The First Principles Study Of Perovskite Oxide Heterostructures

Since the perovskite oxides have the unique oxygen octahedra structure with great flexibility to hold a series of A and B ions with similar sizes, they exist extensively on the earth. Due to many degrees of freedom in these materials, such as lattice, orbital, spin, charge, and their sensitiveness to stress, composition, temperature and some else factors, there are various physical properties of technical importance in perovskite oxides, such as ferroelectricity, antiferroelectricity, piezoelectricity, metallic conductivity, insulating behaviour, ferromagnetism, antiferromagnetism etc. When two different bulks are fabricated to constitute a heterostructure, the degrees of freedom from two lattices would couple with each other, generating a variety of interesting phenomena on the interfaces, such as the two dimensional electron gas, high temperature superconductor, giant magnetoresistance, magnetoelectricity and so on. In this thesis, the density function theory is applied to study the properties of perovskite oxide heterostructure, and thus to design new materials and predict their new properties. This thesis is organized as follows:In the first chapter, the structure and classification of the ABO3-type perovskite are introduced first. Then their properties are described briefly. Finally, some novel phenomena observed on the interface of heterostructures are emphasized, which lay the grounds for the following investigation.In the second chapter, the theoretical basis of first-principles calculation is briefly introduced. Through the presentation of HF approximate, UK theory, HS equations and exchange density functions, the expressions of density function theory are obtained, which can be applied in the first principle calculations. Then the Berry Phase theory is simply introduced as well as VASP package.In the third chapter, PbTiO3 is taken as an example to study the polarization of perovskite oxide films strained by substrate. By using the first principle theory, the polarization of PbTiO3 films strained by the substrates of [001], [110] and [111] orientations are calculated. Moreover, two substrates of SrTiO3 and LaAlO3 are selected for comparison. The results show that it is the tetragonality but not the substrates the main factor to influence its polarization.In the fourth chapter, the properties of (YFeO3)n/(YTiO3)n superlattices are predicted. First the properties of two bulks are studied. The band alignment is used to analyze its density of electronic state, and results show that charge transfer would not occur in superlattices. Then the electron structure of (YFeO3)n/(YTiO3)n superlattices is calculated. Surprisingly, the charge transfer do occur both in n=1 and n=2 systems with one electron transferring from Ti’s 3d orbital to Fe’s 3d orbital, formed Ti4+-Fe2+. Second, the superlattices’sysmmetry is analyzed. The (YFeO3)i/(YTiO3)i superlattice is centrosymmetric while the (YFeO3)3/(YTiO3)2 superlattice is noncentrosymmetric. After the Berry Phase calculation, the results show that the polarization is zero in n=1 superlattice but ～1.01 μ.C/cm2 in n=2 superlattice, which is compatible with the theory analysis. Moreover, in the (YFeO3)2/(YTiO3)2 superlattice, by calculating the partial contribution to the total polarization, it predicts that the polarization may also persevere when the system is metallic.In the fifth chapter, our present work is summarized and the future work is proposed.