Exploration Of Superconductivity In The New Fe-based Eu112 System And Several Compounds With Related Structure

Since its discovery in 2008,iron based superconductors reached a great upsurge rapidly.Iron based superconductors mainly include iron pnictides and iron chalcogenides.The parent compounds of the iron arsenides are usually antiferromagnetic at low temperature.Superconductivity will be induced,once the antiferromagnetic order is suppressed through carrier doping or under pressure.The antiferromagnetic order and superconductivity compete with each other in iron arsenides.When superconducting transition temperature reaches the highest value through doping,the Fe-related antiferromagnetic order will be destroyed thoroughly.However,the newly found Ca112-type Fe-based（Ca,La）FeAs₂ superconductors in late 2013 showed a novel phenomenon that superconductivity is accompanied by the antiferromagnetic order in the whole doping region.Ca112-type iron based superconductor crystalizes in an unusual monoclinic crystal structure.The crystal structure can be understood as a variant of CaFe₂As₂ in which one of the FeAs layers is replaced by an independent As layer consisting of tiled zigzag As chains.Superconductivity with Tc⁴7 K was induced in La and Sb co-doping system.Nuclear magnetic resonance results showed that the Fe-related antiferromagnetism co-exists with superconductivity,and even enhanced in the overdoped region when superconductivity is suppressed in Ca_1-xLa_xFeAs₂（x≥0.15）system.This results in a different phase diagram from other systems of iron superconductors.Unfortunately,all of the Ca112-type iron based materials are doped with rare earth metals,and the parent compound CaFeAs₂ is still absent.Research in Ca112-type superconductors is restricted due to this incompleteness in material system.In this thesis,we replaced Ca with Eu in Ca112 system,and discovered the parent compound EuFeAs₂.After plenty of attempts,we finally adopted CsCl as flux and cultivated high-quality EuFeAs₂（Eu112）single crystals.We picked a 100μm sized crystal for single crystal X-ray diffraction analysis,and the structure of EuFeAs₂ is refined to be orthorhombic with a space group of Imm2.The crystal structure of EuFeAs₂ has changed,compared to that of（Ca,La）FeAs₂ with space group of P2₁/m.While,the independent As plane is still comprised of tiled As zigzag chains.From resistivity,magnetic susceptibility and heat capacity measurements,we proposed that EuFeAs₂ has three phase transitions:the structural phase transition at 106 K,the Fe-related antiferromagnetic phase transition at 93 K,and the Eu-related antiferromagnetic phase transition at 45 K.Hydrostatic pressure up to 2 GPa is used to research on the resistivity under pressure for EuFeAs₂ single crystal.The temperatures of the structural phase transition and the Fe-related antiferromagnetic phase transition rise along with the pressure increasing.However,the temperature of the Eu-related antiferromagnetic phase transition exhibits the opposite behavior.No sign of superconductivity is observed under the pressure of 2.11 GPa.To induce superconductivity in Eu112 system and to seek the origin of superconductivity,we conducted a series of doping research on the parent compound EuFeAs₂.Through La-doping in the Eu site,both of the antiferromagnetic orders are suppressed to some extent,but not destroyed even in over-doped area.Superconductivity is completely accompanied by both antiferromagnetic orders,which results in a novel phase diagram.We also prepared a series of Co-doped samples to study the evolution of Fe-related antiferromagnetic phase transition.Substitution 3%of Fe by Co suppresses the Fe-related antiferromagnetic order significantly,and induces superconductivity with a Tc¹3 K.With doping level increases to 10%,Tc increases to 28 K.La-doping in the Eu site can not destroy the Fe-related antiferromagnetic order thoroughly,and induces superconductivity with lower Tc.While,Co-doping in the Fe site suppresses the Fe-related antiferromagnetic order quickly,and induces superconductivity with higher Tc.Two different doping effects show a suppression of superconductivity from the Fe-related antiferromagnetic order,which is consistent with the competition between superconductivity and antiferromagnetic order in other systems of iron based superconductors.We also researched on the Sm-doping effect in Ca₁₀Pt_nAs₈（Fe₂As₂）₅（n=3,4）,whichhave a similar structure with 112 system.For n=3,the undoped sample shows superconductivity with Tc¹3 K,which can be enhanced up to 22 K through Sm-doping;while for n=4,the undoped sample shows superconductivity with Tc²2K,and suppressed to 12 K through Sm-doping.In addition,we also conducted some intercalation research on the layered materials with Van Der Waals interlayer interaction.Taking Sr₂CuO₂Cl₂ as an example,we introduce a chemical method of soaking the original materials into organic solution to realize the intercalation of alkali metal.In Sr₂CuO₂Cl₂,Van Der Waals interlayer interaction exists between the Cl double layers.Alkali metal intercalation results in a lattice expansion,supplies electrons for superconducting layer,and induce superconductivity with a Tc⁵ K in Sr₂CuO₂Cl₂ successfully.In conclusion,we synthesized a new iron based parent compound EuFeAs₂,and broke the restriction due to the lack of parent compound in Ca112 system.On this basis,we studied the evolution of the properties of EuFeAs₂ through La-doping and Co-doping,respectively.We found that La-doping in the Eu site can not destroy the Fe-related antiferromagnetic order thoroughly,while Co-doping in the Fe site suppresses the Fe-related antiferromagnetic order quickly,and induces superconductivity with higher Tc.We also researched on Sm-doping effect in Ca₁₀Pt_nAs₈（Fe₂As₂）₅（n=3,4）.For n=3,superconductivity is enhanced through Sm-doping;for n=4,superconductivity is suppressed through Sm-doping.Finally,we researched on the alkali metal intercalation of Sr₂CuO₂Cl₂ with Van Der Waals interlayer interaction,and induced superconductivity with a Tc⁵ K in Sr₂CuO₂Cl₂successfully.