Metal-insulator Transition And Electron Transport In Perovskite Oxide Heterostructures

Owning to the multi-coupling between the degrees of freedom at the interfaces and the broad compatibility of the oxygen sublattices of dissimilar materials,perovskite ox-ide heterostructures have been shown to exhibit novel physics that is absent in bulk constituents.This feature makes perovskite oxide heterostructures hold promise of the future electronics and spintronics applications.Among these materials,LaAlO₃/SrTiO₃ interfaces have attracted lots of interests since the discovery of two-dimensional electron gas at the interfaces,and have been confirmed to display superconductivity,magnetism,and tunable Rashba spin orbit coupling.Although it has been extensively studied more than ten years,the origin of two-dimensional electron gas at this interface is still under debate.Paralleling to the study of LaAlO₃/SrTiO₃ interfaces,a new subject named mul-tiferroics has been established since the preparation of BiFeO₃/SrTiO₃ thin films.This subject renews the study in ferroelectric tunnel junctions and extends the spintronics into perovskite oxides.Benefited from the interdisciplinary research,the novel multi-ferroic tunneling junctions have been proposed as multifunctional systems to support the multi-state storage of information,however,the practical applications are still far less than our expectations.In this Doctorate Dissertation,on the basis of reviewing the recently reported ad-vances in the above two aspects,we make an in-depth understating of the possible fac-tors that contribute to the two-dimensional electron gas at LaAlO₃/SrTiO₃ interfaces,and develop a strain engineering method to dynamically modulate the two-dimensional conductivity at LaAlO₃/SrTiO₃ interfaces.Utilizing the confining effect of LaAlO₃ and SrTiO₃,transition metal oxide monolayer is designed through interface termination engineering.In addition to the previous study,a novel multiferroic tunnel junction de-vice is developed to exhibit robust tunneling magnetoresistance effect and tunnelling electroresistance effect.Specifically,we firstly study the two-dimensional electron gas at?110?LaAlO₃/SrTiO₃ interfaces with a critical thickness of four layers,and find the band bending at the interfaces resulted from the coexistence of La and Ti is a key to two-dimensional electron gas.The band bending is also found at the?001?LaAlO₃/SrTiO₃ interface.Driven by our first-principles predictions and experimental evidences,we develop an electrostatic field model to explain the band bending and propose a way to bridge the gap between polar discontinuity mechanism,defect-induced mechanism,and cation mixing mechanism.Secondly,we design a magnetostrictive material-based LaAlO₃/SrTiO₃ interface that uniaxial strain can be applied to?001?LaAlO₃/SrTiO₃ interface through direct current magnetic field.Our first-principles calculations show that the uniaxial com-pressive strain can enhance the interfacial conductivity while the uniaxial tensile strain tends to decrease the conductivity.Our experimental measurements are in good agree-ment with the results.Furthermore,LaAlO₃/SrTiO₃ interface under the uniaxial strain shows anisotropic transport properties,and these transport properties are sensitive to the type of applied uniaxial strains.In other words,we reach a dynamical way through con-trolling the magnetic field to modulate the anisotropic interfacial conductivity at?001?LaAlO₃/SrTiO₃ interfaces.Thirdly,confining effect at LaAlO₃ and SrTiO₃ interfaces is shown to be a new way to design transition metal oxide monolayers with novel properties.Take MnO2 mono-layer sandwiched at the perovskite oxide interfaces for instance,we find proper interface engineering can give rise to the reconstruction of electrostatic field in heterostructurevs,and consequently introduces unusual metal-insulator transition and magnetic transition at the monolayer scale.Unlike?001?LaAlO₃/SrTiO₃ and?110?LaAlO₃/SrTiO₃ in-terfaces in which critical thickness is required to achieve the two-dimensional electron gas,the highly spin-polarized two-dimensional electron gas in monolayer MnO2 is in-dependent on the thickness of capping layers.In the last research part of this Dissertation,we report the four resistance states in multiferroic tunnel junction made of "tricolor" ferroelectric barrier and SrRuO₃ electrodes.The "tricolor" ferroelectric barrier is actually the stacking structure of three perovskite oxides,i.e.SrTiO₃,BaTiO₃,and CaTiO₃.This three-component com-pound features intrinsic asymmetric ferroelectricity,and its ferroelectricity can survive at ferroelectric/metal interface.Using "tricolor" as the barrier,the multiferroic tun-nel junctions with symmetric electrodes show four well-defined resistance states.The robust tunneling electroresistance effect and tunneling magnetoresistance effect are as competitive as those in the widely studied asymmetric multiferroic tunnel junctions.