UC Berkeley

Topological structures in condensed matter are fascinating for fundamental research and applications due to their phase transitions with unique properties. Notably, the topological structures in ferroic materials can facilitate the miniaturization of electronic devices and could create future novel functional devices. Ferroelectricity and magnetism are two intriguing properties of ferroic materials. The switchable spontaneous properties of ferroic materials also play important roles in the topological structures. Ferromagnetic topological structures have been studied in-depth decades ago, and today the magnetic skyrmions have potential for racetrack memory in the future. As for the ferroelectrics, although two-dimensional topological structures such as domain walls are well investigated, scientists successfully predicted the three-dimensional topological structure in low-dimensional ferroelectrics a decade ago. This finding attracted significant attention and stimulated demand for more studies. Until recently, the manipulation of charge and lattice degrees of freedom in atomically precise and low-dimensional ferroelectric superlattices can stabilize exotic polar structures. For example, in PbTiO3/SrTiO3 superlattices, emergent polar toroidal phases such as the vortex phase can be produced; these phases exhibit phase competition and emergent chirality.

This dissertation studies the phase transition and intrinsic properties of polar vortices in artificial ferroelectric heterostructures and understand mechanisms underlying the property. In Chapter 1, the basic knowledge of ferromagnetic and ferroelectric topological structures is introduced and how domain structures are formed. In general, the multi-domains separated by domain walls would arrange the homogeneous polarization distribution in ferroic materials. Tuning the boundary conditions such as the mechanical and electrical boundary conditions, the inhomogeneous polarization distribution can be generated in low-dimensional ferroelectrics.

Recently, the discovery of polar vortices in PbTiO3/SrTiO3 superlattices as well as their phase coexistence and chirality indicate the challenges of the emergence, phase transition, and property of the inhomogeneous phase. However, the role of interfaces in evolving the vortex phase in these superlattices, the associated electrostatic and elastic boundary conditions they produce, and the chiral domains with different handedness in the ferroelectrics have remained unclear. Therefore, the dissertation investigates a toroidal phase in a SrTiO3/PbTiO3/SrTiO3 trilayer along with its phase transition and different chiral properties.

Chapter 2 describes the experimental methods used in this work. The trilayer heterostructure fabrication is achieved with pulsed laser deposition. Transmission electron microscopy and synchrotron X-rays are used as structural characterizations. Piezoresponse force microscopy is used to probe the polar structure. The chirality measurement is achieved with second harmonic generation in circular dichroism. The polarization distribution and mechanism are simulated with phase-field modeling.

Chapters 3 and 4 explains how the toroidal phase, which is arranged in arrays of alternating clockwise and counterclockwise polar vortices, arises from the effects of depolarization fields and tensile strain. The chapter observes how varying the thickness of the confined PbTiO3 layer results in the vortex phase emerging from the ferroelectric phase. Intriguingly, the origin of the vortex state only emerges at the head-to-head domain boundaries in ferroelectric a1/a2 twin structures. Further, by varying the total number of confined PbTiO3 layers that are moving from trilayers to superlattices, it is possible to manipulate long-range interactions among multiple confined PbTiO3 layers. This manipulation also has an impact on the stabilization of the vortex state and is key to the important role that elastic energy plays in mediating the formation of these structures. This approach offers a new understanding of how the different energies work together to produce this exciting new state of matter and contribute to the design of novel states and potential memory applications.

Chiral materials possess extraordinary right-or-left-handedness that can be chosen as the active object with different responses for broad applications in science. In ferroic materials, topological defects such as magnetic skyrmions that have chirality can be controlled by the electric field for future spintronic and memory applications. However, chiral domains with different handedness in the ferroelectrics have never been observed. Chapters 5 and 6 discuss how polar vortices as three-dimensional (3D) polarization textures exhibit collective behaviors of chiral domains in the confined ferroelectrics. The polar vortices are identified to possess antiparallel axial polarizations that perform microscopic helical rotation of electric polarization by probing the atomic displacement using atomic-resolution scanning transmission electron microscopy (STEM). Mirror symmetries are used to verify the handedness. As a result, left-and-right-handed chiral vortices and the achiral region are found as the gradient chirality. The competition between left-and-right-handed vortices arises at the achiral region, which forms the possible precursor of the skyrmions-type structure. Strikingly, this thesis uses circular dichroism measurement with second harmonic generation to confirm the left-and-right-handedness of helicity and achiral regions macroscopically. These multi-states that are under handedness manipulation by the electric field can open an energy-efficient route and have significant potential for information processing.