First-Principles Calculations On Low-Dimensional Magnetic And Topological Materials

First-principles calculations based on the density functional theory are the frontier of condensed matter physics.By calculating or simulating various materials or quantum dynamics,people have predicted novel physical properties in materials and explained the essence of many physical phenomena.Nowadays,the first-principles methods have become a routine in materials science.Two-dimensional（2D）materials,magnetic materials,and topological materials,which possess essential physical mechanisms and rich potential applications,have been extensively focused on and explored.In this thesis,we have investigated the background of magnetism and topology in low-dimensional systems based on the understanding of the solid-state theory.We have studied the square pentagon Mo S₂（sp-Mo S₂）,which is an intrinsic 2D magnetic topological insulator,and puckered pentagonal VTe₂（PP-VTe₂）,which is a narrow-gap magnetic semiconductor with multiferroic coupling.Besides,considering the successful cooperation with experimentalists,we are also interested in manipulating physical properties in Bi Fe O₃,La Sr Mn₂O₆,and Mo S₂ systems for their tunable electronic behaviors.Novel atomistic patterns will provide new insights in achieving two-dimensional magnetism and topology.In the first work,we find that an atomically thin Mo S₂monolayer with squared pentagon structure motifs,distinct from all conventional Mo S₂monolayers,has unique electronic band structures with Dirac cones comprising of d orbitals.The sp-Mo S₂ monolayer is an intrinsic ferromagnet with a high Curie temperature and behaves as a half semi-metal,which possesses spin-polarized Dirac fermions around the Fermi level.Moreover,when strong spin-orbit couplings are included,the magnetic Dirac band is gapped,hosting quantized conductance channels.2D magnetic crystals are very important because of their potential applications in spintronics and valleytronics.In the second work,we report a novel monolayer magnet namely PP-VTe₂,intriguing atomic structures of which can be considered as a pyrite structure when thinned to 2D limits.The stability of PP-VTe₂ was firmly validated from first-principles calculations.The PP-VTe₂ exhibits strong intrinsic ferromagnetism and semiconducting property and an unusual magnetic anisotropy with large magnetic exchange energies was found distinct from the half-metallic bulk pyrite VTe₂ phase.More interestingly,the multiferroic coupling between its 2D ferroelasticity and in-plane magnetization is further identified in PP-VTe₂,lending it unprecedented controllability with external strains and electric fields.In a word,we have not only successfully predicted 2D materials with intrinsic magnetic or topological properties,but also revealed exotic crystalline configurations and magnetic topology in low dimensions.Therefore,these continuous innovations will not only promote development of condensed matter research but also inspire people to discover the physical nature behind novel phenomena,and develop broader applications.