored-noise Langevin equation can be used in the context of molecular dynamics as a tool to obtain dynamical trajectories whose properties are tailored to display desired sampling features. In the present paper, after having reviewed some analytical results for the stochastic differential equations forming the basis of our approach, we describe in detail the implementation of the generalized Langevin equation thermostat and the fitting procedure used to obtain optimal parameters. We discuss in detail the simulation of nuclear quantum effects, and demonstrate that, by carefully choosing parameters, one can successfully model strongly anharmonic solids such as neon. For the reader's convenience, a library of thermostat parameters and some demonstrative code can be downloaded from an on-line repository

Vascular Smooth Muscle Cell Plasticity and Autophagy in Dissecting Aortic Aneurysms.

Objective- Recent studies suggested the occurrence of phenotypic switching of vascular smooth muscle cells (VSMCs) during the development of aortic aneurysm (AA). However, lineage-tracing studies are still lacking, and the behavior of VSMCs during the formation of dissecting AA is poorly understood. Approach and Results- We used multicolor lineage tracing of VSMCs to track their fate after injury in murine models of Ang II (angiotensin II)-induced dissecting AA. We also addressed the direct impact of autophagy on the response of VSMCs to AA dissection. Finally, we studied the relevance of these processes to human AAs. Here, we show that a subset of medial VSMCs undergoes clonal expansion and that VSMC outgrowths are observed in the adventitia and borders of the false channel during Ang II-induced development of dissecting AA. The clonally expanded VSMCs undergo phenotypic switching with downregulation of VSMC differentiation markers and upregulation of phagocytic markers, indicative of functional changes. In particular, autophagy and endoplasmic reticulum stress responses are activated in the injured VSMCs. Loss of autophagy in VSMCs through deletion of autophagy protein 5 gene ( Atg5) increases the susceptibility of VSMCs to death, enhances endoplasmic reticulum stress activation, and promotes IRE (inositol-requiring enzyme) 1α-dependent VSMC inflammation. These alterations culminate in increased severity of aortic disease and higher incidence of fatal AA dissection in mice with VSMC-restricted deletion of Atg5. We also report increased expression of autophagy and endoplasmic reticulum stress markers in VSMCs of human dissecting AAs. Conclusions- VSMCs undergo clonal expansion and phenotypic switching in Ang II-induced dissecting AAs in mice. We also identify a critical role for autophagy in regulating VSMC death and endoplasmic reticulum stress-dependent inflammation with important consequences for aortic wall homeostasis and repair

Dynamic structure factor and dielectric function of silicon for finite momentum transfer: Inelastic x-ray scattering experiments and ab initio calculations

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Quasiparticle interfacial level alignment of highly hybridized frontier levels: H2_2O on TiO2_2(110)

Knowledge of the frontier levels' alignment prior to photo-irradiation is necessary to achieve a complete quantitative description of H$_2$O photocatalysis on TiO$_2$(110). Although H$_2$O on rutile TiO$_2$(110) has been thoroughly studied both experimentally and theoretically, a quantitative value for the energy of the highest H$_2$O occupied levels is still lacking. For experiment, this is due to the H$_2$O levels being obscured by hybridization with TiO$_2$(110) levels in the difference spectra obtained via ultraviolet photoemission spectroscopy (UPS). For theory, this is due to inherent difficulties in properly describing many-body effects at the H$_2$O-TiO$_2$(110) interface. Using the projected density of states (DOS) from state-of-the-art quasiparticle (QP) $G_0W_0$, we disentangle the adsorbate and surface contributions to the complex UPS spectra of H$_2$O on TiO$_2$(110). We perform this separation as a function of H$_2$O coverage and dissociation on stoichiometric and reduced surfaces. Due to hybridization with the TiO$_2$(110) surface, the H$_2$O 3a$_1$ and 1b$_1$ levels are broadened into several peaks between 5 and 1 eV below the TiO$_2$(110) valence band maximum (VBM). These peaks have both intermolecular and interfacial bonding and antibonding character. We find the highest occupied levels of H$_2$O adsorbed intact and dissociated on stoichiometric TiO$_2$(110) are 1.1 and 0.9 eV below the VBM. We also find a similar energy of 1.1 eV for the highest occupied levels of H$_2$O when adsorbed dissociatively on a bridging O vacancy of the reduced surface. In both cases, these energies are significantly higher (by 0.6 to 2.6 eV) than those estimated from UPS difference spectra, which are inconclusive in this energy region. Finally, we apply self-consistent QP$GW$ (scQP$GW$1) to obtain the ionization potential of the H$_2$O-TiO$_2$(110) interface.Comment: 12 pages, 12 figures, 1 tabl

Electronic Structure Calculation by First Principles for Strongly Correlated Electron Systems

Recent trends of ab initio studies and progress in methodologies for electronic structure calculations of strongly correlated electron systems are discussed. The interest for developing efficient methods is motivated by recent discoveries and characterizations of strongly correlated electron materials and by requirements for understanding mechanisms of intriguing phenomena beyond a single-particle picture. A three-stage scheme is developed as renormalized multi-scale solvers (RMS) utilizing the hierarchical electronic structure in the energy space. It provides us with an ab initio downfolding of the global band structure into low-energy effective models followed by low-energy solvers for the models. The RMS method is illustrated with examples of several materials. In particular, we overview cases such as dynamics of semiconductors, transition metals and its compounds including iron-based superconductors and perovskite oxides, as well as organic conductors of kappa-ET type.Comment: 44 pages including 38 figures, to appear in J. Phys. Soc. Jpn. as an invited review pape