Transition metal oxides under X-ray vision

In this thesis, thin films of transition metal oxides are studied using a variety of X-ray spectroscopy techniques. Complex oxide heterostructures consist of interlocking perovskite building blocks that are combined at the unit cell level. In this way, an unprecedented control over the physical properties of the material is realised. Specifically, through appropriate choice of material combinations, an individual tuning of the electronic, orbital and spin degrees of freedom is achieved. This ability, combined with the reduced dimensionality of the systems, promotes the role of the interface. In a number of material combinations, these properties are completely absent in the parent compounds. In this research, novel, emergent electronic and magnetic states of matter are investigated.
The first part examines the effect of interfacial band offset in a material combination in which a large electronegativity mismatch is realised,namely the isopolar LaCoO₃ | LaTiO₃ interface. This is found to result in an interfacial electron transfer from Ti to Co, an effect which can be exploited to achieve nanoscale control over the number of d electrons in Co via interfacial engineering.
The second part discusses interfacial charge transfer driven ultimately by a different phenomenon: that of avoiding a polar catastrophe in ultrathin (polar) LaMnO₃ films, grown on (non-polar) SrTiO₃ substrates.
The third and final part of this thesis investigates the metal-insulator transition in VO₂, employing recent advances in the growth of the VO₂ thin films on nanosheets that enable X-ray spectroscopy experiments to be carried out in transmission.