The Design And Research Of ALaCuO₃FeAs（A=Ca,Sr,Ba）: Potential Candidates For High Tc Superconductors

Since the iron-based high-temperature superconductors have been discovered in2008, more and more iron pnictide Superconductors were found, and much research has been devoted into this series of Superconductors. In the research it is found that there are many analogies between the iron pnictide and copper oxide superconductors including that they both have a superconducting layer, Fe2Pn2(Pn=As, P, Se) layer for the former and CuO2layer for the later. Although the comparative studies between the only two types of high-temperature superconductors (HTSCs) have provided some guidance for the research direction of HTSCs, the mechanism of high-Tc superconductivity wasn’t found. Thus, we should find more new superconducting materials besides copper oxide and iron-based superconductors.In the two types of HTSCs, the parent compounds both have a tetragonal structure at room temperature and are composed of superconducting layers and charge reservoirs, and some elements, such as La、S、Ba、Ca、Nd, are shared in the charge reservoirs of two types of HTSCs. Moreover, in two types of parent compounds, the lattice parameters in the ab plane are both close to0.38-0.4nm. For these reasons we consider that the crystal cells of the iron pnictide and copper oxide superconductors can be combined, and we intend to design a new class of materials which contain both CuO2layer and Fe2As2layer. If this class of materials were prepared successfully, the CuO2layer and Fe2As2layer will make a synchronous response while the outside environment changes. Then through the research we can get more clear information and data to find and understand the intrinsic properties of HTSCs free from the influence of environment factors, and because of their special structures which contain CuO2layer as well as Fe2As2layer, the new class of materials may have prominent superconductivity with high Tc.In this paper, we designed a new series of candidates for superconducting parent compounds that contain CuO2layer and Fe2As2layer—ALaCuO3FeAs (A=Ca, Sr, Ba), and optimized their geometry structure using Wien2k based on FPLAPW method. Compared the total energy of new materials ALaCuO3FeAs with their isomers and the single-phase materials that made up of the atoms in ALaCuO3FeAs, We found the energy of ALaCuO3FeAs is least of all, which indicates that the crystal structures of ALaCuO3FeAs are stable and they are easy to synthesize. Then we calculated the density of states and energy band structures of the three new materials. The electronic structures of the three materials get from the calculation are similar to iron-based superconductors, and they show that the series of ALaCuO3FeAs are metal and there are three hole-like Fermi surfaces and two electron-like Fermi surfaces respectively along G-Z and M-A directions. On the Fermi energy, the density of states is mainly from the numbers of Fe3d electrons and secondly from Cu3d electrons. In the calculation results, It is suggested that in ALaCuO3FeAs the CuO2layer and Fe2As2layer are conducting layers, and the other atoms make up of charge reservoirs. In Fe2As2layer, there are direct hopping of electrons from Fe atom to Fe atom and indirect hopping from Fe to As to Fe, while in CuO2layer, there is only super-exchange interaction between Cu atoms through O atom. It is indicated that the conductivity and effect of two conducting layers are similar to that in HTSCs, and the series of ALaCuO3FeAs may also be superconducting after doping.Subsequently, we studied the changes in the electronic structure of SrLaCuO3FeAs after hole and electron doped. In the hole doping on La site, holes are brought to CuO2layer which results in the decrease of DOS at Fermi level and the conductivity of CuO2layer is enhanced, which is similar to the situation in La2CuO4with Sr doped. And at the same time, the increase of the N(Ef) of Fe2As2layer indicated Sr(La1-χSrχ)CuO3FeAs is at least another iron-based superconductor. In the electron doping on Sr site, the DOS of Fe3d electrons drops when the DOS of Cu3d electrons remains nearly unchanged, which made the DOS of Fe3d and Cu3d electrons closer to each other and the CuO2layer would conduct with Fe2As2layer together. In the calculation it is showed that whatever hole or electron doping was carried in the series of ALaCuO3FeAs there would be some special features in physical property and the research of this series of potential superconducting parent compounds is valuable.