Resonant spectroscopy on strongly correlated materials

All strongly correlated materials exhibit fascinating physical properties, like Mott insulator transitions, high temperature superconductivity, Kondo and heavy fermion behavior, colossal magnetoresistance, etc. These properties make them attractive for developing novel applications. To achieve this ambitious goal, the nature of the strong electronic correlations must be investigated. Soft x-ray resonant inelastic scattering (RIXS) and resonant inverse photoemission (RIPES) are precious in this context, because they provide detailed information on the fundamental low energy excitations. The high resolution RIXS experiments which represent the main subject of the present thesis required a lot of experimental work for testing, commissioning and fine-tuning two very sophisticated instruments: the soft x-ray ADRESS beamline at the Swiss Light Source (SLS) and its dedicated RIXS spectrometer SAXES. Thanks to this work, the design goal of reaching a world-record resolution better than 100 meV below 1k eV has been successfully achieved. Chapter 3 and 4 present RIXS measurements of NiO, MnO, CuO and other cuprates performed both at the TM L3, O K edges. The unprecedented resolving power of SAXES allows us to accurately identify the local electronic excitations at the ~eV scale as well as new low energy excitations at the ~100 meV scale, originated from the magnetic exchange interaction. Chapter 5 is devoted to the RIPES study of the isostructural Kondo systems CeX9Si4 (X=Ni,Co), and presents a possible explanation for the remarkable differences in the magnetic and transport properties of these two related compounds.