Interfacial Design And Properties Study Of Perovskite Superlattice

The electronic properties of strongly correlated materials have been a focus session in the field of condensed matter physics. Perovskite oxides provide an excellent platform for investigating the intriguing behavior of electrons, and therefore have become a hot topic in the field. Phenomena such as high-Tc superconductivity, p-wave superconductivity, colossal magneto resistivity and multiferroics, etc., have been found in this family. Even the trace of topological insulator can be found in perovskite materials. All these physical phenomena originate from the specific electronic structure in perovskite oxides. In-depth theoretical investigations are necessary for the comprehension of the physical picture underlying these phenomena, and help the design of practical devices.In this work, first-principles simulations are performed to investigate three different perovskite superlattices, all of which are based on the strontium titanate (SrTiO3). The electronic structures and physical properties, including band structure, magnetic properties and ferroelectricity, are investigated. An effective method to manipulate the magnetism in the material by the external strain is proposed theoretically and realized experimentally. The main results are listing as following.1. The superlattice composed by strontium ruthnate (SrRuO3) and strontium titanate (STO) has a ferromagnetic ground state even under the ultrathin limit (1:1). By applying external strain, the magnetic property can be controlled. As the tensile strain ranging from0to8%, the SRO/STO superlattice undergoes three phase transitions:The first two are structural phase transitions, induced by the rotational distortion of the oxygen octahedra; the last one is a Mott transition, which depends on the on-site Coulomb correlation of the Ru-4d orbital. The magnetic moment change with respect of applied strain is found in the experiment, which is quantitatively consistent with that in the theory.2. The interface of the lead titanate and strontium titanate superlattice (PbTiO3/STO) has been systematically studied. Interfaces of two types caused by the lacking of TiO2(Ruddlesden-Popper type) or the excessiveness of TiO2(Magneli type) both suppress the electric polarization of the material. An electrostatic model is established. The charge transfer at the interface is quantitatively computed. This interfacial charge is found to act as the depolarizing charge.3. In order to have deeper understanding of the growth of the lanthanum aluminate and strontium titanate interface (LaAlO3/STO), several intrinsic point defects on STO surface are examined. These defects include oxygen vacancy, substitution and anti-site defect. The phase diagram of the STO surface under different growing condition is provided. Furthermore, the LAO/STO heterointerfaces with these defects are investigated. The combined La and O vacancies on both sides of the LAO layer are found to effectively avoid dielectric breakdown of the LAO layer. This is due to the defect-induced internal electric field counteracts the intrinsic internal field in the LAO layer.