Low-Dimensional Construction And Emergent Transport Properties Study Of 5d Strong Spin-Orbital Coupling Perovskite Oxide SrIrO₃

The interaction and coupling between lattice,charge,spin,and orbital degrees of freedom in transition metal oxide thin films give rise to rich physical properties.As a prototypical 5d oxide,SrIrO₃（SIO）has a relatively weak electron-electron coupling（EEC,～0.5 e V）and strong spin-orbital coupling（SOC,～0.4 e V）in comparison with common 3d complex oxides.The interaction between EEC and SOC leads to rich physical phenomena,such as metal-insulator transition,paramagnetic-antiferromagnetic transition,and Mott-Slater crossover.The electron-electron interaction behaviors and resulted physical properties at 3d-5d interface are attractive.For instance,interfacial charge-transfer induced ferromagnetism,strong anomalous Hall Effect are being systematically studied.Besides,SIO has relatively strong spin Hall effect（SHE）and spin Hall angle(θ_SHE),which offers a promising future in oxide spintronics,like spin-orbit torque anisotropy induced by symmetry and magnetization switching induced by current and crystal orientation.However,the physical mechanisms of the interactions at the 3d-5d interface are still not clear,and there are still controversies and disagreements in certain aspects of the current research.In addition,the current means of regulating spin-charge conversion are based on magnetic materials,requiring the use of polarized photons or magnetic fields to manipulate the spin states.The generation,detection,and control of spin currents in non-magnetic materials are crucial for designing spintronics devices.Enriching and improving the basic research of SrIrO₃ films,thereby improving the efficiency of spin-charge conversion and developing new spin detection methods,is of great significance for promoting the application of oxide spintronics.Based on the above discussion,we carry out from the following three perspectives.Firstly,we systematically studied the growth condition of high-quality,atomically-sharped SIO（001）films grown in different substrates.We found that it is fully strain until 140-unit cell（u.c.）SIO on Sr TiO₃（STO）substrates,while the lattice structure of SIO changes from orthorhombic（20 u.c.）to monoclinic（140 u.c.）.For（La AlO₃）_0.3（Sr₂Al Ta O₆）_0.7（LSAT）substrates,SIO film is fully relaxed when thickness is higher than 80 u.c..Interestingly,when grown on Nd GaO₃（110）substrates,strain gradient would appear when SIO thickness is higher than 24 u.c.,which could be considered as continuously change of lattice constant with varied layers The in-plane（IP）lattice constant changes small while out-of-plane（OOP）lattice constant changes fast.The possible origin could be attributed to the mismatch of thermal coefficient.Through analysis,we believe strain gradient determines the continuously change of metal-nonmetal crossover（MNC）temperature(T_cross).Secondly,spin texture in momentum space is highly related with spin-charge conversion.We introduce the research progress with respect to spin texture in momentum space and emphasize topological spin textures and Rashba-type spin textures,which determines by inversion symmetry broken（ISB）and strong SOC.Afterwards,we systematically introduce a new paradigm for detecting spin textures named nonlinear magnetotransport effect（NLME）and emphasize the strength of mapping spin texture by giving examples.However,tuning spin textures at Fermi surfaces has not come true yet.We systematically studied the NLME of SIO films with or without strain gradient.We prove the existence of NLME due to inversion symmetry breaking at the interface in SIO without strain gradient.However,SIO films with strain gradient were demonstrated to show two groups of NLME.One of them is brought by interface symmetry broken while the other is determined by strain gradient.Through first-principles calculations,we conclude that strain gradient has a prominent role to tune the nontrivial spin textures at Fermi surface.Our work offers an example of mapping spin vector warping at Fermi contour,and proves that strain gradient can introduce inversion symmetry broken in centrosymmetric system and could be an efficient tuning method for nontrivial spin textures in momentum space.Besides,the NLME of 140 u.c.SIO on STO was studied.We found the second order rectification coefficient is one order of magnitude higher than that in SIO films grown on NGO substrates,mainly because of the change of lattice symmetry.Finally,we investigate the electrical transport properties of high-quality[（SrIrO₃）n/（Ca MnO₃）n]m superlattices and systematically reveal the interaction at 3d-5d interface.Besides,we illustrate the 3d-5d interaction and the anti-correlation character between electron-electron correlation（EEC,intrinsically 2D behavior）and SOC（intrinsically 3D behavior）,i.e.,enhanced EEC can suppress SOC and vice versa.3d electrons have high EEC but weak SOC while 5d electrons have high SOC but weak EEC.In 3d-5d superlattices without charge transfer,5d EEC would enhance at interfaces because of 3d-5d interaction.Since the SOC is intrinsically 3D,the interaction between SOC and EEC results in the EEC enhancement beyond the 2D limit.This indicates the enhanced 5d-EEC has a certain penetration depth in 5d-SrIrO₃.