First-principles Studies Of Electronic Structures And Magnetic Properties At Several Pervoskite Hetero-interfaces

With the advances in thin film growth technology, high-quality perovskite epitaxial heterostructures with atomically sharp interfaces can now be achieved by pulsed laser deposition and molecular beam epitaxy. The control of interface at the atomic level has led to novel interfacial properpitaxy. The control of interface is not expected in their parent compounds. This opens possibilities for future applications in oxide-based electronics and spintronics. In particular, over the decade, polar/nonpolar interfaces have become one of the focuses in condensed matter physics and materials science research. A characteristic feature is the polar discontinuity at the interface which may result in the reconstruction of the charge, spin, orbital and lattice degrees of freedom across the hetero-interfaces. One of the most intriguing epitaxial oxide heterostructures to date is LaAlO3/SrTiO3, which exhibits a nontrivial two-dimensional electron gas (2DEG) at the interface, with exotic magnetic and even superconductivity properties. Despite extensive efforts in both theory and experiment, there is no consensus regarding the dominant mechanism responsible for these interface properties in this system. In this respect, it is helpful to study other similar perovskite epitaxial hetero-interfaces to understand the origin of the interface phenomena. Predicting and engineering novel interface properties that can be exploited in novel all-oxide electronic devices are expected.Motivated by this, the structural and electronic properties of several pervoskite hetero-intefaces were studied using first-principles calculations based on density functional theory in this thesis. Main achievements are listed as follows.1. The electronic structures of (001) epitaxial LaGaO3/SrTiO3(LGO/STO) heterostructures were calculated. The effects of ionic relaxation on electronic characteristics of both n-type (LaO)+/(TiO2)0and p-type (GaO2)-/(SrO)0interfaces are investigated. For the n-type LGO/STO interface, all of the studied interfaces are metallic before ionic relaxations. A insulator-metal transition occurs when the thickness of LaGaO3exceeds a critical value of6unit cells after ionic relaxations. Whereas for p-type LGO/STO interface, they are metallic before atomic relaxation and become insulating after atomic relaxation. This indicates that the polar distortions due to ionic relaxations partly delay the onset of the electronic reconstruction. The asymmetry between n-type and p-type interface can be ascribed to the difference of their interfacial geometry and effects of polar distortions. On the one hand, different interfacial atomic arrangement leads to different band alignment, yielding different electronic reconstruction:charge transfer is more difficult to occur for the p-type interface than the n-type interface. On the other hand, stronger polar distortions for the p-type interface further prevent the appearance of charge transfer.2. Electronic and magnetic structures of LaAlO3/SrMnO3(LAO/SMO) heterostructures were studied. Two-dimensional electron gas is predicted to occur at the interface of the two band insulators, due to electron transfer driven by the interface polar discontinuity. The transferred electrons occupy the Mn eg orbitals of the SMO layers close to the LAO/SMO interface. The SMO layers show different types of magnetic orders due to the electrostatic electron doping into Mn eg orbitals. The modulation on the magnetic structure in interfacial SMO is studied from two aspects. Firstly, the magnetic structures of interfacial SMO monolayers can be controlled by the LAO overlayer thickness. Electrons transferred into SMO are restricted in the monolayer closest to the interface in (LAO)1/(SMO)5, but may penetrate into two SMO monolayers in (LAO)5/(SMO)5. The preferential occupation of the x2-y2orbitals facilitates an in-plane ferromagnetic alignment of Mn spins, leading to a half-metallic spin polarized2DEG in (LAO)i/(SMO)5, while the Mn spins in the two monolayers adjacent to the (LAO)5/(SMO)5interface are aligned in an A-type antiferromagnetic manner. Secondly, the magnetic order of interfacial SMO layers can be controlled by the in-plane strain which can modulate the occupation of Mn eg orbitals. The results show that a tensile strain stabilizes the A-type antiferromagnetic order and a compressive strain stabilizes the C-type antiferromagnetic order at the interfacial SMO layers in (LAO)5/(SMO)5.3. Electronic and magnetic structures of LaMnO3/SrTiO3(LMO/STO) heterostructures with LaO/TiO2interface were studied. The results show that the LMO layer in all heterostructures favors ferromagnetic state. When the thickness of LMO layer is up to6unit cells, electrons transfer from LMO to STO layer. The transferred electrons occupy the conduction band of STO, mainly composed of Ti3d orbitals. The polar distortions due to ionic relaxation decrease with increasing LMO film thickness. This results from the competition between elastic strain energy, electrostatic energy, and electronic reconstructions. In addition, the electrostriction observed in LMO layers directly indicates the presence of the built-in electric field in the polar LMO layer.