Theoetical Prediction And Properties Of Topoloical Metallic Boron Mateials

The topological Dirac material has a unique electronic structure-its energy band has a linear cross point near the Fermi level,and it is linear dispersion in every direction near the cross point.More importantly,such band crossing properties are protected by crystal symmetry,so they have very attractive application prospects in electronic devices and other fields.With the development of research,a large number of topological Dirac materials have been found theoretically or experimentally.However,these topological Dirac materials often contain heavy metals,which are not only expensive to manufacture,but also not friendly to the environment,which is not conducive to their practical application.Boron,as a light element,is abundant in the earth,non-toxic,harmless and widely used.At present,it has become a very valuable subject to explore the pure boron topological Dirac materials with excellent properties.Based on this,three novel topological Dirac nodal metals are predicted by using the bottom-up assembly method with boron nanomaterials as the basic elements.The results are as follows:?1?The stable 3D-borophene was designed by stacking and assembling the synthesized 2D borophene.The calculation of its electronic structure shows that there is a linear Dirac state near the Fermi level in 3D-borophene,and its band inversion is protected by mirror symmetry.In the first Brillouin region,there are two periodic continuous nodal lines,which are typical topological Dirac nodal semi metals.In addition,by calculating the Fermi velocities in different directions of the inverted space,we find that the Dirac fermions of 3D-borophene have strong anisotropy and show the characteristics of the second kind of cone in some directions.Based on the tight binding model,we found that the projection of 3D-borophene's nodal line on?010?plane formed a typical tympanic topological surface state.Such surface states are expected to be detected by future photoelectron spectroscopy experiments.?2?A new type of three-dimensional boron isomer Cmcm-B₄ with orthogonal symmetry was designed by stacking the honeycomb boron synthesized in the experiment.The structure has good energy,dynamic and thermal stability.The calculation results of the electronic structure show that Cmcm-B₄ is also a typical topological Dirac nodal semimetal,and its nodal lines form different nodal rings in the Brillouin region.Similar to 3D-borophene,Cmcm-B₄ has high Fermi velocity?from 0 to1.1×10⁶ m/s?and strong anisotropy,showing the characteristics of the second type of Dirac state in some directions.Due to the existence of the energy band with gentle dispersion near the Fermi level,i.e.heavy fermion,the superconductivity of Cmcm-B₄ is studied based on the calculation of the acoustoelectric coupling coefficient.It is found that the superconductivity transition temperature of Cmcm-B₄ is 6 K,which belongs to the conventional superconductive material.?3?The three-dimensional stable boron isomer,Cmm2-B₄₀,was designed based on the zero dimensional B₄₀ nanocluster synthesized in the experiment and assembled from the bottom to the top.The cage structure of B₄₀ is basically maintained in the crystal.The analysis of electronic structure and the calculation of topological properties show that Cmm2-B₄₀ is also a topological Dirac nodal line semimetal.There are two periodic continuous nodal lines in the Brillouin region.The energy band inversion is protected by time inversion symmetry and space inversion symmetry.At the same time,the nodes on the nodal line show strong electric transport anisotropy.In some directions,they have the characteristics of the second type of Dirac state.The Fermi velocity is as high as 3.4×10⁵m/s.According to the calculation of topological properties,the surface state of tympanic membrane protected by topology is formed on the?100?surface.The above results will not only help to expand people's understanding of boron isomers,but also provide a new idea for the research and development of light topological Dirac materials,that is,the bottom-up assembly of nano materials.