Seminars Archive

Shedding new light on two textbook transition metal oxides: Anatase TiO₂ and Tetragonal CuO

Abstract
Transition metal oxides 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 exploited in new devices. Anatase TiO2 has been proposed for many applications from transparent conducting panels to photovoltaic- and photocatalytic- devices, as well as memristors. However, little is known about the dynamics of the photoexcited or doped-in charge carriers in this textbook insulator. In the first part of this presentation, we show that the number of conduction electrons in anatase, and their nature, can be controlled by UV light, in a controllable and reversible way. Oxygen vacancies created by UV light liberate electrons that populate the bottom of the conduction band. At low densities, these electrons are dressed by lattice vibrations, and behave as a gas of weakly interaction large polarons. At larger densities, the quasiparticles spatially overlap, losing their identity, and eventually dissolve into a weakly correlated Fermi liquid. The cupric oxide CuO 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. Recently, CuO was synthesized by epitaxial growth on SrTiO3 substrates in a higher symmetry tetragonal structure much closer to the one observed in most cuprates (Siemons et al. PRB 79, 2009). At beamline 7 of the Advanced Light Source, we have grown tetragonal CuO thin films by pulsed laser deposition and revealed the dispersion of the first ionization state, a Zhang Rice Singlet (ZRS). We show that the ZRS preferentially propagates on two almost independent corner sharing sub-lattices of this edge sharing CuO system. Further we find clear indications for the ZRS interacting with a fluctuating spin background.