The Application Of The First Principle In Iron-based Superconductor

Since the discovery of the iron based superconductor, it has not been revealed the mechanism of superconductivity though it has been explored for five or six years. Different from the conventional superconductors, iron-based superconductors are layered compounds and its Fermi surface dominated mainly by the iron 3d orbital.The parent compounds have complex Fermi surface and are poor metal.Based on density functional theory, we study the properties for two kinds of iron base materials and similar materials.Based on the density functional theory, using the ultra-soft pseudopotential and GGA-PBE methods, we calculated crystal structure, band structure, density of states, Mulliken bond population of CaFeAs2, BaFe2As2 and SrFe2As2. Our results show that four bands crossed the Fermi level, which are mainly from the contribution of Fe 3d.The arsenic layerhas an important influence forthe Fermi surface of CaFeAs2. The Fermi surface is dominated mainly by Fe atom and As atom. We find that CaFeAs2 is a relatively soft brittle material with poor thermal conductivity, which is the same as other superconductor systems. With the increasing pressure, the lattice parameter of CaFeAs2 change by nonlinear way. Unlike Ba Fe2As2 and SrFe2As2, with the increasing of pressure, the As-Fe-As bond angle of CaFeAs2 close to the ideal value, and the Fermi surface remains unchanged. So it is can be predicted that CaFeAs2 would have a structure or magnetic transitions under pressure and the transformation temperature of superconductor may reach the maximum value at 20 GPa.By comparing the parent material of the iron arsenic superconductor, it is found that, compared with other superconducting materials, the iron arsenic superconductors are relatively soft materials. The bulk modulus and the lattice parameters are related, in general, the smaller the lattice parameter, the larger the bulk modulus.Based on the density functional theory, using the pseudopotential method and GGA-PBE, we calculated the crystal structure, bands structure, density of States, Mulliken bond population of CuFeAs, Li FeAs and NaFe As. Different LiFeAs and NaFeAs, As-Fe-As bond angle of Cu FeAs deviates significantly the ideal value; Only a band across the Г point, central region of the Brillouin hole pocket and lack of a Fermi hole type cylinder; Density of states around Fermi level are dominated mainlyby Fe and Cu atoms. No superconductivity is found for CuFeAs. Unlike other iron arsenic superconductors, the calculation of elastic constants showed that CuFeAs is a relatively hard plastic material. Pressure induce a small variation to the lattice parametersof CuFeAs, and with pressure increasing, the As-Fe-As bond angles deviate from the ideal value.And the Fe-As bond length do not change within 0~15 GPa, and the Fermi surface is mainly contributed from Fe and Cu atom at 15 GPa; In conclusion, we can predict that pressure does not induce CuFeAs superconducting transition.Iron-based superconducting have many common characteristics, especially the characteristics of the Fermi level and the Fe-As key, has an important influence on superconductivity. The effect of the crystal structure on the iron-based superconductivity is the interaction between the ions in the layers and the interaction between the layers and the layers. Similar to the structure of the iron based materials, and some have superconductivity, some no superconductivity; structure of different ferrous materials, due to the electronic structure, the internal chemical bonds are similar, but there are superconductivity. The common characteristics of iron-based superconductors provide important references for the study of Fe based materials. And pressure changes in the conditions of the Fermi surface, the Fe as bond characteristics change, as the theoretical prediction of superconducting transition temperature changes provide an important basis.