Doping Effect And Physical Properties Study On The Iron-Arsenic Compounds

Iron based superconductors have widely potential applications and huge value in the future industry owing to its metallicity,relatively large upper critical field(Hc2)and critical current density(Jc).The study on the way and media of its paring provides unprecedented chances to understand the microscopic mechanism of high temperature superconductivity.Thus the research of Fe based superconductors and possible superconducting parent compounds has become one of the hotspots in the material science communities recently,as a result,many new type of Fe based compounds and superconductors have been developed.For example,CaFe4As3 forms infinitely long ribbons along b axis and a rectangular network in the ac plane which is completely different with the parallel stacking of conducting layers in the traditional superconductors and totally undergoes two AFM transitions with temperature decreasing,another distinguished sample is Ca10(Pt4As8)(Fe2As2)5(Ca10-4-8),it uses the Pt4As8 that has Skutterudite structure as intermediary layer and raise the chemical complexity of the superconducting pnictides to a new level.The main contents of the dissertation are listed below:1.Single crystals of CaFe3.64X0.36As3(X = Pt,Ga)were grown using Sn flux,after the phase purity and crystal structure check by X-ray diffraction,we measured the comprehensive physical properties of samples synthesized including the temperature dependant electrical resistivity,magnetoresistance,Hall effect,magnetic susceptibility and heat capacities.Two magnetic transition temperatures remain unchanged upon Pt or Ga doping,as confirmed by specific heat,electrical resistivity and magnetic susceptibility,suggesting the magnetic structure of CaFe4As3 is insusceptible to non-magnetic dopants.The electrical resistivity of CaFe4As3 is reduced by approximately half with Pt dopant but increases by an order of magnitude with Ga doping,consistent with the changes in their Hall coefficients,which indicates the effects of Pt and Ga doping give us a different modification on physical properties and electronic structure in CaFe4As3.2.Ba(Fe1-xRux)2As2(0?x?0.4)single crystals were grown out of FeAs flux and their electrical and magnetic properties were systematically studied?Upper critical field and irreversible field for the superconducting samples were derived and phase diagram was acquired.Through comparison we can find that there exists a relatively large superconducting region in the diagram of Ru doped Ba-122 system which is very similar to another isovalent doping system BaFe2(As1-xPx)2,however the maximum superconducting transition temperature for BaFe2(As1-xRux)2 is lower than that of BaFe2(As1-xPx)2,which could be attributed to much more disorders introduced by Ru into Fe sites.We also measured the longitudinal magnetoresistance for Ba(Fe1-xRux)2As2 samples and compared the measurement result with that from the spin-polarized calculations by Shilpam Sharma.It is showed that the spin fluctuation play an important role in the superconductivity emergence of Fe based superconductors.The longitudinal magnetoresistance measurement is a convenient and practical way to judge whether the superconductivity is caused by AFM or FM spin fluctuation in the iron based system before we use further advanced nuclear test technique such as neutron diffraction method to determine their precise or detailed magnetic structures.3.We synthesized and done physical property measurements of the Ca10(Pt4As8)(Fe2As2)5 single crystals and find that although the intermediary layer Pt4Asg is metallic,there still has a remarkable behaviour disparity in the temperature dependence of the in-plane(?ab)and out-of-plane(?c)resistivity and a large anisotropy in ?ab and ?c of Ca10(Pt4As8)(Fe2As2)5 which could be attributed to a strongly anisotropic 2D electronic structures and the existence of a certain amount of stacking faults inside the crystal.The out-of-plane semiconducting-like resistivity behavior and highly anisotropic property along different direction of crystal are very common to see in the hole-type high-Tc cuprates but never encountered in Fe based superconductors before.Thermal conductivity measurements on Ca10(Pt4As8)(Fe2As2)5 show it is a low-charge carrier density system which are consistent with Hall effect measurements results,also the heat in Ca10(Pt4As8)(Fe2As2)5 is mainly carried by phonons,electron's contribution can be negligible.At last we proposed that layered structure with large electrical transport anisotropy of Ca10(Pt4As8)(Fe2As2)5 rarely seen among other Fe-based superconductors known by far implies it may be a new good candidate and have potential perspective application on the fabrication of high frequency microelectronic device such as next generation intrinsic Josephson junctions.