Electronic Transitions in Perovskite Oxide Heterostructures
- Author(s): Wong, Franklin
- Advisor(s): Suzuki, Yuri
- et al.
The focus of this dissertation is on tuning electronic phase transitions in perovskite oxide heterostructures. The recurring theme is the exploitation of characteristics inherent to heteroepitaxy that are unavailable in the bulk materials of identical chemistry; these inherent characteristics are epitaxial strain and interfacial electronic reconstruction. In contrast, the effects of chemical substitution, for example, are not defining traits of heterostructures as they can be routinely studied in bulk materials, and therefore will not be included in this work.
Heteroepitaxial lattice deformation is identified to be crucial in two systems: coherently strained LaTiO3 films on SrTiO3 substrates and La2/3Sr1/3MnO3 films on LaAlO3 substrates. One the other hand, interfacial charge transfer is the responsible for the insulator-metal transitions in LaAlO3 films on SrTiO3 substrates, or LaAlO3/SrTiO3 heterointerfaces. The charge, orbital, spin, and lattice "degrees of freedom" are intimately linked in perovskite oxides, particularly those containing transition metals. The lattice can be modified through heteroepitaxial strain, and the charge state can be modified through interfacial electronic reconstruction.
It is shown that epitaxial strain can induce an insulator-metal transition in LaTiO3, which is a Mott insulator in the bulk that is near an electronic instability owing to its orbital disorder. It is confirmed that metallic conduction occurs throughout the entire thicknesses of the LaTiO3 films, as opposed to being confined to the surfaces or interfaces. It is possible to exploit heteroepitaxial strain as a knob to modulate the electronic bandwidth. Strain values that can be introduced routinely in thin films are greater than those which are easily accessible in hydrostatic or uniaxial stress studies of bulk materials.
Epitaxial strain also is explicitly linked to the enhanced magnetoresistive properties of La2/3Sr1/3MnO3 films, particularly at low temperatures. The key effect appears to be the stabilization of a more insulating phase that may coexist with the double-exchange metallic ferromagnetic phase, which is stable in the bulk. The more insulating phase in turn becomes unstable upon the application of a magnetic field, resulting in higher magnetoresistance over a wide range of temperatures. While such a mechanism is certainly believed to be pertinent for many bulk manganites, it is not expected to be relevant for the specific chemical composition studied. Similar to the case of LaTiO3, the orbital degrees of freedom in La2/3Sr1/3MnO3 are important for the tunability of such an electronic transition controlled by thin-film lattice deformation; an added ingredient is that there exist two competing magnetic exchange mechanisms that are sensitive to chemical bonding. Both the orbital degree of freedom and competing exchange interactions make the electronic properties of manganite films very sensitive to external lattice perturbations.
Finally, metallic LaAlO€3/SrTiO3 heterointerfaces between two nominally undoped insulators are examined. Interfacial polarity is believed to induce electron transfer from LaAlO3 to SrTiO3. Low temperature magnetotransport studies reveal signatures of strong spin-orbit interaction in this quasi-two-dimensional metallic channel. The spin-orbit coupling in the heterostructures is enhanced greatly by interfacial electric fields; hence, it represents a property that is fundamentally non-bulk-like. The transport data presented in this work suggest that disorder and carriers are introduced concurrently at the heterointerface, and also shed light on the origin of metallicity.