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UBC Theses and Dissertations

Charge localization phenomena in correlated oxides Comin, Riccardo


Charge segregation is very common in correlated oxides, spanning from the extreme limit of the Mott insulator, characterized by strong charge localization and suppressed charge dynamics, towards more mildly correlated states of matter, characterized by partial charge reorganization (charge-density-wave). In this thesis work I have investigated how the spatial organization of valence charge evolves with electrostatics (carrier doping) and chemistry (going down in the periodic table). For this reason, the focus is on the experimental study of the strongly-correlated 3d-oxides and the spin-orbit coupled 5d-oxides. These investigations have been performed with a bundle of state-of-the-art spectroscopic techniques in the field of quantum materials, namely: angle-resolved photoemission (ARPES), low-energy electron diffraction (LEED), resonant elastic X-ray scattering (REXS), X-ray diffraction (XRD), scanning tunnelling microscopy (STM) and optical spectroscopy. To support experimental data, we have used theoretical tools such as conventional density functional theory (DFT) for the 5d-oxides and developed ad-hoc approaches for the more complex 3d-materials. The all-around study of underdoped high-Tc Bi-based cuprates allowed us to shed new light on the universality and origin of charge-ordering instabilities in these materials and understand their interplay with superconductivity and pseudogap. These phenomena have been investigated in detail in underdoped samples of Bi2201, using a various experimental techniques (ARPES, REXS, XRD, LEED, STM) and various tailored theoretical approaches. The results of these studies are presented in Chapters 2 and 3. In the 5d-based iridates (in particular, Na₂IrO₃) we have revealed and characterized a novel form of Mott-Hubbard physics. This has been possible thanks to the combination of ARPES and optics for probing the electronic structure near the Fermi energy, and DFT for providing the theoretical framework to understand the electronic ground state in these materials. Ultimately, this approach helped demonstrate the crucial role of spin-orbit interaction in driving a novel Mott phase in materials where the Mott criterion might be violated (Chapter 4). Altogether, the resulting phenomena discovered in copper- and iridium based oxides have revealed novel unconventional aspects of the physics of correlated materials, thus paving the way for future explorations of the complex but fascinating jungle of transition metal oxides.

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