Date of Graduation

5-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Laurent Bellaiche

Committee Member

Huaxiang Fu

Second Committee Member

Salvador Barraza-Lopez

Third Committee Member

Mark Arnold

Keywords

ferroelectricity, linear-scaling, negative capacitance, optical activity, oxide heterostructure, topological defects

Abstract

Ferroelectric (FE) nanostructures have attracted considerable attention as our abilities improve to synthesize them and to predict their properties by theoretical means. Depolarizing field effects at interfaces of FE heterostructures are particularly notable for causing topological defects such as FE vortices and negative dielectric responses in superlattices. In this thesis, I employ two large-scale atomistic techniques, the first-principles-based effective Hamiltonian (HEff) method and the linear-scaling three-dimensional fragment (LS3DF) method. I use these methods to explore optical rotation in FE vortices, electro-optic effects in FE vortices and skyrmions, and voltage amplification via negative capacitance in ferroelectric-paraelectric superlattices. We employ HEff in Monte Carlo and molecular dynamics schemes to maximize spontaneous optical rotation in a BaTiO$_3$/SrTiO$_3$ nanocomposite. For a small bias field, maximal optical rotation was realized at room temperature. The result has acquired greater relevance since Ramesh and coworkers observed ``emergent chirality" in FE vortex arrays in PbTiO$_3$/SrTiO$_3$ superlattices. In a similar nanocomposite as above, we use the combined HEff and LS3DF method to study how band gap and band alignment evolves along the path from a polar-toroidal to an electrical skyrmion state. Temperature control of the vortex provides substantially larger range of control of bandgap and band alignment than field control of the skyrmion. Using temperature and electric fields to manipulate polarization and bond angle distortion in both constituent materials provides an additional handle for bandgap engineering in such nanostructures.

We then use HEff to study BaTiO$_3$/SrTiO$_3$ superlattices as a platform for negative differential capacitance. We implement an atomistic framework amenable to simulation of negative capacitance in strained superlattices. In these systems, we predict misfit epitaxial strain control allows for broadly extending the operable temperature range for negative. By manipulating this strain, we observed switching of negative capacitance behavior between both constituent materials of the superlattice at low temperature.

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