Exploration Of Novel Physical Properties At The LaAlO₃/SrTiO₃ Heterointerface

With the rapid development of science and technology,the performance of elec-tronic devices based on conventional semiconductor has been approaching the theo-retical limit.The development of low-dimensional systems thus attracts great interest.Two-dimensional systems with a wealth of novel physical properties and high tunabil-ity have a great potential for developing novel electronic devices in the future.The transition-metal oxide heterostructure interface is one of typical two-dimensional sys-tems.Researches about the transition-metal oxide heterostructure interfaces have made great progress,since Ohtomo and Hwang found that a two-dimensional electron sys-tem(2DES)with high mobility can form at the interface of perovskite oxide insulators LaAlO3(LAO)and SrTiO3(STO)in 2004.The LAO/STO interface has extremely spectacular interesting physical properties,including the existence of a critical thick-ness of LAO to induce a conductive interface,nonmonotonically gate-tunable spin-orbit coupling(SOC)and superconductivity,the coexistence of superconductivity and mag-netism,and gate-tunable ferroelastic order.However,due to the complexity of this interface,origins of many properties are still unclear,and the possible correlation or interplay between different properties is also still elusive,and new properties remain to be exploited.In this work,we grew high-quality LAO/STO heterostructures using pulsed laser deposition.Then,metal/LAO/STO tunneling devices and Hall devices were prepared with the LAO/STO samples to deeply explore novel physical properties in the interfa-cial 2DES.Our works contain three main aspects as follows:First,in the Ti/LAO/STO tunneling junctions,we find three kinds of many-body effects,including superconduc-tivity,the Altshuler-Aronov effect,and the Kondo resonance.These many-body effects can be tuned effectively by temperature,magnetic field,and gate voltage.In particular,it is the first time to observe the Kondo resonance at a correlated oxide interface system.By quantitative analyses,we found that the Kondo coupling strength and the resonance lineshape parameter can be significantly tuned by gate voltage,and show an obvious critical phenomenon at the Lifshitz transition point of the 2DES.This unusual behavior can be attributed to orbital selectivity of the Kondo effect.Second,in the Nb/LAO/STO tunneling junctions,we observe two kinds of phase-coherence effects:Josephson ef-fect and phase-coherence multiple Andreev reflection-induced reflectionless tunneling effect.Temperature,magnetic field,and gate voltage can effectively realize a switch between the two phase-coherence effects.In particular,the strength of reflectionless tunneling shows a nonmonotonic dependence on the gate voltage,which may be linked to the nonmonotonic gate-dependence of Rashba SOC in the 2DES.SOC can suppress the interband scattering to extend the quasiparticle dephasing time,which can enhance the reflectionless tunneling effect.In addition,our theoretical calculation shows that SOC can have a significant effect on Andreev reflection in the in-plane superconduc-tor/2DES junctions.Third,in the Hall devices based on the LAO/STO,we observe an anomalous transverse voltage near the superconducting transition,which can be at-tributed to the guided vortex motion induced by the ferroelastic domain walls at the interface.Our findings suggest that the LAO/STO interface can provide an ideal platform for exploiting and manipulating diverse novel electronic phases,and some unique proper-ties at the interface can act as good tools to explore some basic physics.Therefore,the exploration and application of novel physical properties at the LAO/STO interface can provide a great opportunity for developing novel electronic devices.