First Principles Study On Two-dimensional Topological Materials CdAs And RuS₂

In recent years,two-dimensional（2D）topological insulators and 2D topological semimetals,as new quantum states,have been received great attention of material researchers.Topological insulators are internally insulated but have non-trivial topological edge states that can be turned on,while topological semimetals have non-trivial low-energy excited electronic states and topological edge states.These peculiar properties make these two types of materials have great application potential in high-speed electronic devices with low energy consumption in the future.At present,2D topological materials have been successfully prepared and confirmed by experiments that they are still very rare.Especially,there are few theoretical and experimental reports on 2D topological semimetals with multiple types and multi-node rings.Therefore,it is very necessary to explore more new 2D topological materials in theory.In this paper,we mainly predict three new 2D topological materials:Dirac semimetal Cd As-164,Type-I and Type-II coexisting three-node rings semimetal Cd As-187（since the stoichiometric ratio of Cd and As is the same,different space group numbers are used to distinguish them）and topological insulator Ru S₂.The main contents are as follows:1.Based on first-principles calculations and the RG²search method,we propose a new 2D Dirac semimetal:Cd As-164.The results of binding energy,phonon dispersion,mechanical analysis,and molecular dynamics simulation show that it have good stability of energy,dynamics,mechanics and thermodynamics.When the spin-orbit coupling（SOC）is not considered,there are six equivalent Dirac cones in the entire first Brillouin zone.These Dirac cones are not only robust under the disturbance of applied biaxial strains in the range of-5%-+5%,but also have anisotropic Fermi velocities（2.02×10⁵m/s and 8.67×10⁵m/s）.By symmetry analysis,we found that these Dirac cones are protected by the vertical mirror symmetryσ_y.In addition,the edge states obtained from the one-dimensional semi-infinite structure in the direction of（010）prove that it is a topologically non-trivial material.When SOC is included,the Dirac cone opens a tiny band gap（26 me V）and converts it into a topological insulator with Z₂=1.Since these tiny band gaps are negligible under thermal disturbance at room temperature,the material still has semi-metallic states and the non-trivial edge states also verify that it is topological.2.We predict a new 2D nodal ring semi-metallic material:Cd As-187.It has good dynamic stability,mechanical stability and thermodynamic stability,which indicate that this material is expected to be synthesized in future experiments.When the SOC is not considered,there are two Type-I nodal rings and one Type-II nodal ring near the Fermi level,and these rings are protected by the horizontal mirror symmetryσ_h.Moreover,under the action of applying biaxial strain,the three closed nodal rings can be effectively adjusted.The construction of an effective k·p model through symmetry and the edge states in the（010）direction prove its topological non-trivial nature.When considering the SOC,the crossing points of the nodal rings along the high symmetry line all open the tiny band gaps that are negligible at room temperature,so we believe that it is still three nodal rings semimetal.Both the Z₂index and the non-trivial edge states show that it is also a topological material.3.Based on first-principles calculations,we predict a topological insulator:Penta-Ru S₂monolayer.Its binding energy is 4.38 e V,which is higher than the binding energy of the prepared monolayer black phosphorus and silylene,which indicates that it has a higher experimental synthesis possibility.The material not only has good dynamic stability,mechanical stability and thermodynamic stability,but also has anisotropic mechanical properties.When the SOC is not considered,it has four Dirac cones distributed in the first Brillouin zone,and they are all protected by the vertical mirror symmetryσ_y.However,the Ru atom is a kind of heavy atom,and its SOC effect is stronger.When considering SOC,all of its Dirac points are destroyed,and eight accidental band crossing points appear near the Fermi level.Because they are not protected by lattice symmetry,the global band will be opened under strain,so that the 2D Ru S₂behaves as a typical topological insulator.The non-trivial edge states and the spin-polarized edge states prove that it is topological.