Electronic excitations and magnetism of materials : ab initio calculation of K-edge X-Ray absorption and X-Ray Circular Magnetic dichroism ; Excitations électroniques et magnétisme des matériaux : calcul ab initio de l'absorption X et du dichroïsme circulaire magnétique au seuil K

Experimental measurements of X-ray absorption (XAS) and magnetic circular dichroism(XMCD) are widely used to study electronic excitations and magnetism in materials.Theoretical interpretations of those measurements play a crucial role in the understandingof those studies. We have developped new methods to calculate X-ray absorptionand XMCD at the K edge based on the use of the density functionnal theory (DFT), ofpseudopotentials and of the continued fraction approach. Those methods allow a parameterfree treatment of the core hole.We applied those methods on a wide range of materials, from weak to strong electroniccorrelation, in order to see if a monoparticle approximation could describe XAS andXMCD spectra.We rst sudied the X-ray absorption of graphene, a weakly correlated material,in order to determine if the interlayer state responsible for the supraconductivity ofintercalated graphites could be seen in XAS, as suggested by recent experiments. We thencalculated the strongly correlated systems NiO, CoO, CuO, La2CuO4 and Ca2−xCuO2Cl2.We showed the presence of non-local excitations in the dipolar part of the absorption spectrain those systems, and proposed an independant method to measure the charge-transfergap based on the XAS of a hole-doped system. We showed the importance of many-bodyeects in the edge region and attributed them to multi-determinant eects with the helpof a concolution with photoemission formula. Last, we successfully calculated the XMCDspectra of iron and cobalt under pressure, and discussed of the relative importance ofdipole and quadrupole transitions, of anisotropy and of the accuracy of sum rules. ; Les mesures d'absorption X et de dichroïsme circulaire magnétique sont largementutilisées pour étudier les excitations électroniques et le magnétisme des matériaux. Cesétudes dépendent de manière cruciale de l'interprétation théorique des mesures expérimentales.Nous avons développé de nouvelles méthodes de calcul de l'absorption X (XAS)au seuil K et du dichroïsme magnétique circulaire (XMCD) ...