Effect Of Electric Gating And Photoexcitation On A-LaAlO₃/KTaO₃ Interfaces

As a typical strongly correlated electronic system,complex transition metal oxide heterostructures suffer from an extensive investigation due to their variety of emergent phenomena occurring at their interfaces.One typical example is the discovery of two-dimensional electron gas?2DEG?at the interface between two band-gap insulators LaAlO₃?LAO?and SrTiO₃?STO?.This system exhibits many exotic properties,presenting a promising platform for the exploration of both fundamental and applied research.Most of the previous works focused on STO-based2DEGs,this thesis takes the amorphous-LaAlO₃/KTaO₃?a-LAO/KTO?oxide heterointerfaces as the research object,and we hope to reveal the transport properties of the 2DEGs with 5d orbital characteristics by systematically investigating the effect of electric gating and photonexcitation on this system.In the followings are the main achievements:1.We obtained high quality 2DEG by fabricating an amorphous LAO layer on a?001?-orientated KTO substrate at high temperatures.Metallic 2DEG with the carrier density as low as²?10¹² cm^-2 and the Hall mobility as high as²150 cm²/Vs is obtained successfully.We found our 2DEG exhibits strong gating effect,transiting from a well metallic state into a Kondo state by applying a gating field.Remarkably,the spin relaxation time is also strongly tuned by back gate.This result indicates the great potential of 2DEGs at a-LAO/KTO interfaces as oxide devices with spintronic functionality.2.We systematically investigated the effects of the fabrication condition on the performance of the 2DEGs at a-LAO/KTO interfaces.We found an abnormal dependence of the transport properties on fabrication condition:the 2DEG obtained at low temperature and high vacuum exhibits the best metallic character and the highest mobility of⁶.6?10³ cm²/Vs,which is much higher than that of the 2DEGs at the a-LAO/STO interfaces.Possibly,5d electrons are more extended than the 3d ones and have a smaller effective mass.Moreover,a threshold carrier density?or Fermi level?is observed,above which two species of charge carrier coexist.The present work opens new avenues for the exploration for high mobility 2DEG at KTaO₃-based oxide interface.3.We systematically investigated the effects of electric gating and photonexcitation on 2DEGs at a-LAO/KTO interfaces with totally different band fillings.A relationship between the Rashba spin-orbit coupling and band filling has been established in the Fermi level ranging from¹3 meV to³13 meV,showing the maximal spin splitting energy of 30 meV,which is much higher than that of the2DEGs at the a-LAO/STO interfaces.Remarkably,the Rashba spin-orbit coupling exhibits a steep peak and the composition of mobile electrons undergoes a crossover from one to two species as Fermi energy exceeds a critical band filling.At this point,an additional species of charge carriers with a 50-fold enhanced Hall mobility emerges.As evidenced by density functional theory calculations,these unusual physical properties stem from the distinct hierarchic band structure of KTO-based2DEGs composed of 5d electrons.These results further deepens our understanding of the electron correlated 2DEGs with 5d orbitals,laying a foundation for the development of microelectronics and spintronics based on oxide interfaces.