Infoscience

Thesis

The nature of the quasiparticles in two benchmark transition metal oxides: a spectroscopic study of anatase TiO₂ and tetragonal CuO

Transition metal oxides (TMOs) are emerging strong players in many domains, ranging from superconductivity, to microelectronics to spintronics to light harvesting for photovoltaics. Beyond their non-toxicity, low corrosiveness and low price, they exhibit a whole range of exciting electronic properties, which could be realistically exploited in new devices. Typically, TMOs are governed by strong correlation of its 3d electrons, often dressed by lattice or magnetic excitations and thus leading to complex electronic behavior. This work is focused on the electronic structure of two benchmark TMOs - anatase TiO₂ and tetragonal CuO - spectroscopically investigated by angle resolved photoemission (ARPES) and resonant inelastic x-ray scattering (RIXS). Anatase TiO₂, a 3d0 system, has been proposed for many applications from transparent conducting layers to photovoltaic- and photocatalytic- devices, as well as memristors. For the performance of these devices, the charge carrier lifetime and their control is of primordial importance. By means of ARPES, the possibility to achieve a fine control of the mobile charge carrier concentration through x-ray beam doping is demonstrated. We show that the conduction electrons in anatase, and their nature, is determined by significant electron-phonon-coupling and by the number of oxygen defects. At low defect densities, charge carriers behave as a gas of weakly interacting large polarons. At larger densities, the polarons spatially overlap and dissolve into a weakly correlated Fermi liquid. The role of the electron-lattice coupling is further resolved by RIXS. The spectral signatures of phonons hint towards isotropic electron-phonon-coupling in anatase. Phonon frequency and electron-phonon coupling both show low doping sensitivity. The cupric oxide CuO, a 3d9 system, exhibits an insulating ground state with a correlation-induced charge-transfer gap and antiferromagnetism. It is, in principle, the most straightforward parent compound of the doped cuprates, and therefore has been theoretically studied as a model material for high temperature superconductivity. Bulk CuO crystallizes in a low-symmetry monoclinic form, in contrast to the rocksalt structure typical of late 3d transition metal monoxides. In this work, CuO was synthesized by epitaxial growth on SrTiO3 substrates in a higher symmetry tetragonal structure (T-CuO) much closer to the one observed in most cuprates. ARPES identifies its first ionization state as a Zhang Rice Singlet (ZRS). This is the first observation of the ZRS on a quasi 2D edge-sharing cuprate system with possible implications for magnetism and potential superconductivity in the doped phase.

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