The Calculational Design Of Topological Semimetal Materials

Topological semimetal materials represent a new class of quantum states,which are widely used in the studies of quantum relativistic quantum mechanics and quantum computers.It has drawn lots of attentions from researchers and became a hot topic in condensed matter physics and materials science.However,the quantum states in this kind of materials are hard to be observed in experiment,and would be harder when their electron band structures are complicated.Therefore,more topological semimetal materials are needed and the theoretical studies are more necessary.During the Phd period,my work is to design new class of topological materials and study their properties,mainly focused on their electron band structure and topological feature.From now on,I have found several kinds of new topological materials,including the large gap topological insulator M3Bi2(M=Ca,Sr,Ba),topological metal NaBi,Dirac node line semimetal Be,Weyl ferroelectric semimetal HgPbO3.The large gap topological insulator M3Bi2 and topological metal NaBi are obtained from the study of Bi binary system,through first-principles calculations in combination with high throughput evolutionary structural searches.In topological insulator and topological nodal point semimetal,the topological non-trivial state can only exist in the system with strong spin orbital coupling effect.Considering that Bi is owns the strongest spin orbital coupling effect among the stable elements in the periodic table of elements.So we studied the Bi binary system through first-principles calculations in combination with high throughput evolutionary structural searches to find out such topological materials.M3Bi2 only has been mentioned in the early studies of phase diagram,without any structure data.According to our calculations,the M3Bi2 belongs to rhombohedral structure.Further band structure calculations suggest that M3Bi2 is a topological insulator with a large gap of 0.3 leV,which is one of the largest gap among all known topological insulators.NaBi was found in 1932,being a tetrahedral metal in with topological feature.Because the pseudo-gap exists everywhere in the band structure of NaBi,the topological feature of NaBi is same to topological insulator.Furthermore,the phonon calculations show that NaBi also has extremely low lattice thermal conductivity and be super conductivity in low temperature.The topological feature in topological nodal line semimetal is quite different from topological nodal point semimetal and topological insulator.The topological nodal line state can only exist in the materials in which the spin orbital coupling effect can be ignored.It means that the topological nodal line semimetal should be constructed by light elements.At the same time,a full filled orbit is also required in topological nodal line semimetal,which ensures the material show semimetal feature and the nodal line occurs nearby the Fermi energy level.Then the light alkali earth metal is the first consider case satisfying all the conditions required in topological nodal line semimetal.The first case we studied is pure metal Beryllium.Through first principle calculations method,we found Dirac node line in pure metal Be.Furthermore,pure metal Be has many special physics phenomenon on its(0001)surface,such as anomalous surface states,large Friedel oscillation and large electron-phonon coupling.These features were found one after the other since 1960s,but their mechanism is still unknown.By analyzing the band structure of Be,we found the Dirac node line will induce topological non-trivial drumhead like surface state on Be(0001)surface,and all the special physics phenomenon can be related to the existence of the topological non-trivial drumhead like surface state.We have also designed a new kind of ferroelectric Weyl semimetal.Weyl semimetal is a kind of topological nodal point semimetals,which can only exist in the system without inversion or time reversal symmetry.The noncentral symmetric Weyl semimetal has more interesting feature than Weyl semimetal without time reversal symmetry,so Weyl semimetal design focus on noncentral symmetric system.The ferroelectric feature also in noncentral symmetric system,whether the Weyl point can exist in ferroelectric metals are quite interesting.A topological and ferroelectric phase transition can be expected in such materials,which can be used as a quantum on-off.The ferroelectric metal are always metallic oxide,so we studied the metallic oxide HgPbO3.HgPbO3 was first synthesized in 1973,but the existence of inversion symmetry in is still in controversy.Through first principle calculations method,we found the structure without inversion symmetry is more stable in the ground state,Weyl points and polar axis coexist in this structure,HgPbO3 is a ferroelectric Weyl semimetal.