First-principles Calculations Of Several Two-dimensional Materials With Spin-spiral And Topological Properties

Since graphene was exfoliated successfully in 2004,two-dimensional materials have gradually become a research hotspot in condensed matter physics and material science.Among many two-dimensional materials,magnetic materials and topological materials have potential applications in spintronics and high-speed low-dissipation devices because of their spin properties and nontrivial topological states,respectively.With the successful fabrication and characterization of monolayer antiferromagnetic FePS₃,monolayer Ising ferromagnetic CrI₃ and bilayer Heisenberg ferromagnetic Cr₂Ge₂Te₆,two-dimensional（2D）magnetic materials have been rapidly developed.In recent years,two-dimensional spin textures,such as skyrmions,bimerons,and spin spirals,have been promoted.Among them,the spin spiral,which is a periodically modulated spin-rotating state,attracts increasing attention.The long-range magnetic ordering breaks the spatial inversion symmetry and induces spontaneous polarization.Therefore,spin-spiral materials are also known as type-II multiferroic materials,which facilitate the introduction of magnetoelectric coupling into nanoelectronic devices.Up to now,experimental and theoretical investigations on spin-spiral materials are rare.Therefore,searching for more two-dimensional spin-spiral materials and exploring their properties in depth is one of the most interesting directions.Topological materials have been widely concerned by researchers because of their many unique properties.Up to now,topological materials include topological insulators,topological crystalline insulators,topological semimetals and so on.In recent years,with the successful synthesis and characterization of several materials such as Cu₂Si,CuSe and AgTe in experiments,two-dimensional Dirac nodal line fermions have aroused the interest of researchers.Since only several materials have been confirmed experimentally and theoretically,it is necessary to find new two-dimensional Dirac nodal line fermions by combining experiment and theory.In this dissertation,several two-dimensional spin-spiral materials are investigated using first-principles calculations and four-state method simplified by considering symmetry.At the same time,monolayer AuTe with Dirac nodal line fermions character is researched in cooperation with the experiment.The main results are briefly described as follows:Firstly,predicting a two-dimensional direct bandgap semiconductor with spin-spiral magnetic ordering:monolayer CoO with a honeycomb lattice.By using density functional theory（DFT）calculations,the monolayer CoO with a honeycomb lattice shows a planar structure.The bandgap is 2.36 eV.The phonon dispersion relations indicate that the monolayer CoO is kinetically stable.The magnetic ground state of the monolayer CoO is found to a spin-spiral magnetic ordering with spins rotating in the xy-plane by calculation methods based on the generalized Bloch theorem.The spins point from the Co atoms to O atoms along the Co-O bond.The resulting polarization is completely in xy plane.The valence band maximum is mainly contributed by the out-of-plane d orbitals of the Co atom and the p orbitals of the O atom,while the conduction band minimum is mainly contributed by the s and in-plane d orbitals of the Co atom and the s orbital of the O atom.Further investigations present that the spin-spiral state and the direct bandgap character are both robust under biaxial compressive strain（-5%）to tensile strain（5%）.The bandgap varies only slightly（80 meV）under either compressive or tensile strain up to 5%.This work suggests monolayer CoO is a spin-spiral material with robust semiconductor character and spin-spiral ground state,and provides potential fabrications and property investigations on substrates with different lattice mismatch.Secondly,exploring three two-dimensional spin-spiral materials:monolayer PdCl₂,PdBr₂,and PtCl₂ in T phase.DFT results show that these three materials are all semiconductors with band gaps of 0.69 eV,0.39 eV,and 0.70 eV.The phonon dispersion relations indicate that all three materials are kinetically stable.These three materials are spin-spiral materials by calculation methods based on the generalized Bloch theorem.Meanwhile,U values have no qualitative effects on the magnetic ground state.These materials have the same energy for the clockwise and counterclockwise rotating spiral states by the space inversion operation.Further calculations show that the easy magnetization planes locate on the inclined surfaces that are quasi-parallel to the halogen-metal-halogen determined surfaces with angles of 57.0°,60.8°and 65.4°to the z-axis for monolayer PdCl₂,PdBr₂ and PtCl₂.The spin-spiral magnetic ordering breaks the spatial inversion symmetry and generates spontaneous polarization.The in-plane polarization intensities of the PdBr₂ and PtCl₂ monolayers are one order of magnitude larger than the out-of-plane polarization intensities.They are comparable to the recently reported polarization values of monolayer Fe OCl.This work provides candidates for investigating spins rotating on unconventional planes in spin-spiral materials.Lastly,investigating the magnetic and topological properties of monolayer AuTe with a honeycomb structure.By using first-principles calculations,we investigated the Au_xTe_y compound which was fabricated by direct tellurizing Au（111）surface.We calculate the geometric structures of Au_xTe_y monolayers.Compared with the experimental results,it is determined that the material fabricated on Au（111）surface is the monolayer AuTe with a honeycomb structure.The calculation of spin polarization shows that the magnetic ground state of the monolayer AuTe is nonmagnetic state.Due to the interaction between monolayer AuTe and the Au（111）surface,monolayer AuTe exhibits a slightly buckled structure.Theoretical simulation results show that the buckling height is about 0.39（?）.The monolayer AuTe material is found to exhibit a Dirac nodal line fermionic feature protected by xy mirror reflection symmetry.The Dirac nodal line is derived from the crossing of two downward-opening energy bands with even parity and one upward-opening energy band with odd parity.The projected energy band results show that the two downward-opening bands are mainly contributed by the s,d_xy and d_x²-y² orbitals of Au and the p_x and p_y orbitals of Te,while the upward-opening band is contributed by the d_xz and d_yz orbitals of Au and the p_zorbital of Te.Considering the spin-orbital coupling,bandgaps appear in the Dirac nodal line.The corresponding electronic energy band result presents that the gapped Dirac nodal line caused by the buckling is in good agreement with the experimental results,which is closer to the Fermi surface than those reported in CuSe and AgSe.This work provides a new member to the family of two-dimensional materials with Dirac nodal line fermions character.