Amorphous-film-induced Electron Gas And Superconductivity At Heterointerfaces Of Complex Oxides

The study of electron gas and superconductivity at the interfaces of complex oxides is a hot topic in the field of condensed matter physics.It plays an active role in exploring the interface emergent phenomenon induced by competition and coexistence of various quantum orders,understanding the physical mechanism of high temperature superconductivity and two-dimensional superconductivity,and developing new electronic devices beyond traditional semiconductor devices.Therefore,it is of great scientific significance to deeply understand the interface conduction mechanism of complex oxides,develop corresponding efficient control methods,and explore novel physical phenomena at more interfaces.This thesis mainly focuses on amorphous-film-induced electron gas and superconductivity at heterointerfaces of complex oxides.We find that capping amorphous oxide film can transfer charge into the interface of complex oxides heterojunctions.Further experiments allow us to prove that the transferred electrons come from the capped amorphous films.Based on this conclusion,we capped amorphous LaAlO3 films on KTaO3 substrates to induce interface electron gas and interface superconductivity.It is the first time that we discover interface superconductivity from the amorphous LaAlO3/KTaO3(110)heterojuction.Also,we find it’s possible to tune the superconducting-to-insulating transition in amorphous LaAlO3/KTaO3(111)interface by field effect.This thesis contains the following four parts,specificly:First,surface amorphous oxides can induce charge transfer into complex oxide heterointerfaces.A variety of SrTiO3-based heterojunctions are capped with different amorphous oxide films.It is found that more than 2×1014 cm-2 electrons can be induced into heterointerfaces that are originally either conducting or insulating.By a thorough examination of various combinations between different heterostructures and different amorphous oxides,it’s concluded that the charge transfer from amorphous films to interfaces is universal.Second,we perform in-situ transport measurements during depositing amorphous LaAlO3 on SrTiO3 substrates and reveal the creation of two-dimensional electron gas for depositing amorphous LaAlO3 film on SrTiO3 substrates.We explore that only 12.5%monolayer coverage of amorphous LaAlO3 film on SrTiO3 can already generate conducting electron gas.By changing the growth condition of amorphous films and the substrates,we conside that electrons transferred from the oxygen vacancies in film,rather than SrTiO3 substrates,dominate the formation of electron gas.These findings provide a new way to understand the long-standing debate on the origin of the two-dimensional electron gas at SrTiO3-based heterojunctions.Third,we find interface superconductivity by capping amorphous LaAlO3 films on KTaO3 substrates.With the help of the conclusion that amorphous films can induce electrons,we explore the superconductivity in amorphous LaAlO3/KTaO3(110)heterojunctions for the first time,and its’superconducting transition temperature is 0.93 K.The upper critical field shows large anisotropy and the superconductivity transition fits Berezinskii-Kosterlitz-Thouless(BKT)phase transition,both of which suggest that the interface superconductivity is two-dimensional in nature.Our result provides a new platform for the study of 2D superconductivity at oxide interfaces.Finally,we report an extraordinary field effect of the superconducting amorphous LaAlO3/KTaO3(111)interface.The critical temperature of LaAlO3/KTaO3(111)interface superconductivity can reach 2 K.By applying a gate voltage across KTaO3,the interface can be continuously tuned from superconducting into insulating states.As the Hall-effect measurement shows,the electric gating has strong effect on mobility but a minor one on carrier density.The resistance saturates at the lowest temperatue on both superconducting and insulating sides,which indicates a interfaces superconductivity related quantum metallic state.