First-principle Studies Of Electronic Reconstruction At The Heterointerfaces Of Transition Metal Oxides

Due to the strong coupling between lattice, spin, charge and orbital degrees of freedom,transition metal oxides show many interesting physical and chemical properties, includingpiezoelectricity, pyroelectricity, ferroelectricity, high temperature superconductivity,colossal magnetoresistance effect, and magnetoelectric multiferroics. These uniqueproperties provide potential applications in morden electronics. Recently, owning to theadvances developed in epitaxial growth, transition metal oxide heterostructures haveattracted intense interest in condensed matter physics and materials science. By constructinga heterogeneous structure, novel properties, differing from their bulk components, appear.Interestingly, these properteis are gate-tunable, providing avenue towards functionalelectronic devices. In this paper, we propose and investigate three transition metal oxideheterostructures.First, we investigate the electronic and magnetic reconstructions at SrTiO3(111) and(110) surfaces. Itinerant ferromagnetism with high spin-polarization at Fermi level isrevealed at both SrTiO3(111) and (110) surfaces. The surface spin-polarization can reach ashigh as86%and97%for (111) with SrO3termination and (110) with SrTiO termination,respectively. The highly spin-polarized states arises from O2p orbitals and Ti3d states fortwo-dimensional electron gas and hole gas, respectively. In addition, the itinerantferromagnetism revealed at these polar surfaces could be described within the Stoner criteria,where large orbital occupation of surface Ti3d states or O2p states is found to be the origin.Further thermodynamic stabilities calculations show that they are experimentally accessibleundercertain certain deposition conditions, showing their potential applications inspintronics.Second, by replacing SrTiO3in the well-known heterostructure of LaAlO3/SrTiO3with the isostructure G-type antiferromagnet EuTiO3, we have achieved half-metallic two-dimensional hole gas at the p-type LaAlO3/EuTiO3heterointerface. Such a half-metallic state arises from large exchange splitting of almost localized Eu4f orbitals and the high density of states even at low concentration of carrier doping. On the other hand, n-type LaAlO3/EuTiO3heterointerface is normal ferromagnetics, where the bottom of Ti d band is partially occupied with a negatively spin-polarization driven by the strong hybridization between localized Eu4f states and itinerant Ti3d electrons. Interestingly, when n-type and p-type heterointerfaces appear simultaneously, a magnetic field driven insulator-to-metal transition accompanied with colossal magnetoresistance effect is found. In addition, the unusual band structure with spin separation between electron and hole may provide a real material system to realize emergent phenomena based on spin-dependent exciton condensation and finally dissipationless spin transport.Last, we have studied electronic and magnetic reconstructions at the polar surface of infinite layered systems of SrFeO2. It is revealed that the electronic reconstruction of surface Fe ions is the origin for the surface magnetic reconstruction, where for bulk SrFeO2, the electronic state of Fe ions is (dz2)2(dxzdyz)2(dxy)1(dx2-y2)1,while for the surface Fe ions it is (dz2)1(dxzdyz)2(dxy)1(dx2-y2)1,The inter-layer magnetic exchange couplings via(dxzdyz)2,(dxy)1,(dx2-y2)1orbits are unchanged with antiferromagnetic ordering developed. Along z direction, exchange coupling between surface Fe (dz2)1state and sub-surface Fe (dz2)2state prefers to ferromagnetic coupling. Therefore, the system exhibits surface spin-flip antiferromagnetic ordering. For the (001) surface of isotructure CaCuO2, spin-flip ferromagnetic ordering is revaled.