Physical Properties And Material Design Of Strongly Correlated Nickel-based Oxide Heterostructures

In recent years,there has been extensive research on nickel-based oxide heterostruc-tures,especially ultra-thin heterojunctions and superlattices.In this thesis,we designed nickelate RNiO3 or RNiO2(R=La,Nd)related oxide heterostructures,combined with the first-principles method and the maximum localized Wannier functions downfolding technique,studied their electronic structures and model Hamiltonians.The obtained results are compared with those of cuprate superconducting materials to find possible ways to realize high-temperature superconductivity of nickel-based oxides.In Chapter 1,we first introduced the structure of perovskite,oxygen octahedral rotation and d-orbital physics in perovskite;then we introduced the related knowledge of RNiO3 nickelate,including phase diagram,basic electronic configuration,metal-insulator transition and its regulation,and insulator-metal transition;then focusing on superconductivities and topological properties,the research progress of LaNiO3-based heterostructures is briefly described;finally,we briefly introduced the latest progress in the study of superconductivity of infinite layer Ni+oxide,from both experimental and theoretical aspects.In Chapter 2,we mainly introduced the first-principles calculation methods.First,starting from the energy band theory based on the single electron approximation,we summarized the free electron gas and nearly free electron gas models,the tight-binding model and the Wannier function,as well as the maximum localized Wannier function method;then we briefly described density functional theory,including Born-Oppenheimer approximation,Hartree-Fock approximation,Hohenberg-Kohn theorem,Kohn-Sham equation,and several commonly used exchange correlation functionals;then we briefly introduced the development of density functional theory including DFT+U model;finally,the calculation code and calculation method used in this thesis are briefly ex-plained.LaNiO3 is a very special material in the trivalent nickel family,because it is only member that exhibits paramagnetic metallic properties at any temperature.However,due to the inability to achieve large Ni-eg orbital polarization,LaNiO3-based super-conductivity has not yet been realized experimentally.Using the parameter-free den-sity functional approximation and the maximum localized Wannier function method,in Chapter 3,we studied in detail the orbital polarization and orbital engineering of the LaNiO3-based heterointerface system.The results have shown that even without in-troducing additional correlation effects,a single-occupied band structure and Fermi surface can be achieved,that is,a complete Ni-eg orbital polarization,which has important enlightening significance for the superconductivity research of Ni3+-based heterointerfaces.Then starting from the tight-binding model,we studied the origin of the completely polarized single-band Fermi surface and its similarities and differences with cuprate superconducting materials.The final magnetic state calculation results show a stable antiferromagnetic ground state.These results sheds light on a new direc-tion for the preparation of nickelate superconducting materials.In 2019,Hwang et al first discovered infinite layer Ni+-oxides superconductivity.Their experimental measurements reveal that a superconducting transition occurred at around 15 K.Theoretical calculations indicate that the complex energy band structure involving multiple orbits around the Fermi level may be one of the reasons for its low superconducting transition temperature.Therefore,in order to increase the supercon-ducting transition temperature of Ni+oxide,in Chapter 4,based on NdNiO2,we de-signed four Ni+-based heterostructures and carried out energy band design and orbital occupation research.The parameter-free first-principles calculation results show that the energy bands of the two heterostructures have typical single-band char--acteristics.Subsequently,we conducted a p-d model Hamiltonian study on these two materials and compared the obtained tight-binding parameters with those of cuprates.The final magnetic state calculation results show that their nearest magnetic exchange coupling constant J[?(EFM-EAFM)]is much higher than the infinite layer type NdNiO2,so they all have a relatively more stable antiferromagnetic ground state.Our research shows that these two heterostructures may be potential high-temperature superconduct-ing materials.In the last chapter,we summarize the main research conclusions.Our results show that several nickel-based heterostructures we designed may be potential superconduct-ing materials with higher transition temperatures.Meanwhile,we also look forward to subsequent research that can consider introducing more types of functionals and the influence of correlation effects on energy bands and Fermi surfaces.Experimental re-searchers can also try to synthesize the heterostructures we propose to further explore the possibility of them becoming superconducting materials.