Infoscience

Thesis

Lattice, spin and orbital excitations in selected transition metal oxides: a resonant inelastic x-ray scattering investigation

Transition metal compounds represent a unique class of solids characterized by a surprising richness of physical properties. They display many phenomena and represent the main area of interest for the research on strongly correlated materials. Many novel ideas such as spintronics, magnetoelectricity, multiferroicity and high-Tc superconductivity arose from transition metal compounds and form a fruitful field of research, promising and having already established important applications. This thesis focuses on two of these phenomena: ferroelectricity and high-Tc superconductivity. BaTiO3 is a well studied and technologically relevant ferroelectric material characterized by a high temperature for the paraelectric to ferroelectric transition. The temperature dependent RIXS investigation, reported in this thesis, provided a further clarification on the microscopic mechanism at the basis of its ferroelectric phase transition. Lowering the temperature, the RIXS spectra show a transfer of spectral weight from the elastic to the charge-transfer spectral features, indicative of increasing Ti 3d-O 2p hybridization. When the incident photon of the RIXS process is tuned to select transitions to the Ti 3d eg manifold, the quasielastic RIXS response exhibits a tail indicative of phonon excitations. A quantitative analysis of the line shape permitted the estimation of the electron-phonon coupling strength, placing BaTiO3 in the intermediate coupling regime. A vast area of research on high-Tc superconductivity concerns cuprate materials. In this work, the attention is focused on two different examples of this family: the benchmark La(2-x)SrxCuO4 compound and the alternative cuprate structure of Ba2Cu3O4Cl2. An excitation-energy dependent RIXS investigation on La(2-x)SrxCuO4 revealed a double peak structure located at a 2 eV. This feature shows fluorescent behaviour excluding the identification to purely dd excitations. Cluster calculations suggest significant contributions from the Zhang-Rice singlet (ZRS) state. A Cu L3 edge RIXS investigation on Ba2Cu3O4Cl2 provided the signature of the spin waves of this oxhychloride. These results compared with previous RIXS data on cuprates allowed to selectively determine the relevant parameters of the model Hamiltonian at the basis of the description of this class of materials.

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