First-principles Study Of Low-dimensional Systems

When one or more of the dimensions of a solid are reduced sufficiently,the result-ing structure is then called a low-dimensional structure（or system）.The surface and in-terface of the material,graphene and nanoribbons belong to low-dimensional systems.Electrons or holes confined in low dimensional system having a dramatic change in their behavior.Compared with Sr Ti O₃（STO）,KTa O₃（KTO）has a lower carrier effective mass and a stronger spin-orbit coupling effect,and thus low-dimensional characteris-tic of its surface and interface are deserved to more extensively study.In addition,the edges of graphene nanoribbons have been extensively studied for their novel properties.Through the first-principles calculations,we have investigated the effect of the biaxial strain on the Rashba spin splitting of the KTO slabs,studied undoped KTO-sandwiching heterostructures and the coexisting two-dimensional electron（2DEG）and hole gases（2DHG）at its interfaces,achieved edge magnetism through self-edge reconstructions in graphene armchair nanoribbons（AGNRs）.The in-plane strain-dependent Rashba spin splitting of the ultrathin KTO films was investigated with the strain ranging from-5%（compressive）to 8%（tensile）.The Rashba effects remain almost unchanged under tensile strain,while a giant Rashba spin splitting can be obtained by applying compressive biaxial stress.This giant Rashba spin splitting can be kept in the presence of capping layer and/or substrate.Such the giant Rashba effect can be used to generate spin-polarized photocurrents.Undoped and stoichiometric STO/KTO/Ba Ti O₃（BTO）heterostructures was es-tablished,in which two types of interfacial configurations are made by（Sr O）⁰/（Ta O₂）⁺-（KO）^-/（Ti O₂）⁰and（Ti O₂）⁰/（KO）^--（Ta O₂）⁺/（Ba O）⁰.The coexisting 2DEG and 2DHG can be obtained at these two heterostructures,having an advantage in carrier effective mass expected to realize high carrier mobility.Modified AGNRs was obtained by edge reconstruction along the edge of AGNRs with carbon only.The reconstructed edge consists of carbon pentagons（AP_w）or a hy-brid of carbon hexagons and pentagons（HP_w）,and the other edge preserve the pristine armchair edge unchanged.Study presented that their geometric structure and electronic properties were both changed.The resultant nanoribbons are narrow-gap semiconduc-tors and the reconstructed edges host quantum heisenberg antiferromagnetic spin-1/2chains,and thus ballistic quantum spin transport can be achieved along the reconstructed edges.