Structural Design Of Novel Two Dimensional Materials And Theoretical Investigations On Their Topological Properties And Magneto-optical Effects

The successful isolation of graphene in 2004 has triggered a vibrant field of two-dimensional(2D)materials,and led to dozens of monolayer substances to be discov-ered.Due to their extraordinary electronic,optical and chemical properties,2D mate-rials are promising for a variety of applications ranging from optical devices to sensors and energy storage systems.Besides,the combination of spin-orbit coupling(SOC)and quantum confinement effect in 2D materials results in several fascinating physical phenomenon,such as quantum spin Hall(QSH)and quantum anomalous Hall(QAH)ef-fects as well as magneto-optical effects.The symmetry protected edge states supported by QSH and QAH insulators are back-scattering forbidden and energy-dissipationless.Therefore,they are highly desired for applications in low-energy transport,spintron-ic devices,and topological quantum computing.In this dissertation,we investigated the topological properties,magneto-optical effects and the underlying physical mecha-nism for several novel 2D materials based on first-principle calculations and Wannier function method.The main studies and results can be summarized as follow:(1)The structural and electronic properties along with the band topology of stanene on a Ge(111)substrate are investigated.When ignoring the SOC effect,the band struc-ture of stanene/Ge(111)system displays a typical Dirac cone at ? point in the vicinity of the Fermi level.Upon the inclusion of SOC effect,a gap of 52 meV is opened at the Dirac cone.Aided with the result of Z2 invariant calculations,the stanene/Ge(111)system has been proved to sustain the nontrivial topological phase,with the prove being confirmed by the edge state calculations of stanene ribbons.Additionally,we find that the band inversion between the pz states of Sn and px,y states of Ge(111)substrate leads to the occurrence of QSH state in stanene/Ge(111)system.This finding can serve as guidance for epitaxial growth of stanene on substrate and render stanene feasible for practical use as a topological insulator.(2)The electronic,magnetic,and magneto-optical(MO)properties of few-layer and bulk Cr2Ge2Te6 structures are investigated.Competing contributions of both magneto-crystalline anisotropy energy(MCE)and magnetic dipolar anisotropy energy(MDE)to the magnetic anisotropy energy(MAE),are found.Interestingly,we find that the per-pendicular magnetic anisotropy is preferred in all the considered systems except the monolayer(ML)where an in-plane magnetization is favored because here the MDE is larger than the MCE.Crucially,this explains why long-range FM order was observed in all the few-layer Cr2Ge2Te6 except the monolayer.In the visible frequency range,large Kerr rotations up to 2.2° in these magnetic materials are predicted.Besides,the calculated maximum Faraday rotation angles in these 2D materials are also large,being up to?120°/?m.These findings thus suggest that with large MO effects plus signif-icant MAE,atomically thin films of Cr2Ge2Te6 would find valuable applications in 2D magnetic,magneto-electric and MO devices such as high-density data-storage and nanomagnetic sensors.(3)The magnetic,electronic and topological properties of a planar pentagonal MoS2 monolayer(termed as 1P-MoS2)are investigated.The 1P-MoS2 is found to be a ferromagnetic insulator with a Curie temperature of 466 K predicted under the mean field approximation.Electronic structure calculations reveal a Dirac half-metal state in 1P-MoS2 when SOC is ignored and a band-gap opening of 18 meV upon the consid-eration of SOC effect.Further analyses on Berry curvature,quantum anomalous Hall conductivities(AHC),and chiral edge states confirm the existence of QAH state in 1P-MoS2,where the nontrivial Chem number is estimated to be C=-2.Additionally,dynamical and mechanical stabilities of 1P-MoS2 is verified by phonon spectrum and elastic constant examinations.