Research On Structure And Superconductivity Of High Temperature Superconductors And Magnetic And Transport Properties Of Doped Manganites Sr_1-xCe_xMnO₃

High-temperature superconductivity has attracted much research attention and become a hot topic in condensed matter physics due to the significance in foundation sciences and the potential applications.The cuprates with high-temperature superconductivity show many intriguing physical properties,such as pseudogap, charge ordering,etc.It's urgent to investigate and understand these novel physical phenomena in the strongly correlated electron systems.The cuprates belong to the doped superconductors,in which hole or electron carriers are introduced into CuO₂ plane by element substitution or change of oxygen content.It has a significance of studying the doping effect on the structure and superconductivity of cuprates.Perovskite manganites have been actively studied in view of their special electronic and magnetic properties including colossal magnetoresistance(CMR) as well as the potential applications.In recent years,there has been some interest in studying the properties of electron-doped manganites because both electron-doped manganites and hole-doped manganites may open up very interesting applications in the emerging field of spintronics such as p-n junctions with CMR effect.Additionally, the studies on electron-doped manganites are significant to enrich the physical properties of the phase-separated manganites,and further shed light on understanding the CMR mechanism.The contents in this dissertation are mainly focus on two parts:firstly,we study the doping effects on the structure and superconductivity of high-temperature superconductors Bi₂Sr₂Ca₂Cu₃O_z(Bi-2223) and La₃CaBa₃Cu₇O_y(La-3137) systems, then the magnetic and transport properties of the electron-doped manganites Sr_1-xCe_xMnO₃.The dissertation is divided into five chapters,and the main contents of each chapter are summarized as follows:In chapter 1,we briefly review the fundamental properties and the research history of superconductors,and make a brief introduction to the structural characteristics and superconducting mechanism of high-T_c superconductors Bi₂Sr₂Ca₂Cu₃O_z(Bi-2223) and La₃CaBa₃Cu₇O_y(La-3137).We also briefly review the research history of magnetoresistance and the related physical properties of colossal magnetoresistance(CMR) manganites.In chapter 2,the effect of double doping with equivalent substitution levels of K⁺ for Sr²⁺ and Nd³⁺ for Ca²⁺ and that of single doping for the series of Bi_1.8Pb0.4Sr_2-xK_xCa_2-yNd_yCu₃O_z(x=0,0.01,0.02,y=0,0.01,0.02) were investigated. The structure and physical properties were studied by the measurements of the x-ray diffraction(XRD) technique,field emission scanning electronic microscopy (FE-SEM),resistivity,magnetization and Raman spectrum.The room temperature XRD measurements indicate that the Bi-2223 phase is the dominant phase and also that the Bi-2212 phase can always be detected as a minor phase.The fitting of XRD using the standard Rietveld technique indicates that the samples can be indexed by a tetragonal lattice with space group I4/mmm.The superconducting transition temperature T_c^zero of the samples varies from 99 to 110 K for different doping.The superconductivity and the critical current density(J_c) of the sample single doped with K are improved compared with the undoped one,which is mainly due to the improvement of grain boundary links.In chapter 3,we mainly discussed the structure and superconductivity of La₃Ca_1-xSr_xBa₃Cu₇O_y(0≤x≤1.0) system.Structural and superconducting properties were studied by XRD technique,resistivity,magnetization,FE-SEM and Raman measurements.Sr can replace up to 100%of Ca ions without the formation of any impurity phases based on the analysis of XRD measurement.Resistivity and magnetization measurements indicate that both the superconducting transition temperature T_c and the critical current density J_c(H) decrease monotonously with Sr content.The difference in ionic radius between Sr²⁺ and Ca²⁺ is believed to be responsible for the structural changes and suppression of superconductivity through the introduction of oxygen vacancies and disorder when Sr replaces Ca.In chapter 4,the structural,magnetic,electrical properties of Sr_1-xCe_xMnO₃ (0.05≤x≤0.30) system were studied.The XRD results indicate that the partial substitution of Ce for Sr in SrMnO₃ stabilizes the perovskite-type structure.The results of magnetic and transport measurements show the potential existence of an intrinsic magnetic inhomogeneity in the electron-doped system(the antiferromagnetic insulator AFI and ferromagnetic metal FMM phases coexist),i.e.phase separation, which is due to the A-site doping disorder effect.Ce substitution for Sr induces the A-site ion disorder distribution and enhances the fluctuation of antiferromagnetic insulating(AFI) and ferromagnetic metallic(FMM) phases.Finally,the brief summary and prospect of this dissertation are given in chapter 5.