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Abstract

More than twenty five years have passed since the discovery of the first copper oxide based superconductor La2-xBaxCuO4 in 1986, and the intriguing physics of cuprates superconductors continues to fascinate and challenge scientists from all over the world. These relatively new compounds belong to a major class of materials known as strongly correlated materials that show the most disparate physical properties: Mott metal-insulator transitions, High temperature Superconductivity, Kondo and heavy fermion behavior, colossal magnetoresistance, etc. These properties make them attractive for developing new applications, but before this goal is reached a deep investigation of the fundamental electronic properties and the understanding of their tunability is fundamental. In particular high resolution RIXS experiments which represent the main subject of this thesis, became a very powerful tool to investigate the electronic and magnetic properties of strongly correlated materials and in particular high temperature superconductor. The experiment takes advantage of the photon tunability of the synchrotron radiation and the recent technological achievements in the developement of dedicated instrumentation for the construction of the spectrometer (i.e. SAXES). SAXES is installed on the ADRESS beamline at the third generation synchrotron radiation SLS (Swiss Light Source) in Villigen (Switzerland). It operates in the 400-1500 eV energy range and it is designated to have a resolving power higher than 10000 in the whole operational range. In Chapter 1 the RIXS physical process, the distinction between direct and indirect RIXS, the experimental configuration at the ADRESS beamline and a theoretical development for the calculation of the RIXS process will be presented. Chapter 2 is dedicated to an introduction to the vast and still open field of the electronic and magnetic properties of cuprates. The model Hamiltonians for cuprates (Hubbard, Heisenberg, pd and tf Hamiltonians), the Zaanen-Sawatzky-Allen scheme, the rearrangement of the atomic levels due to the crystal field, the core level spectroscopy together with a brief introduction to the problem of the low dimensional quantum magnetism will be presented. Chapter 3 is dedicated to the experiments on the parent compounds of HTSCI have performed at the ADRESS beamline. A systematic study of the crystal field excitations {dd) as a function of the scattering geometry and incident polarizations have been carried out on different samples. An ionic model is presented in order to interpret the Cu L3 edge RIXS in layered cuprates. Then the central topic of the present thesis will be presentend. Measurements on parent compounds of high temperature superconductor along the two high symmetry directions revealed that a correct description of the measured magnon dispersion, needs to take into account an extended Hubbard model with higher order interaction than the nearest neighbour one. I will also present an attempt to enhance the visibility of the spectral feature by a deconvolution method. Chapter 4 presents a wide RIXS study of the superconducting Bi2212 family. I will present our RIXS measurements on Bi2212 superconducting samples along both the antinodal and nodal directions. I will compare our results to other RIXS measurements existing in literature on superconducting samples along the antinodal direction only. Importantly I will show that the observed dispersion along the antinodal direction is very different with respect to the spectra taken along the nodal one. This problem prompted the search for an alternative theoretical model to the one already proposed. The theoretical refinements to this approach are in progress but, already at the actual level they can reproduce the observed differences between the spectra along the two high symmetry crystallographic directions. In Chapter 5 I will show preliminary measurements on cuprite with high resolution. The sharper lineshape with respect to previous measurements, will allow a better comparison with band structure calculations. Moreover these measurements, open the possibility to measure the valence band dispersion of cuprite and related seminconducting materials at the Cu L3 edge. In the appendixes I will present the Lucy-Richardson method for the deconvolution of RIXS data and an explicit calculation for the correction of the self absorption effect in RIXS experiments.

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