Transport Properties And Microstructure Strain Control Of Epitaxial Ba₂IrO₄ Thin Films

Since the last century,high-temperature superconducting materials have received widespread attention as materials with great application prospects in the future.However,the current physical mechanism of high-temperature superconductivity is still unclear and needs further study.The 5d transition metal iridate oxides has many novel physical properties due to its d-orbital electrons under the combined action of the strong spin-orbit coupling effect and the electron correlation effect.It is expected to achieve high-temperature superconductivity through electronic doping.Cuperate high-temperature superconductivity is based on the 3d orbital electrons of copper atoms,so the exploration of 5d orbital electrons is of great significance to the study of the mechanism of high-temperature superconductivity.At present,most researches on 5d iridate oxides have focused on antiferromagnetic Mott insulator Sr₂IrO₄.In Sr₂IrO₄,Fermi arc and V-shaped energy gap have been observed in Sr₂IrO₄,which are similar to high-temperature superconducting copper oxide.However,experimentally,there are no two direct evidences of high-temperature superconductivity observed in Sr₂IrO₄-zero resistance and complete diamagnetism.This may be because the rotation of the IrO₆ oxygen octahedron in Sr₂IrO₄ leads to weaker ferromagnetism in the IrO₂ plane,which inhibits the formation of electronic Cooper pairs,so that high-temperature superconducting states have not been found in this system.Compared with Sr₂IrO₄,Ba₂IrO₄ is structurally more similar to La₂Cu O₄.And there is no IrO₆ octahedral rotation in Ba₂IrO₄,and the IrO₂ surface is an ideal antiferromagnetic surface.Therefore,the potential high-temperature superconductivity in the Ba₂IrO₄ system is worthy of further exploration.However,since Ba₂IrO₄ is a metastable phase,the bulk body usually needs to be synthesized under high pressure conditions.The growth and preparation of single crystal films are difficult,and the film quality is very susceptible to growth conditions.It is also difficult to achieve effective carrier doping in Ba₂IrO₄,and the doped samples also need to be under high pressure to achieve the metal to insulator transition.All these have brought great difficulties to the research,making the current research on the Ba₂IrO₄ system still relatively small.The quality of the Ba₂IrO₄ film is very important for the study of the potential high-temperature superconductivity of the system.In the research on Ba₂IrO₄ thin film,the growth quality of thin film is optimized by controlling the growth parameters.The transport properties of the thin film were controlled by the method of epitaxial strain and oxygen vacancy doping,and the polarization phenomenon of thin film under strain control are observed.the main work and contributions of this thesis are as follows:1)The Ba₂IrO₄ film was grown and prepared by molecular beam epitaxy,and the quality of the epitaxial film was further improved by optimizing the growth temperature and growth oxygen pressure.Experiments show that the quality of the high-quality Ba₂IrO₄ film is basically unchanged after being stored in a vacuum-sealed and dry environment for two months.This allows us to further study the potential high-temperature superconductivity of the Ba₂IrO₄ system and have a good foundation for conducting a series of tests to characterize the film.2)The transport properties of Ba₂IrO₄ films are controlled by epitaxial strains and Ca H₂ annealing doped with oxygen vacancies.Experiments show that the Ba₂IrO₄ film epitaxially grown on different substrates,with the increase of in-plane compressive strains,the conductive properties of the film gradually increase;we initially explored the Ca H₂ annealing parameters of the Ba₂IrO₄ film,and the experimental results show that annealing at a suitable temperature for a certain time,the resistivity of the film is significantly reduced.3)The microstructure of Ba₂IrO₄ film was characterized by scanning transmission electron microscope.And it is found that the crystal structure of the Ba₂IrO₄ film grown on the Sr Ti O₃ substrate has a certain distortion,and the Ir atoms in the film have a certain offset relative to the center position in the bulk Ba₂IrO₄.Under the larger lattice mismatch,this kind polarization phenomenon of Ba₂IrO₄ caused by atomic displacement is worthy of our further investigation.