Design And Control Of Physical Properties At Non-isostructural Oxide Interfaces

The natural presence of broken inversion symmetry at complex oxide interfaces enables a plethora of intriguing phenomena such as spin-momentum locking,spin-orbit torque,and topologically nontrivial spin textures.The interplay among the charge,spin,lattice and orbital degrees of freedom in transition metal oxides gives rise to the abundant functionalities and consequently provides an exciting test-bed for studying the novel emergent phenomena,and in this way acts as a versatile platform to generate,control and detect spin currents or spin textures.With the rapid development of atomic-level control of heterointerfaces and the in-situ detection technology,the interface engineering in oxide heterostructures is no more confined to the conventional strain engineering.In fact,many approaches,including the defect engineering and the symmetry mismatch engineering,have been applied to the precise control of interfacial effects.Under these circumstances,it is particularly important to search strategies for constructing and designing oxide heterointerfaces,which is also the starting point and the core issue in this research.In this dissertation,we successfully induced a series of interface emergent phenomena through the non-isostructural complex oxide hetero-interface design,including the large perpendicular magnetic anisotropy,the interface structures-selective orbital reconstruction,the band order anomalous two-dimensional electron gas and the enhanced interfacial Dzyaloshinskii-Moriya interaction（DMI）.The main contents and conclusions are shown as below:1.Based on first-principles calculations,a perpendicular magnetic anisotropy up to 1.9×10⁷ erg/cm³ was obtained in a perovskite/brownmillerite-type La_0.5Sr_0.5MnO₃/LaCoO_2.5 superlattice and remained as large as 1.0×10⁷ erg/cm³ under a tensile strain of+4%.Further studies indicated that the metal-to-insulator transition at interfacial La_0.5Sr_0.5MnO₃ layer and the giant perpendicular magnetic anisotropy originated from the enhanced Jahn-Teller Q₃ mode at the oxygen tetrahedron/octahedron interface.By combining the projected density of states with the second-order perturbation theory,we identified the orbital pairs that strongly affect magnetic anisotropy and showed that adjusting orbital mixing was an effective approach to tune magnetic anisotropy.2.Based on first-principles calculations,we revealed that the valence,spin state and orbital occupancy of the interfacial atom could be effectively modulated by artificially modifying the interface structures in La Ni O₂/La MnO₃ superlattices.A relative orbital occupancy change ratio up to 15%（-21%）was observed at type A（B）interface with preferential occupation of d_{（3z）²-r²}((d_x²-y²) orbital,which is greater than those at perovskite/perovskite interfaces（<5%）.These results demonstrated the strong impact of oxygen coordination manipulation based on asymmetric interface design on orbital configurations and electronic properties,providing new approaches to explore emergent orbital-driven phenomena.3.Based on first-principles calculations,we predicted the presence of two-dimensional electron gas（2DEG）in Sr Cu O₂/SrTiO₃ heterostructures.Besides,we found that one-unit-cell-thick Sr Cu O₂ layer was enough to generate 2DEG and the calculated carrier density of SCO-capped 2DEG was three times larger than that in the corresponding LAO-capped 2DEG with the same film thickness.Moreover,the new Ti O₅ square pyramid at the interface dramatically modulated the crystal field splitting of Ti atom.In this way,the band order anomaly of interfacial conducting states emerged,adopting the d_xy<(d_3z²-r²<d_xz/yz band order rather than(9<(9_/in conventional LaAlO₃/SrTiO₃ systems.Similar interfacial conducting behaviors were also observed in Sr Cu O₂/KTa O₃ heterostructures.4.We predicted that the different interfacial configurations in non-isostructural Sr Cu O₂/Sr Ru O₃ heterostructures effectively tuned the valence state,spin state and magnetic anisotropy of interfacial Ru atom based on the first-principles calculations.The low oxygen vacancy formation energy ensured the experimental feasibility of different interface structures in Sr Cu O₂/Sr Ru O₃ heterostructures.Besides,a strong DMI of 3.5 me V/Ru and a D/J ratio of 0.63 were obtained at the Cu O₂-Sr-Ru O₂interface,being the biggest in the perovskite oxides family so far.The DMI is tunable,monotonically decreasing with the increase of the content of the apical oxygen ions in the interfacial layer,and attained the minimal value of 0.1 me V/Ru at the Cu O₂-Sr O-Ru O₂interface.Based on the model analysis,we concluded that the enhanced DMI originated from the enhanced interface symmetry breaking and the occupation of d_3z²-r² orbital at Cu O₂-Sr-Ru O₂ interface.