The First-principles Study Of Topological Semimetals And Iron-based Superconductors

The investigations relate to novel properties as well as application of exotic materials are the focus of condensed matter physics.Due to the particular topological character of band structure,topological materials can host very fascinating electronic properties.For instance,topological insulators showing linear dispersion and spin helical metallic surface states inside the bulk insulating gap,Dirac/Weyl semimetal showing fermi arc on the specific surface and nodal line semimetal showing drumhead like surface state.Therefore,topological materials have attracted considerable interests.On the other hand,the discovery of FeAs-based superconductors?IBSCs?with transition temperature above 40K in 2008 has opened a new era in the study of unconventional superconductivity.Important developments in IBSCs is helpful for understanding the mechanism of high-Tc superconductor.The first principle calculations based density functional theory?DFT?is very important in the study of IBSCs and topological materials.It has been widely used to electronic structure analysis,ground state determination and new materials predication.In present thesis,first principle method and k·p model are utilized to study the topological semimetals and iron based materials.By performing first-principle electronic structure calculations for Sc Ta N₂,we find that it is an Dirac semimetal with the Dirac points locating at Fermi level exactly,and it would transform into strong topological insulator phase by breaking rotational symmetry.It could also transform into Weyl semimetal state by introducing external magnetic field.Our studies show that Dirac semimetal is a critical point for topological phase transition?Based on first-principles calculations and an effective k·p model,we theoretically propose topological nodal-line?NL?semimetal states in the family of non-centrosymmetric ternary transition metal pnictides,TT'X?T,T'=Transition metal,X=P,As?,when spin-orbit coupling?SOC?is ignored.A single?centre nodal loop on the k_z=0 plane is protected by mirror-reflection symmetry,exhibiting intriguingly site dependent tilt in the momentum space.Remarkably,unique double Dirac-cone like surface states appear on the?001?surface of these materials,which is distinct from the usual drumhead like surface states for previously studied NL compounds.When SOC is considered,the gaps open along the NL and these materials transition to full energy gap insulators with Z₂ topological invariants?1;000?.In combination with the fact that this family of materials was observed to be conventional superconductors with transition temperature?Tc?above 10 K,our results may provide an appropriate platform to study the interplay between topological NL fermions and superconductivity.Electronic structures and magnetism properties of Cu Fe As and CuFe Sb are investigated by using first-principles calculations.We found that CuFe As and Cu Fe Sb share similar electronic structures and magnetic properties.Unlike the antiferromagnetic isostructure Li Fe As,the ground state of both compounds is ferromagnetic state driven by the Stoner ferromagnetic instability.Their ground state is very sensitive to the height of anion?As or Sb?,translating from the ferromagnetic state to the stripe antiferromagnetic ordering when the anion height is smaller than a critical value.Such magnetic phase transition can be understood by the J₁-J₂ Heisenberg model.Reducing the anion height will decrease the nearest-neighbor interaction J₁ but increase the next-nearest-neighbor interaction J₂.The competing between the anion height dependent antiferromagnetic superexchange mediated by As?Sb?and the ferromagnetic direct exchange between Fe results the variations of magnetic structure with anion height.