Study of ferroelectric oxides and field effect in complex oxides heterostructures

With the rapid development of technology, the need for novel materials and state-of-theart devices is growing fast. Complex oxides which have strongly correlated electrons are favorable candidates for materials industry, due to their rich phase diagrams and multiple functions. Especially, ferroelectric oxides is very promising materials in the industry for storage, due to their bistable polarization states triggered by external electrical field. This thesis is centered on ferroelectric oxides, analyzing their lattice structures and investigating the interface of ferroelectrics and other complex oxides to examine the potential of the heteostructures in the application in electronic devices.

The most notable feature of ferroelectric oxides is their reversible spontaneous polarization, which is strongly related to the lattice distortion in the crystal. In chapter 2, we report a neutron diffraction study of NaNbO3 [sodium niobate], to comprehensively understand the origin of ferroelectricity. We find that the structure evolves six phases from 930K to 15K in long-range, while only three real ground states are detected at short-range.

We then study the ferroelectric/manganites heterostructures and explore its potential in the application in field effect transistors and ferroelectric memories. In chapter 3, we first describe the insulator-to-metal phase transition is realized in La0.8Sr0.2MnO3 [strontium doped lanthanum titanate oxides] under the field-effect of the ferroelectric Pb(Zr0.2Ti0.8)O3 [lead titanate oxides] layer. Moreover, we find that the interfacial transition plays a very crucial role in the tunneling electroresistance effect (TER) of the heterostructures.

In chapter 4, we test the similar function at the interface of ferroelectrics and cuprates. We describe that the epitaxial strain and oxygen stoichiometry both lead to substantial changes in superconducting properties and these two factors are strongly coupled. And we demonstrate that the interface superconductivity is achieved in overdoped cuprates La1.6Sr0.4CuO4 [strontium doped lanthanum cuprate oxides] under field effect, with high Meissner volume and Tc [superconducting transition temperature] around 15K. Therefore, our results provide deep understanding on ferroelectric materials and open more opportunities for their application.