First-principles Study On Two Dimensional Electron Gas At The Surface And Interface Of Perovskite Oxides

The discovery of two-dimensional electron gas?2DEG?at the interface be-tween LaAlO₃?LAO?and SrTiO₃?STO?has led to extensive research on this system and perovskite oxide heterostructures.However,the physical mechanism of 2DEG has always been a heated debate.At present,the widely cited physi-cal mechanisms include polar discontinuities at the interface,oxygen vacancies,cation mixing,and substrate induced octahedron distortion.In this dissertation,we study the strain and oxygen vacancies control on the two dimensional carrier gas at the surface and interface based on KTaO₃?KTO?,and strain control on the electronic properties of superlattices based on Mott insulator YTiO₃?YTO?by first-principles calculations based on the density functional theory.we study in-plane strain-tunable?-5?5%?the two dimensional carrier gas at the polar KTO surfaces.We observe the presence of a thin 2D electron/hole gas at the top/bottom layers of KTO surfaces,respectively,due to the polar alternating?KO?^-...?TaO2?⁺layers along the[001]direction.When tensile strain is applied in the ab plane,the location of 2DEG and two-dimensional hole gas?2DHG?has not changed.However,as the compressive strain increases,the 2DEG gradually shifts from the top to the bottom,and the 2DHG gradually shifts from the bottom to the top.In this process,the 2DEG and 2DHG disappear with the strain value of-3?-4%,the system exhibits insulating properties with band gaps of 0.53?1.40 eV,respectively,because,with the increasing compressive strain,the cation-anion relative displacements gradually increase in each layer along c direction,resulting that the ferroelectric polarization between cation and anion ions balances out the polar atom layers effect of KTO surfaces along[001]direction.we constructed EuO/KTO superlattice,consist of ferromagnetic semiconduc-tor EuO and KTO,containing oxygen vacancies at the TaO₂/EuO and KO/EuO interface,respectively.It is found from calculation results that 2DEG will appear at the interface due to the presence of oxygen vacancies at the interface.Due to the d-f exchange interaction in Eu atoms and the hybridization between the Ta d and the Eu d at the Fermi surface,spin-polarization of the two-dimensional electron gas at the TaO2/EuO interface occurs.The Ta atoms in the TaO₂layer closest to the interface are arranged in a ferromagnetic arrangement with a mag-netic moment value of up to 0.18?_B.However,the spin polarization of 2DEG at the KO/EuO interface is close to zero.We calculated in-plane strain-tunable the structure and electronic state of the YTO/STO superlattice.It is found that an insulator–metal transition occurs when a compressive strain is applied,and our structural analyses reveal that the electronic transition accompanies a structural phase transition.No transition occurs for tensile strain.In addition,we found that when the U value is reduced,similar insulator-metal transitions occur in the system.Therefore,the controlling parameter is the U/W ratio for such insulator-metal phase transition.These findings can improve our understanding of the structural and electronic properties of such oxide surfaces and heterostructures.