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Theses

Propriétés spectrales et optiques des Matériaux corrélés.

Abstract : The major concern of this thesis is the calculation and interpretation of spectral and optical properties of correlated materials. We extend the variety of physcial quantities that can be accessed by modern many-body techniques for realistic materials, such as LDA+DMFT. We introduce an analytical continuation scheme that enables us to derive the real-frequency self-energy from imaginary time quantum Monte Carlo calculations in the most general (cluster) context. This furthermore allows for the calculation of the optical conductivity We have devised a formalism that relies on a formulation of the solid in terms of a localized basis set. The commonly used Peierls substitution approach is generalized to the case of multi-atomic unit-cells. This results in an approach of great versatility, since the entire formulation is independent of the underlying electronic structure method. We have applied these novel techniques to several compounds of interest: *Vanadium dioxide VO2* We find that while the metallic phase is characterized by strong signatures of correlations, the insulator is, as concerns its excitation spectrum, close to a description within a one-particle approach, that we define. Our picture of the insulator emerges as a "many-body Peierls" scenario. In a full-orbital setup, we calculate the optical conductivity of both phases, and find our theoretical result in satisfactory agreement with recent experiments. From the conductivity we, in particular, deduce the colour of the compound. *Vanadium sesquioxide V2O3* In our analysis, we find a correlation enhanced crystal-field splitting at the origin of its metal to insulator transition that occurs upon Cr-doping. Besides making predictions for angle-resolved photoemission experiments, we further evidence an orbital selectivity in the quasi-particle coherence temperature, which in particular allows for an understanding of the temperature dependence of recent optical measurements. *Rare-earth sesquioxides RE2O3 (RE=Ce, Pr, Nd, Pm)* These compounds are wide-gap Mott insulators that are not well described in density functional theory, due to the localized character of the RE4f orbitals. In our analysis, we, in particular, track the influence of these localized 4f orbitals as a function of the filling along the rare-earth series and find quantitative agreement for the evolution of the optical gap and a reasonable overall shape of the optical conductivity.
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Submitted on : Friday, July 23, 2010 - 1:59:08 PM
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Jan Martin Tomczak. Propriétés spectrales et optiques des Matériaux corrélés.. Physique [physics]. Ecole Polytechnique X, 2007. Français. ⟨pastel-00003163⟩

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