Design,Property And Synthesis Exploration Of The Candidates For High Tc Superconductor: R₄CuO₇Fe₂Se₂?R=Rare Earth Elements?

Since thefirst discovered in 1986,high Tc superconductors?HTS?have attracted great attentions,and the mechanism of high Tc superconductivityhas been one of the most difficult problems in physics.The HTS can be classified into two series:cuprate superconductors and iron pnictide superconductors.Many intensive studies have been carried out on thestructure and electronic propertiesof these two kinds of HTS,but a universaltheoretical system is still missing to explain the abnormal phenomena,so far.If one can put at least two kind of superconducting layers or similar ones intoonematerial,the properties of the two different typical superconducting layers can be studied in the same environment,which can promote the research of the mechanism of HTS.After summarized the results of the characteristics ofthe structures and properties of HTSs in the literature,we designa new series HTS candidates which contains both CuO₂ layer and Fe₂Se₂O layer based on R₂CuO₄ and R₂O₃Fe₂Se₂?R=rare earth element?.Using first-principle calculation method,we compare the ground state energy of the designed R₄CuO₇Fe₂Se₂ with that of R₂CuO₄and R₂O₃Fe₂Se₂ after the geometrical optimization of their structures.The results show that the energy of Pr₄CuO₇Fe₂Se,as an example,is lower than the sum of their two parent materials,indicating that the crystal structures of R₄CuO₇Fe₂Se₂ are stable.We carry out first-principle calculation on the designed Pr₄CuO₇Fe₂Se₂ and its parent compounds,and predict its physical property.Comparing the calculation results in the cases with and without spin polarization as well as with antiferromagnetic structures,we can conclude that the new designed material is more likely an antiferromagnetic structure,which is consistent with our expectation.The calculated local electronic density of states and band structures show that both Cu and Fe bands cross the Fermi surface,just like existing cuprate and Fe-based superconductors.It's a typical multiband structure with a very strong anisotropy,which shows conductivity in CuO₂ layer and Fe₂Se₂O layer.The electronic properties of Pr₄CuO₇Fe₂Se₂is similar with that of Pr₂CuO₄,with the Fe₂Se₂O layer taking effect,too.Furthermore,the active range of CuO₂ layer and that of Fe₂Se₂O layer in energy bands are not quite coincident,which make it possible to investigate the two special layers in one material,under the same condition.Finally,we carry out several necessary experimentsto explore the synthetic process of R₄CuO₇Fe₂Se₂.Both one step reaction method and multistep reaction method have be attempted,but the product of solid state reaction are RFeO₃ and RCuSeO,instead of expected R₄CuO₇Fe₂Se₂.According to the electronegativity of every element in both reactants and products,we deduced that the key of this reaction is the valence of Fe.We should make the reactants reacting in a reducing atmosphere,which can restrain the reductibility of Fe or Fe²⁺,and the reaction could take place inaccordance with our expectations.