First Principle Study On Two New Types Of Topological Materials CdX(X=F、Cl、Br、I) And NaAlSi(Ge)

Topological insulator is a new quantum material state,which is completely different from the traditional sense of"metal"and"insulator".The bulk electronic structure is characterized by insulation,and the surface state protected by topology is characterized by non-dissipative ballistic transport.This non-dissipative trajectory transport feature has potential applications in future quantum computers and spintronics.With the deepening of the research on topological insulators,topological semi-metals have also come into people’s attention and become a new hotspot in the field of condensed matter physics and materials.So far,a large number of topological insulators or topological semi-metals have predicted in theory,some of which have been verified experimentally.From the development process of the research,theoretical prediction plays an important role in experimental preparation and observation.Therefore,purposeful design and prediction of new topological materials not only enrich the family of topological materials,but also provide a platform for discovery and experimental research of new topological properties.In this paper,two new types of topological materials are designed and predicted based on the first principle method.The first kind of material is two-dimensional CdX（X=F,Cl,Br,and I）with topological insulator characteristics,in which CdI has large band gap and is a candidate material for high temperature quantum spin Hall effect.The second kind of material is NaAlSi（Ge）family with topological semi-metallic characteristics,which is expected to be used as electrode materials for topological sodium ion batteries.The main contents of this paper are as follows:（1）Based on first-principles calculations,we have designed and predicted a new type of two-dimensional topological materials:transition-metal halides materials CdX（X=F,Cl,Br,and I）.The calculated results of phonon spectra prove the dynamic stability of these materials.After that,we studied the electronic properties of the two-dimensional CdX material.The band structure of the two-dimensional CdX material is similar to that of graphene.Without considering the spin-orbit coupling（SOC）,there are two equivalent Dirac cones at the high symmetry points of K and K’in the Brillouin zone of CdX,which are shows Dirac semi-metallic properties.When SOC is considered,the two-dimensional CdX materials behave as topological insulators or topological metals with nontrivial topological Z₂ invariants.Especially,the band gap of CdI reaches up to 143 meV,which makes this material hopeful to be a candidate material for quantum spin Hall effect at room temperature.By modulatingthe lattice field of this material,we find that its topological characteristics are derived from the energy band inversion induced by the lattice field interaction,while the SOC only opens its global or local band gap,and has no effect on its topological properties.In addition,we also discussed the substrate requirements and the electronic properties on the substrate for the growth of these materials.The results show that the topological properties of these two-dimensional materials can be maintained on BN substrates.The results show that it is possible to fabricate these new topological materials by experiments.（2）We find that the prepared NaAlSi（Ge）materials is a kind of double nodal lines systems with unconventional topological surface states.In this system,there are two nodal lines（one is type-I nodal line and another is hybrid nodal line）around high symmetry points ofΓand Z,which formed two nested double nodal lines structure.Moreover,this hybrid nodal lines is different from the previously reported hybrid nodal lines.Along the high symmetry lines ofΓ-X andΓ-Y in Brillouin zone,the hybrid nodal line are composed of type III points which constituted by flat band,while the other directions are composed of type I points which constituted by bands with opposite slopes.The research shows that the double nodal lines structure in this system is a stable topological state protected by mirror symmetry.Considering the high sodium ions contained in the systems,we studied the migration behavior and charge storage capacity of the materials.The results show that NaAlSi（Ge）has a low sodium ion migration barrier and a high charge theoretical reversible capacity of 476（238）mAhg^-1.The above results indicate that NaAlSi（Ge）system is expected to be electrode material for topological sodium ion batteries.Our prediction of NaAlSi（Ge）not only provides a new candidate material for exploring novel physical phenomena of hybrid nodal lines in future experiments,but also provides a possible experimental material for studying novel electrode materials for topological sodium ion batteries.