First-principles Study On The Transition Metal Oxide Heterostructures

With the rapid progress in computational technique and theoretical method, first-principles calculation has become one of the most important scientific tools to interpret exotic features in materials and make functional design. The perovskite heterostructures have attracted a great amount of interests in last decade due to the rich physical functionalities such as ferroelectricity, magnetism, superconduc-tivity, multiferroic behavior and two dimensional gas(2DEG) at the interfaces. In this thesis, we use first-principles calculations to investigate the physical nature of the heterostructures and propose several models to make designs, including predictions of promising heterostructures for photocatalysis, strained ferroelectric devices to tune the 2DEG and possible superlattices for topological notrival Chern insulators.In chapter one, we give an introduction to the theoretical method for the electronic structure calculation, from Hartree-Fock theory to density functional theory, including the essential issue of the density functional theory:choice for the exchange correlation functional. In the end of this chapter, we briefly describe two widely used simulation packages.In chapter two, we briefly introduced the background knowledge of LaAlO3/SrTiO3(001) (LAO/STO(001)) interface. First, the 2DEG at the inter-face and its mechanisms have attracted the most attentions, we have short discus-sions for the three mechanisms:Polar catastrophe model, Oxygen defect model and Ionic intermixing model. Subsequently, the other interesting phenomenons such as ferromagnetism, superconductivity, and the coexistence of ferromagnetism and superconductivity are introduced.In the third chapter, we proposed based on first-principles density func-tional theory calculations that a nano-scale thin film based on a polar-nonpolar transition-metal oxide heterostructure can be used as a highly-efficient photocata-lyst. This is demonstrated using a STO/LAO/STO sandwich-like heterostructure with photocatalytic activity in the near-infrared region. The effect of the polar nature of LaAlO3 is two-fold. First, the induced electrostatic field accelerates the photo-generated electrons and holes into opposite directions and minimizes their recombination rates. Hence, the reduction and oxidation reactions can be insti-gated at the STO surfaces located on the opposite sides of the heterostructure. Second, the electric field reduces the band gap of the system making it photoactive in the infrared region. We also show that charge separation can be enhanced by using compressive strain engineering that creates ferroelectric instability in STO. The proposed setup is ideal for tandem oxide photocatalysts especially when com-bined with photoactive polar materials.In chapter four, we investigate the existence and stability of the 2DEG under the application of a biaxial strain on the LAO/STO(001) heterostructure. The compressive strain induces ferroelectric (FE) polarization in STO, which allows for the tunability of the 2DEG by reversing the STO polarization orientation. We show that the formation of the 2DEG is unstable when LAO and STO have the same polarization direction. On the other hand, the 2DEG will always form if the two polarizations are in the opposite directions regardless of the LAO thickness, which is in contrast to the unstrained interface that has a critical thickness for stabilizing the 2DEG. We show that the underpinnings of this behavior are due to charge passivation and band gap alignment.In chapter five, buckled honeycomb lattices (LXO)2/(LAO)4(111), (X= Transition Metal) are designed to explore the topological nature of the perovskite heterostructures. The discovery of quantum anomalous Hall (QAH) insulators, whose robust surface charge and spin currents were explained in terms of a bulk topological invariant known as the Chern number has attracted interest both as a novel electronic phase and for its anticipated applications in electronic devices. Honeycomb lattice systems such as we discuss here, formed by heavy transition-metal ions, have been proposed as topological insulators, but identifying a viable example has been limited by the complexity of the problem, appearing as an as-sortment of broken symmetry phases that must be studied and that sometimes thwart the topological character. The complexity has its positive aspect as well, providing many knobs to tune to obtain the desired properties. Here we use the flexibility of choice of spin-orbit and interaction strength, with tuning by strain, to design two Chern insulator systems with bandgaps up to 132 meV. Full structural relaxation is necessary, and the resulting lack of any symmetry whatsoever does not hamper the QAH character. Recent growth of insulating, magnetic phases in closely related materials supports the likelihood that synthesis and exploitation will follow.In the last chaper, a brief summary and outlook are given.