First-principles Study On Topological Properties In New Transition Metal Monochalcogenides

At low temperatures and a high magnetic field,the fractional quantum Hall states,in which quantum-mechanical effects dominate,are beyond the paradigm of Landau’s symmetry-breaking theory,giving rise to quantum topological states of matter.These topological states can be divided into long-range quantum entanglement states with topological order,or short-range quantum entanglement states without topological order.However,an extreme temperature and a strong magnetic field are required for the topological order system.Therefore,searching for classical topological states with short-range quantum entanglement and symmetry-protected in various systems has become one of the important research fields in condensed matter physics.The high correlation between crystal symmetry and classical topological states has enabled several research groups to complete the high-throughput confirmation of topological electronic materials based on symmetry indicators,and constructing a vast and rich database of topological electronic materials.So far,it has become an important research topic to select ideal topological candidate materials specifically and purposefully.This dissertation predicted several classes of topological candidate materials in transition metal monochalcogenides with excellent properties through theoretical calculations outside the framework of high-throughput searches.The main research contents are summarized as follows:1.Based on theoretical calculations and symmetry analysis,the stable existence of Dirac nodal states is predicted in the two-dimensional transition metal monochalcogenide compounds MX（M=W,Mo,X=Te,Se）with space group No.11.In the absence of spin-orbit coupling,the system exhibits two-fold degenerate Weyl nodal line states under time-reversal symmetry and two-fold screw symmetry protection.After considering spin-orbit coupling,under combining the two-fold screw symmetry,time-reversal symmetry,and spatial inversion symmetry,the system exhibits four-fold degenerate Dirac nodal line states.2.The ideal type-II Weyl node states are confirmed in the non-centrosymmetric Mo Te system based on theoretical calculations.The topological nontrivial degeneracy of the bands in this system is at general momentum points,which results in the topological properties of this system cannot be confirmed by the high-throughput methods.Through strain tuning,the idealized modulation of Weyl points and topological phase transitions between topological semimetal and compensated semimetal can be achieved.3.The X₂Te Se（X=Mo,W）candidate systems are obtained by the metal-intercalation,which can realize the coexistence of Weyl nodes and nodal ring against spin-orbit coupling effect.In particular,the nodal ring of the W₂Te Se system is a hybrid nodal ring.In the Mo₂Te Se system,the type of Weyl points is the coexistence of type-I and type-II.Additionally,the novel connection pattern between the Weyl points of the system ensures a longer Fermi arc.4.We focus on discussing the effects of strain modulation on topological electronic properties of the Pmn2₁-Mo Te structure.In the absence of spin-orbit coupling,the system exhibits two type-I nodal rings.After considering spin-orbit coupling,the nodal rings are gapped into 22 pairs of Weyl points with opposite chirality.Under the strain effect,the number of Weyl points is reduced to 4 pairs,and the energy and position of these Weyl points are optimized to different degrees,so that the topological state of the system is more ideal.