First Principles Study Of Novel Two Dimensional Topological Materials And Their Physical Properties

The successful synthesis of graphene and quantum spin Hall materials pave the way for the rising of two dimensional?2D?topological materials.In this article,we focus on four new types of two dimensional topological materials and their electronic properties: 2D topological insulator,quantum anomalous Hall effect,valley-polarized anomalous Hall effect,and topological node-line semimetal.2D topological insulator?TI?,also known as quantum spin Hall insulator?QSHI?,?rstly theoretically proposed in graphene,draw more attention than three dimensional ones due to their only two direction helical edge states whose back-scattering is prohibited by time reversal symmetry?TRS?.Such edge states are potentially applied in nano-electronic devices with dissipationless electron transport channels free from backscattering by any nonmagnetic impurities and defects,just behaving as a two-lane highway.The quantum anomalous Hall eff ect?QAHE?,arising from spin-orbit coupling?SOC?and time reversal symmetry?TRS?broken,shows voltage transverse to the electric current even in the absence of an external magnetic ?eld.Dissipationless boundary states exist in the two edges of materials with QAHE.The zero-?eld dissipationless chiral edge transport channels of QAHE behaving as an information express way for next generation electronics can be used in high speed,low power consumption electronics.In present work,we propose that the QAHE can be realized in anti-ferromagnetic Chern insulator that is different from the traditional ferromagnetic Chern insulator.Besides charge and spin degree of freedoms,valley degree of freedoms of electrons has attracted intense interest recently due to its potential in next generation electronic devices,now called as valleytronics.The valley represents a local maximum/minimum on the valence/conduction band of certain semiconductors inside the first Brillouin zone.The different valley can be regarded as a discrete degree of freedom for low-energy carriers,which is robust against smooth deformation and low-energy phonon thanks to the large separation of the valleys in momentum space.Finally,we predict that both node-line semimetal and Dirac semimetal can exist in the low energy electron of two 2D monolayer B₂ C.Base on these topological phenomea,we have obtained a series of innovational results as follows:1.In present manucript,we predict a new family of QSH insulators based on bilayer hexagonal TlM?M=N,P,As,Sb?.Ab initio molecular dynamics calculationsprove that there are no obvious deformations in the bilayer crystal systems even the temperature up to 800 K.In combination with the phonon spectrum analysis,thermal and dynamic stability of the systems were con?rmed.Further electronic investigation indicates that the special topological non-trivial nature of the systems comes from the band inversion between ??? and p_x,y of M atom with the help of equivalent to bilayer triangular lattice crystal ?eld.Interestingly,such p-p band inversion is independent of spin-orbit coupling?SOC?which is distinctive from conventional TIs.Moreover,the QSH is robust enough to external strain.The effect of SOC open an trivial bulk gap which can reaches 550 meV?TlSb?ensuring their high temperature applications.Their almost ?at hexagonal surface atom con?guration makes it also appropriate to match with hexagonal boron nitride and easily forms topological heterojunction,which paves a new way for high temperature topological edge states detection in experiments;2.By taking two-dimensional transition metal halides?TMHs?as prototype model,we prove that QAHE can be realized in AFM material as long as the material shows spin-polarization and its time reversal symmetry is broken.To produce spin-polarization,we construct a new type 2D TMHs of M1M2Y₆,where the M1 and M2 represent 3d and 4d TM atoms,respectively,and Y is Cl.Our results indicate that under 3%-10% compressive strain in plane the single layer NiRuCl₆ is 2D AFM QAHE insulator,or name as AFM Chern insulator.Moreover,the pristine single layer NiRuCl₆ is a half-metal AFM?HMAFM?with totally compensating magnetic moment.Using spin polarized AFM materials,such as HMAFM,could be a feasible solution to solve the problem of low Curie temperatures of the half-metals materials.The magnetic ground state of NiRuCl₆ is AFM,whereas its valence band maximum?VBM?and conduction band minimum?CBM?are spin polarized,which is an excellent candidate for spin-polarized AFM materials;3.Inspired by the advancement of both germanene and QVHE,we concentrate on how to modulate the valley-polarization of germanene by introducing 3d transition metal?TM?atom on its surface.In comparison with previous reports,we find that appropriate Hubbard U is important to accurately describe the TM-germanene systems.With the help of first-principles Berry curvature calculations,we find that 3d TM atom can break the inversion symmetry of germanene and induce valley-polarization when the spin-orbit coupling?SOC?is included.Valley-polarized AHE can be achieved when Cr,Mn,or Co is adsorbed on the surface of germanene.Moreover,quantum valley Hall effect can be realized in Mn-germanene system just through shifting the Fermi level to specific energy window.The results indicate that the 3d TM adsorption is an effective approach to modulate the valley polarization of germanene;4.We predict that both node-line semimetal and Dirac semimetal can exist in the low energy electron of 2D monolayer B₂ C.More interestingly,a new type of open nodal line and recent popular type-II Dirac fermions can be found around the Fermi level simultaneously.A tight-binding model rely on the basis of B-py,pz and C-py,pz states is derived to describe the low energy electronic of B₂ C sheet.In order to further understand the topological characteristics of these semimetal states,three effective model Hamiltonian around the k-point of semimetal in reciprocal space have been constructed depend on the local symmetry.One of the interesting merit of the system is that the energy windows of these three topological semimetals are different,which are easily distinguished in experiments.