Investigation On The Design Of Novel Two-dimensional Topological Insulators And Quantum Manipulation

Topological insulator(TI),as a new quantum matter discovered in recent years,has become a major technological breakthrough in the field of condensed matter physics,especially in spintronics.Distinguishing from traditional metal and insulator,the bulk phase of TI is insulating,while the surface(edge)of this kind of material possesses a spin-related metallic state,in which the spin and momentum are locked-in.Therefore,the nonmagnetic backscattering is prohibited,which opens up a new avenue for the design of ultrapower-low spintronic devices and quantum computation.In fact,two-dimensional(2D)TIs host an advantage over three-dimensional(3D)TIs,since the electrons at the edges of 2D TIs can only flow along two directions with opposite spins and thus be free from backscattering caused by nonmagnetic defects,whereas the surface states of 3D TI are only free from exact 180°-backscattering.However,so far,various 3D TIs have been fabricated with surface stated observed experimentally,while the 2D TIs still confine to the quantum well and a few systems such as Bi(111)and Bi(110)films.Thus,it is significantly important to solve the bottleneck of 2D TIs development by searching experimentally feasible 2D material with sizable bulk gap and tunable spin quantum Hall(QSH)effectsAnother key point for spintronic application is the exploration of half-metal with 100%spin-polarization.Following the first proposal of half-metallicity in NiMnSb by de Groot,half-metals have received considerable research interest,which show potential application in spin filters,sensors and so on.The pursuit of tunable magnetism in 2D crystals has been a long-sought goal.Magnetic adatoms on the surface and introduction of specific defects or edges are employed to induce half-metalicity in 2D cystals,whereas the experimental realization remains challenging.To develop next-generation spintronic nanodevices,it is crucial to explore 2D material with half-metallcity,high Curie temperature,large magnetic anisotropic energy and experimental fesibility.In this thesis,we predict a series of 2D TIs and half-metal by means of first principles calculations based on density functional theory.We systematically analyze the origin and regulation of topological and half-metallic properties,which provides material basis for experimental progress and therotical support for spintronics device design.The main results are summarized as follow:1.Adequately understanding band inversion mechanism,one of the significant representations of topological phase,has substantial implications for design and regulation of TIs.Here,by identifying an unconventional band inversion,we propose an intrinsic QSH effect in iodinated group-V binary(ABI2)monolayers with a bulk gap as large as 0.409 eV,guaranteeing its viable application at room temperature.The nontrivial topological characters,which can be established by explicit demonstration of Z2 invariant and gapless helical edge states,are derived from the band inversion of antibonding states of px,y orbitals.Furthermore,the topological properties are tunable under strain engineering and external electric field,which supplies a route to manipulate the spin/charge conductance of edge states.These findings not only provide a new platform to better understand the underlying origin of QSH effect in functionalized group-V films,but also are highly desirable to design large-gap QSH insulators for practical applications in spintronics.2.The tunable QSH state of Bi(110)film with black phosphorus(BP)structure,which is robust against structural deformation and electric field,is explored by first-principles calculations.It is found that the 2-monolayer(2ML)BP-Bi(110)film obtains tunable large bulk gap by strain engineering and its QSH effect shows a favorable robustness within a wide range of combinations of in-plane and out-of-plane strains,though a single in-plane compression or out-of-plane extension may restrict the topological phase due to self-doping effect.More interestingly,in view of biaxial strain,two competing physics on band topology induced by bonding-antibonding and px,y-pz band inversions are obtained.Meanwhile,the QSH effect can be persevered under an electric field up to 0.9V/A.Moreover,with appropriate in-plane strain engineering,nontrivial topological phase in 4ML BP-Bi(110)film is identified.3.Knowledge about chemical functionalization is of fundamental importance to design novel two-dimensional topological insulators.Despite theoretical predictions of QSH insulator via chemical functionalization,it is quite challenging to obtain a high quality sample,in which the toxicity is also an important factor that cannot be ignored.Herein,using first-principle calculations,we predict an intrinsic QSH effect in amidogen-functionalized Bi/Sb(111)films(SbNH2 and BiNH2),characterized by nontrivial Z2 invariant and helical edge states.The bulk gaps derived from px,y orbitals reaches up to 0.39 eV and 0.83 eV for SbNH2 and BiNH2 films,respectively.The topological properties are robust against strain engineering,electric field and rotation angle of amidogen,accompanied with sizable bulk gaps.Besides,the topological phases are preserved with different arrangement of amidogen.The H-terminated SiC(111)is verified as a good candidate substrate for supporting the films without destroying their QSH effect.4.Ferrimagnetic half-metal is more promising in spintronic devices than its ferromagnetic counterpart due to its lower stray fields and favorable robustness of magnetism.In comparison to three-dimensional counterpart,the realization on two-dimensional ferrimagnetic half-metal remains blank up to date.Here,based on first-principles calculations and Monte Carlo simulations,we predict a ferrimagnetic half-metallicity in two-dimensional MXene Mo3N2F2 with a Curie temperature of 237K and a considerable magnetic anisotropy energy.The ferrimagnetic coupling is mainly from the interactions of itinerant d electron between different Mo layers,and thus,endows a 100%spin-polarization at the Fermi level with a sizable half-metallic gap of 0.47 eV.Such ferrimagnetic half-metallicity is also robust against external strains.Additionally,diverse magnetic and electronic characters can be controlled,depending on different terminated Mo3N2F2 surface.