Study Of 4d/5d Transition Iridium And Ruthenium Oxides Under High-pressure

4d/5d transition iridium/ruthenium oxides exhibit strong spin-orbit coupling,electronic Coulomb repulsion effect,and crystal field interactions,which cooperate and compete with each other,forming a wealth of novel physical properties,such as the possible spin three-state superconductivity in Sr2RuO4,the Mott insulating state with Jeff=1/2 in Sr2IrO4,the possible quantum spin liquid phenomenon in Na2IrO3 and Li2IrO3,metamagnetic transition of Sr3Ru2O7 under a magnetic field and the accompanying quantum criticality and electronic nematic phase,and possible Weyl semi-metals in pyrochlore iridium oxide,Axion insulators,etc.Pressure is a thermodynamic parameter that can directly act on the degrees of freedom of the lattice,thereby indirectly controlling the degrees of freedom of electric carriers,charges,spin,and topology.It is a clean and effective control method to explore new physical properties and reveal the microscopic mechanism of interactions,especially for iridium/ruthenium oxides.There have been some progresses on the studies of iridium/ruthenium oxides under pressure,while still much unsolved issues such as the metallic state and possible superconductivity in Sr2IrO4.Therefore,we have studied the transport and structural properties of Sr2IrO4,La3Ir3O11,Sr3Ru2O7,Ca3Ru2O7 under high pressure using the diamond anvil cell technology through electrical transport,X-ray diffraction,and Raman scattering experiments.The main conclusions are as follows:(1)Persistent insulating state at Mbar in strongly spin-orbit coupled Sr2IrO4.The results of Sr2IrO4 show that there is a direct and close relationship between the structural and transport properties.At 40.6 GPa,we observe a structural phase transition from the tetragonal phase 141/acdto the orthogonal phase Pbca.Almost at the same time,the insulating ground state of Sr2IrO4 begins to strengthen,forming a "U"-shaped resistance-pressure curve below 50 GPa.A stable insulating state whose resistance value is almost unchanged comes into being as pressure further increased from 70 to 185 GPa.After comparing the abnormal behavior of R and XRD data,we believe that with the appearance of the orthogonal phase,the electronic transition between Ir and Ir is weakened,thereby producing a unique stable insulating state in Sr2IrO4.(2)Pressure-induced anomalous insulating behavior in frustrated iridate La3Ir3O11.The electrical transport results of La3Ir3O11 uncover a semimetal-insulating behavior transition at?38.7 GPa.XRD results show that although the original cubic KSbO3 structure has always existed stably to 73.1 GPa,its bulk elastic modulus and the bond length between Ir atoms in the dimer Ir2O10 have a slope around 38.7 GPa.Consistent with this,a corresponding abnormal pressure interval was also observed near Pc on the Raman spectrum.These show that the insulation behavior under high pressure is closely related to structural distortion.(3)Pressure-induced structural and metal-insulator transition in Sr3Ru2O7.We have carried out related researches of the crystal structure and electronic structure on single crystal Sr3Ru2O7 through high-pressure XRD,Raman scattering and electrical transport experiments.The results show that Sr3Ru2O7 undergoes a structural phase transition from the orthogonal phase Bbcb to the monoclinic phase P2 at?46 GPa,which has lower lattice symmetry and severer local distortion of the RuO6 octahedron.As a result,Sr3Ru2O7 is getting more insulating under pressure.In addition,the softening of M1 and disappearance of M3-6 observed on Raman spectra may represent the decrease of octahedral rotation and the changes of magnetic properties.(3)Pressure-induced stable insulating behavior in Ca3Ru2O7.The electrical transport results of Ca3Ru2O7 exhibits a stable insulation behavior upon 60 GPa accompanied by some new vibration modes in the Raman scattering spectrum.In addition,we also observed the evolutionary anomalies corresponding to the lattice distortion from the XRD results accompanied by the extreme resistance values.We believe that the abnormal insulating behavior may arise from the possible new phase although no signs of structural phase change were observed due to the low pressure of our XRD experiment.In summary,we believe that due to the existence of strong spin-orbit coupling,the physical properties of iridium and ruthenium oxides are significantly affected by the octahedral distortion.Our research results emphasize that in the iridium/ruthenium oxides,the octahedral distortion seriously affects its physical properties.This has played a certain role in understanding the micro-mechanisms of cooperating and competing modes between multiple interactions,further exploring novel properties,and establishing a corresponding theoretical framework.