| Perovskite oxides have attracted considerable attention due to their plenty physical properties.From the microscopic viewpoint,perovskite oxides have many degrees of freedom(e.g.,lattice,charge,spin,and orbital).The couplings and competitions among these degrees of freedom can lead to numerous novel phenomena,such as electronic phase separation,charge transfer,and orbital ordering.From the macroscopic viewpoint,perovskite oxides have many intriguing physical properties,e.g.,high-temperature superconductivity,colossal magnetoresistance,and multiferroicity,which play important roles in the development of quantum devices.More interestingly,if two different materials are further coupled with each other,richer physical phenomena and more controllable performance,and even some new fascinating behaviors which are absent in parent compounds,will be presented through the interfacial lattice and electronic reconstructions.Therefore,it is crucial for both fundamental researches and practical applications to fabricate controllable high quality perovskite oxide heterostructures.To achieve this goal,it is necessary to deep understand the interfacial mechanisms of perovskite oxide heterostructures.In this dissertation,we employed density functional theory(DFT)to investigate the properties of perovskite oxide superlattices,including magnetism,ferroelectricity,magnetoelectric coupling,and topological phase.The whole thesis is organized as follow:In the first chapter,the research background is introduced.First,the basic physical behaviors of perovskite oxides are outlined.Then the physical properties of superlattices are overviewed briefly,including magnetism,ferroelectricity,magnetoelectric coupling,and topological phase.In the second chapter,the theoretical basic knowledge and calculation methods are introduced.First,the approximations of first-principles methods are discussed.Then the developments and relevant knowledge of density functional theory are described.Finally,the modern polarization theory is simply introduced,as well as VASP package.In the third chapter,the magnetism and electronic structures of LaTiO3 films are studied.First,the magnetic orders of LaTiO3 films under epitaxial strain are calculated.Two new magnetic phases are predicted,which enrich the magnetic phase diagram of the RTiO3 family.Then,the structure is extended to the[001]-and[111]-oriented superlattices.The electronic structures are significantly changed.In the fourth chapter,the physical properties of(LaTiO3)n/(LaVO3)n superlattices are studied.First,the[001]-,[110]-and[111]-oriented n=1 superlattices are simulated to address the interfacial lattice reconstruction and electronic reconstruction.The unconventional charge transfer and orbital ordering related metal-insulator transition in the superlattices are well explained.Then the[001]-oriented n=2 superlattice is studied.A way is predicted to achieve the coexistence between polar structure and metallic behavior.In the fifth chapter,the topological magnetic phase in LaMnO3(111)bilayer is studied.First,the first-principles calculation is used to investigate the electronic structures.The Dirac cone can be opened by the spin-orbit coupling.Then,the tight-binding model is calculated to clarify the topological properties.The effects of lattice distortion on the topological magnetic phase are studied in detail.In the sixth chapter,the magnetoelectric coupling in perovskite oxide superlattices is studied.First,a new mechanism for magnetoelectric coupling is proposed based on the carrier-mediated field effect.Then,taking the BiFeO3/SrTiO3 superlattice as an example,the sign of magnetization is turned accompanying the flipping of polarization at room temperature by using first-principles calculations and Monte Carlo simulation.A new route is provided to explore magnetoelectric coupling at room temperature.The seventh chapter is devoted to the conclusion and perspective. |
PDF Full Text Request