Date of Graduation

7-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Fu, Huaxiang

Committee Member

Bellaiche, Laurent

Second Committee Member

Naseem, Hameed A.

Third Committee Member

Salamo, Gregory J.

Fourth Committee Member

Vyas, Reeta

Keywords

Pure sciences; Applied sciences; Ferroelectric; Ferromagnetism; Phonon; Polarization; Srain; Vacancies

Abstract

The studies of ferroelectricity (FE) are of technological significance because of the multitude of applicable properties that ferroelectric materials exhibit. The mastery, and control of these properties necessitate the knowledge of the fundamental physics governing these insulating materials.

In this dissertation I present the results of first-principles investigations of the behavior of the fundamental ferroelectric properties under strain, and in the presence of vacancies. In the first part I introduce the important FE properties, their common behavior, and their numerous valuable applications. Following this background on FEs, a review of theoretical methods is presented with topics such as: Density Functional Theory (DFT), Pseudopotential method, Berry Phase Calculation and Density Functional Perturbation Theory (DFPT). Further, new theoretical approaches are introduced in this dissertation to enable the study of polarization for charged system.

In this work I report behaviors of polarization in rhombohedral (R3m) BaTiO3 (BTO) that do not conform with intuition, or the current state of known behavior of epitaxially strained BTO. These studies reveal a polarization that increases with tensile strain, instead of compressive strain, and a polarization that is anticorrelated with an elongation of the out-of-plane axis. Additionally, the studies indicate strain-driven phase transitions to R3c and Cm upon application of moderate epitaxial compressive (eta=-1.75%) and small tensile strain (eta=+0.375%), respectively. A simple physical explanation, which can be extended to FE materials of the same symmetry, is also provided for this unusual FE behavior.

I also report the studies on the evolution of phonon modes of vibration under strain in tetragonal (P4mm) BTO, revealing that careful analyses are necessary in the assignment of vibration modes in strained system due to different mode ordering between unstrained and strained systems. The splitting between Longitudinal Optical and Transverse Optical vibration mode is rigorously defined in this work, and shown to depend on mode mixing. The evolution of important quantities such as dielectric constant is also presented in this work.

Finally, the results of investigations on the influence of vacancies on ferroelectric and ferromagnetic properties will be presented in this dissertation. First, the studies of vacancy formation energy are highlighted, which shows the type and charge character of the vacancy that are most likely to occur under any given growth conditions. Afterward, I present the effect of vacancies on polarization and polarization switching in tetragonal BTO, demonstrating the relevance of polarization change in charged polar system, and proposing a method of calculating the polarization and an new polarization-switching pathway in FE BTO in the presence of charged vacancies. Then, I reveal the possibility of vacancy-induced ferromagnetism in BTO, and the microscopic origin of this ferromagnetism.

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