Design, Properties Prediction And Synthesis Exploraion Of La-Cu-O-Fe-As: Candidates For High Tc Superconductors

Currently, research on new high-temperature superconductors become increasinglyimportant in the field of condensed matter physics. Since2008, the F-doped LaOFeAswas found, which inspired people’s great interest for the study of newsuperconducting materials. The iron-based superconductor, which is the onlymaterials to compare with the copper oxide high-temperature superconductors, is ofgreat similarities with copper oxide superconductors. Comparing them would bring ussome new ideas in the study of superconductors. Moreover, it will bring us muchmeaningful evidence, but the most essential issue on superconductivity mechanismhave not been solved today. Therefore, more new superconductors should be found tohelp us observe superconductivity in order to find more meaningful evidence.There are many similarities between the iron-based superconductor and the copperoxide superconductor. They both have a tetragonal structure at room temperature, andthe lattice parameters in the ab plane are very close to each other. Moreover, they areboth composed of superconducting layers and charge reservoirs and the elements ofcharge reservoirs such as La, Nd, Sr are similar to each other, which make it possibleto synthesis the new superconductor with both Fe2As2layers and CuO2layers in it.However, there are many differences on reactants, temperature, atmosphere, doping inthe process of synthesis them, which mike it difficult to compare them in a samecondition. Since there are many factors could influence the synthesis of these twokinds of superconductors, they cannot be compared directly in the qualities andproperties of them. To solve this problem, we propose a new idea to design a newkind of superconductor with both Fe2As2layers and CuO2layers. Then， whenexternal conditions change, it can still response with both layers. Therefore, it willsignificantly benefit the study of the issue on superconductivity mechanism.In this paper, a new series of candidates for superconducting parent compounds thatcontain both Fe2As2layers and CuO2layers were designed, and the geometry structurewas optimized using the first principle study. The results show the total energy of newmaterials La-Cu-O-Fe-As is lower than that of the isomers and the single-phasematerials that made of the atoms in it, which indicates that the crystal structures ofLa-Cu-O-Fe-As are stable and they are likely to synthesized. Then, the density ofstates and energy band structures of the new materials were calculated based on the first principle study method. The results show that they are all metal and theelectronic structures of them are similar to each other, and they are also similar withthat of iron-based superconductors. The density of states is mainly from the numbersof Fe3d electrons and secondly from Cu3d electrons. Meanwhile, these three newmaterials all have weak interactions between different layers, which indicate that theyhave obvious stratified structure. The results show that both Fe2As2layers and CuO2layers are conducting layers, and the conductivity of Fe2As2layers is better than thatof CuO2layers. LaO layers make up of charge reservoirs. It is indicated that theconductivity and effect of two conducting layers are similar to that in HTSCs, and theseries of La-Cu-O-Fe-As may also be superconducting after doping. These newmaterials may have both two kinds of superconducting layers and their propertiesmay be more similar to iron-based superconductors.Subsequently, we try to synthesis the new series of superconductors using solid statereaction method, and we expected to get the reaction temperature points by means ofdifferential thermal analysis, and we analysis the structure of the materials by meansof X-ray diffraction patterns method. The X-ray diffraction results show that wesucceed to synthesis the precursors of FeAs and LaAs as well as the high-temperaturesuperconductors parent materials LaOFeAs and La2CuO4. When doping Sr on La site,we succeed to got a Sr-doped La2CuO4superconductor and its Tc is above30K.Considering La as rare earth element has active chemical properties and is easy tointroduce LaH3and La(OH)3,etc, we propose a new method to synthesis “1111”-typeiron-based superconductor LaOFeAs, which can significantly improve the purity ofsamples and increase the efficiency of the whole process. After synthesising the newmaterials for many times, we focused on the study of using LaOFeAs and La2CuO4asthe initial reaction materials. We try to change all the reaction conditions includingtemperature, reaction time, reaction atmosphere, and compare the results underdifferent conditions. All the influence of reaction conditions was compared andstudied, which is valuable as a preparation work in exploring the synthesis method ofnew superconducting materials. Noteworthily, there is a strong peak that is anunknown peak when we study the X-ray diffraction results of the new La-Cu-O-Fe-Asmaterials, and we need to determine its exact exact material composition in the nextwork.