PHASE-FIELD SIMULATIONS OF CONTROLLING AND DESIGNING FERROELECTRIC TOPOLOGICAL STRUCTURES AND HIDDEN PHASES IN SUPERLATTICE SYSTEMS
Restricted (Penn State Only)
- Author:
- Dai, Cheng
- Graduate Program:
- Materials Science and Engineering (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 14, 2022
- Committee Members:
- Venkatraman Gopalan, Major Field Member
Sulin Zhang, Outside Unit & Field Member
Ismaila Dabo, Major Field Member
Long-Qing Chen, Chair & Dissertation Advisor
John Mauro, Program Head/Chair - Keywords:
- Topological phase
Hidden phase
Superlattice
Design strategy
Control - Abstract:
- Artificial superlattices or multilayers with periodic stacking of different materials at the unit-cell level have attracted substantial interest in recent decades among the scientific community. One particular example is (PbTiO3)n/(SrTiO3)n superlattices, which recently were discovered to exhibit a number of novel physical properties such as negative capacitance and improper ferroelectricity and rich non-trivial topological phases including vortices, polar skyrmions, merons, and antivortices. It has recently been demonstrated that emergent metastable states or hidden phases (e.g., the supercrystal in (PbTiO3)n/(SrTiO3)n superlattices and antipolar phases in BiFeO3 layers) can be stabilized by employing different boundary conditions or revealed by applying external stimuli. They provide a new opportunity to explore and understand these transition states and phase diagrams with a wide range of potential applications. In this dissertation, I investigate the appearance of new hidden states under different mechanical and electric boundary conditions and explore the control of the topological phases or new novel microstructures via thermal, or optical, or electric stimuli. I show that the configuration of vortex-antivortex pairs recently discovered in PbTiO3/SrTiO3 superlattices can be ordered among nearby PbTiO3 layers along the stacking direction. This strain-driven order-disorder transition can be considered an electrostatic decoupling process among PbTiO3 layers. With a tensile substrate strain, a disordered vortex state (where the rotation direction of the vortex arrays in the neighboring ferroelectric layers is random) can be induced in PbTiO3/SrTiO3 superlattices. The ordering determines the type of vortex-antivortex pairs. We tune the pairs using an in-plane electric field. I also determine the antivortex and vortex stabilities of different states with respect to external electric fields. While many studies have explored the microstructures and properties of PbTiO3/SrTiO3 superlattices, a complete picture of how strain affects the topological phase transitions and ferroelectric domain structures has not yet been developed. Here, we build an isotropic strain-periodicity phase diagram of PbTiO3/SrTiO3 superlattices by employing phase-field simulations. I discover a new way to control the emergent skyrmion via tuning misfit strain. I also study the evolution of domain structures subject to anisotropic in-plane strains and identify vortex arrays along the [100]pc and [010]pc directions, as well as new labyrinth vortex arrays. This work offers a deeper understanding of this system and can serve as a guide for manipulating such emergent polar structures through strain engineering. Understanding the phase transitions and domain evolutions of mesoscale topological structures in ferroic materials is critical to realizing their potential applications in next-generation high-performance storage devices. We study the behaviors of a mesoscale supercrystal with three-dimensional nanoscale periodicity and rotational topology phases in a PbTiO3/SrTiO3 superlattice under thermal and electrical stimuli using a combination of phase-field simulations and X-ray diffraction experiments. I construct a phase diagram of temperature versus polar state, showing the formation of the supercrystal from a mixed vortex and a-twin state and a temperature-dependent erasure process of a supercrystal returning to a classical a-twin structure. Under an in-plane electric field bias at room temperature, the vortex topology of the supercrystal irreversibly transforms into a new type of stripe-like supercrystal. Under an out-of-plane electric field, the vortices inside the supercrystal undergo a topological phase transition to polar skyrmions. These results demonstrate the potential for the on-demand manipulation of polar topology and transformations in supercrystals using electric fields. The findings also provide a theoretical understanding that can be utilized to guide the design and control of mesoscale polar structures and to explore novel polar structures in other systems and their topological nature. Finally, antiferroelectric materials have attracted much attention due to their applications in a number of energy-efficient devices. However, only few antiferroelectric materials have been explored, and most involve lead. Accordingly, we introduce a new design strategy for the prediction of antiferroelectric materials employing interfacial electrostatic engineering. We begin with the well-known multiferroelectric material BiFeO3 with high polarization. By building up BiFeO3/dielectrics superlattices using density functional theory and phase-field simulations together with experimental observations, we show that a metastable antiferroelectric structure can be induced. At the same time, the misfit strain imposed by substrate provides another pathway for the design of antiferroelectric materials with various domain structures. This work offers a deeper understanding of this system and provides a guide for designing such emergent polar structures via strain, electrostatic, and size engineering.