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Correlated phenomena studied by ARPES : from 3d to 4f systems Zonno, Marta


The physics of strongly correlated materials is at the heart of current condensed matter research. The inclusion of interactions in these materials between electron themselves or with other excitations intertwines various degrees of freedom (orbital, spin, charge and lattice), leading to a number of novel phenomena like Mott-Hubbard and charge-transfer insulators, high-temperature superconductivity and mixed-valence and Kondo physics. This thesis focuses on the study of two classes of correlated materials: copper-oxide high-temperature superconductors, whose correlated physics is driven by the localized nature of the half-filled Cu 3d-orbitals, and the rare-earth hexaborides, which are characterized by the strongly correlated 4f-shell. Recently, it has been shown that the interplay between different mechanisms underlying the formation of the superconducting condensate in the hole-doped bi-layer Bi₂Sr₂CaCu₂O₈₊δ can be addressed in the time domain by means of time- and angle-resolved photoemission spectroscopy (TR-ARPES). Using this technique, the primary role of phase coherence has been established. By exploiting the same dynamical experimental approach, we show that such scenario also describes the ultrafast collapse of superconductivity in the single-layer compound Bi₂Sr₂CuO₆₊δ. Moreover, by performing a comprehensive study on different doping levels of both single- and bi-layer compounds, we provide new insights on the temperature evolution of the nodal quasiparticle spectral weight. The second part of the thesis focuses on electron-doped cuprates, addressing the putative relation between the spectroscopically observed pseudogap and the robust antiferromagnetic order. Employing TR-ARPES as a tool to perform a detailed temperature dependent investigation allows us to explicitly link the momentum-resolved pseudogap spectral features to the evolution of the short-range spin-fluctuations in the optimally-doped Nd₂-xCe₂CuO₄. Lastly, we make use of chemical substitution to investigate the mixed-valent character of the rare-earth hexaboride SmxLa₁-xB₆ series. Our combined ARPES and x-ray absorption measurements reveal a departure from a monotonic evolution of the Sm valence as a function of x and the possible emergence of a mixed-valent impurity regime.

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