All-Electrical Oxygen Doping in Perovskite Oxides

Complex oxides are at the heart of modern functional material developments. In particular, the perovskite ABO3 structure is seen in compounds used in oxide solar cells, resistive memories, fuel cell catalysts, superconducting tapes, quantum bits and programmable magnets, making it one of the most studied material families. One important advantage of these systems is that their properties may be controlled in situ to change between various electronic states, usually by means of thermal or electric conditioning. In this thesis,
the modulation of the properties of some perovskite oxides is demonstrated using simple electrical controls. The model material employed to this end is YBa2 Cu3 O7−δ (YBCO) but results in ferroic manganites indicate that the behavior might be general to the material family. We have unambiguously demonstrated that oxygen vacancies located in the CuO chains in YBCO can be displaced several µm scale distances through a biased diffusive process known as selective electromigration. The rearrangement of these oxygen
chains and the change in relative concentration δ, leads to a related shift in hole carriers in the CuO2 planes. The final outcome is the possibility of tuning the numerous electronic phases displayed by this compound, all via electric actuation. The methodologies discussed, based on current driven oxygen diffusion in planar films, offer the advantage of easy fabrication and control comparable to common gated sample designs that require multi-
step patterning. Reflectivity changes as a consequence of changing levels of oxygenation in the material are used to map the evolution of the phenomenon in real time and to validate a phenomenological finite element method (FEM) model, providing adequate descriptions of the effect as it occurs in these practically useful materials. The understanding generated in this manner allows for reversible resistance switching and tuning of superconducting critical temperature Tc , as demonstrated in YBCO.