UC Berkeley

Central to materials science is the concept that functional properties are derived from structure. By understanding the structure of a material system from the atomic scale to the mesoscale we can understand the emergence of novel phenomena and further tailor these properties for practical applications. Here, I focus on the structure-property relations of ferroelectric vortices formed in superlattices PbTiO3/SrTiO3 with layer thicknesses of only about 20 unit cells. These polar vortices represent the first discovery of a ferroelectric topological texture and were only discovered less than a decade ago. While this discovery has led to an explosion of research on the vortices, there has been a significant lack of work relating to the finer details of the polar vortices. Therefore, in my work, I set about understanding the vortex structure and ordering at various length scales to further understand ferroelectricity and chirality within the system. I first examine the atomic scale structure from the shape of an individual to a vortex to their formation of a one-dimensional lattice. From there I examine domain formation within the system in particular the various domain variants and domain walls present. By understanding the vortex ordering and domain formation I then demonstrate the existence of macroscopic ferroelectricity within the system by scanning probe measurements, electrical characterization, and non-linear optical techniques. I demonstrate how vortex off-centering leads to ferroelectric behavior with multiple switching events that stem from the out-of-plane movement of the vortices. These switching events are then harnessed to demonstrate the non-destructive readout of the ferroelectric state. I then examine ferroelectricity along the axis of the vortices, revealing ferroelectric behavior remarkably different from the ferroelectricity referenced earlier. Despite the fact that both polarization components stem from the same vortex off-centering, I demonstrate the difference in their switching dynamics is due to unconstrained vs. constrained nucleation and growth. Furthermore, I demonstrate that the slower kinetics from constrained growth can be harnessed for a multi-state or analog ferroelectric memory device. Finally, I delve into how the vortex structure also displays structural chirality that is derived from the structure of an individual vortex alongside a secondary chirality originating from the vortex lattice configuration. Overall, my dissertation provides a new, in-depth understanding of the vortex system and demonstrates how their organization leads to emergent ferroelectricity, chirality, collective dynamics, and more.