Spectroscopic Studies of the Electronic Structure in Layered Cuprates and Ruthenates

In materials where electrons interact strongly, a number of exotic and exciting phenomena arise. The mechanisms at the base of many of these phenomena remain debated, as strongly correlated electron physics represents one of the biggest challenges for modern condensed matter physics. Transition metal oxides are a class of strongly correlated systems which exhibit a multitude of physical properties, among which unconventional superconductivity, incommensurate charge and spin ordering, partial anisotropic gapping, strangemetallicity, colossal magnetoresistance, multiferroicity, antiferromagneticMott insulation, etc.. These properties are often observed in close vicinity, when pressure, temperature or chemical composition are varied. As a consequence, it is challenging to disentangle not only their origin, but also their effects on the elementary excitations of the system. In this thesis, high quality single crystals of copper and ruthenium based transition metal oxides (cuprate and ruthenate compounds) were investigated with synchrotron-based spectroscopic techniques. Our angle resolved photoemission spectroscopy (ARPES) studies focused on the normal state of cuprates, at hole content varying from under- to over-doping. First, the low energy single particle excitations in overdoped uprates were verified to fulfill the mathematical conditions for Landau Fermi quasiparticles. In a second study, the evolution of the spectral gap was followed as function of doping and temperature, across the charge order, pseudogap and strangcuprates, ruthenates, strong correlations, RIXS, ARPES, XAS, pseudogap, strange metal, spin orbit coupling, crystal field.e metal phases of a cuprate compound, La1.6¡xNd0.4SrxCuO4. This systematic study allowed the identification of an optimal doping regime for the investigation of the pseudogap physics in cuprates. The orbital structure of single layer ruthenates was explored combining X-ray absorption and resonant inelastic X-ray spectroscopies (XAS and RIXS). Since spectroscopies at the L absorption edge of 4d materials are challenged by insufficient energy resolution, we performed our studies at the oxygen K edge, in the soft X-ray energy range. By exploiting the strong orbital hybridization between the oxygen 2p and the ruthenium 4d states, we obtained high resolution experimental access to the 4d electron properties. The results were interpreted through a simple model Hamiltonian, with analytic solutions that provide a consistent description of the low energy features observed.