Date of Graduation
Doctor of Philosophy in Physics (PhD)
Second Committee Member
Third Committee Member
Fourth Committee Member
Ferroelectrics (FE) and multiferroics (MFE) have attracted a lot of attentions due to their rich and novel properties. Studies towards FE and MFE are of both fundamental and technological importance. We use a ﬁrst-principles-based effective Hamiltonian method, conventional ab-initio packages and linear-scale three-dimension fragment method to investigate several important issues about FE and MFE. Tuning the properties of FE and MFE ﬁlms are essential for miniaturized device applications, which can be realized through epitaxial strain and growth direction. In this dissertation, we use the effective Hamiltonian method to study (i) BaTiO 3 ﬁlms grown along the (110) pseudocubic direction on various substrates, (ii) BaTiO 3 ﬁlms grown on a single substrate along directions varying from  to  via  pseudocubic direction. Optimized physical responses or curie temperatures are found along some special directions or under epitaxial strain of certain range. FE and MFE nanostructures are shown to possess electrical vortices (known as one type topological defect), which have the potential to be used in new memory devices. However, the dynamic mechanism behind them is barely known. We use the effective Hamiltonian method to reveal that there exists a distinct mode which is shown to be responsible for the formation of the electrical vortices and in the THz region. Spin-canted magnetic structures are commonly seen in MFE, which results in the coexistence of two or more magnetic order parameters in the same structure. Understanding the physics behind such coupled magnetic order parameters is of obvious beneﬁt for the sake of control of the magnetic properties of such systems. We employ both the effective Hamiltonian and ab-initio methods to derive and prove there is a universal law that explicitly correlates various magnetic order parameters with the different types of oxygen octahedra rotations. FE or MFE possessing electrical vortices are experimentally shown to have a much lower critical voltage in current-voltage curves. However, the exact underlying reason is unknown. In this dissertation, we take the advantage of the effective Hamiltonian method and linear-scale three-dimension fragment method to study the electronic properties of electrical vortices. Such combined procedure clearly shows the existence of electrical vortices doesn’t decrease the band gap, but increases it instead, which suggests the lower critical voltage in current-voltage curvesis likely to result from the defects inside the vortices.
Gui, Zhigang, "Static and Dynamical Properties of Ferroelectrics and Related Material in Bulk and Nanostructure Forms" (2015). Theses and Dissertations. 1241.