Study On Superconducting Transport Based On Oxide Interfaces

In 2004,Ohtomo and Hwang discovered a conducting two-dimensional electronic system(2DES)at the interface of two oxide insulators,SrTiO3 and LaAlO3,which led to a continuous and extensive study of oxide interfaces.In particular,the interfacial 2DES exhibits rich and novel physical properties,including nonmonotonically gatetunable Rashba spin-orbit coupling(SOC)and superconductivity,and the coexistence of magnetism and superconductivity.The competition and coupling between multiple degrees of freedom and ordered states make oxide interfaces an ideal platform for exploring unconventional superconductivity.In addition,the rich and controllable physical effects at the interface also provide a new opportunity for the realization of novel superconducting transport through the construction of heterostructures.In this thesis,superconducting transport studies are performed based on two typical oxide interfaces,LaAlO3/SrTiO3 and LaAlO3/KTaO3.We grew high-quality LaAlO3/SrTiO3(001)and LaAlO3/KTaO3(111)samples,and used them to fabricate Hall devices and metal/superconductor tunneling devices,to explore the superconductivity at the oxide interface and the superconducting transport in the tunnel junction.The main results obtained are as follows:(1)Study on zero-bias conductance peaks(ZBCPs)based on LaAlO3/SrTiO3We report observations of pronounced ZBCPs in hybrid junctions composed of an oxide heterostructure LAO/STO and an elemental superconductor Nb.Remarkably,the ZBCPs exhibit a domelike dependence on the gate voltage,which correlates strongly with the nonmonotonic gate dependence of the Rashba SOC in the 2DES.The origin of the observed ZBCPs can be attributed to the reflectionless tunneling effect,and their gate dependence can be explained by the enhanced quantum coherence time of electrons in the 2DES with increased momentum separation due to SOC.We further demonstrate theoretically that,in the presence of a substantial proximity effect,the Rashba SOC can directly enhance the overall Andreev conductance in the 2DES/superconductor junctions.Our results have not only discovered a nontrivial connection between SOC and superconducting tunneling transport,but have also paved a systematic way toward engineered topological superconductivity with gate-controllable hybrid structures.(2)Study on superconductivity of LaAlO3/SrTiO3 interfaceBy characterizing the magnetotransport properties of the LaAlO3/SrTiO3 Hall devices,we found that the superconducting critical current at the LaAlO3/SrTiO3 interface can be abnormally enhanced by the parallel magnetic field,and the transition magnetic field is around 10 mT.The results of magnetoresistance and magnetism measurements show that the magnetic domain of LaAlO3/SrTiO3 is also reversed at about 10 mT.Furthermore,the data measured at different gate voltages indicate that the magnitude of the critical current enhancement changes strongly at the Lifshitz point,where the orbital occupancy of the 2DES changes from only dxy to both dxy and dxz,yz·After systematic analysis,we attribute these observations to possible Fulde-FerrellLarkin-Ovchinikov state,which is a novel superconducting state with non-zero pairing momentum.(3)Study on superconducting tunneling spectrum based on LaAlO3/KTaO3 interfaceSuperconductivity at the LaAlO3/KTaO3(111)interface has been reported recently,but its superconducting pairing symmetry remains unclear.Based on this,we constructed metal/LaAlO3/KTaO3(111)tunneling junctions and measured the tunneling conductance spectra.When the thickness of LaAlO3 is relatively thin,we observe symmetrical double conductance peaks and double conductance dips near zero bias.When the thickness of LaAlO3 is relatively thick,we observe a U-shaped superconducting gap.Through systematic theoretical analysis,such coupling-strengthdependent superconducting tunneling behavior can be attributed to the possible p-wave superconductivity at the LaAlO3/KTaO3 interface.Our results have not only pave a new way for exploring unconventional superconductivity,but have also provide a possible platform for the solid-state realization of Majorana fermions based on oxide interfaces.Our findings reveal that oxide interfaces are ideal platforms to explore and tune novel superconducting transport.These results not only promote the development of the studies of correlated electronic systems and unconventional superconductivity,but also have significant implications in various research fronts from superconducting spintronics to topological quantum computing.