| Since the discovery of high-TC cuprate superconductors in1986, high tempera-ture superconductivity has been a hot issue in condensed matter physics. However, people still do not understand the mechanism of high-TC superconductivity. In early2008,the discovery of high-TC superconductivity in iron-based superconductors at-tracts peoples’ attentions. People could deeply understand the mechanism of high-TC superconductivity by comparing the physical properties of cuprate and iron-based superconductor s.In this dissertation, we systematically investigated the anisotropic in-plane re-sistivity of122iron-based superconductors. We also investigated the electron spin resonance(ESR) in EuFe2-xCoxAs2single crystals. We synthesized the high quality single crystals of AxFe2-ySe2(A=K,Cs)superconductors and then studied their pres-sure effect on superconductivity. Finally, we studied the magnetic phase diagram of Ca1-xSrxCo2As2system and speculated their magnetic structures.The whole dissertation is constructed as following:1. Brief Overview of Iron-Based Superconductors and Other Layer Ma-terials with ThCr2Si2structureIn this chapter, the author reviewed the basic physical properties of iron-based superconductors, including crystalline structure, transport properties, magnetic struc-ture, Fermi surface structure, pairing symmetry and so on. We detailedly introduced the in-plane electron anisotropy of iron-based superconductors and its possible origin. We reviewed the properties of KxFe2-ySe2superconductors and the other intercalated FeSe superconductors. The physical properties of some other layer materials with ThCr2Si2structure were also briefly reviewed. We discussed the collapsed tetragonal (cT) phase and uncollapsed tetragonal (ucT) phase in the materials with ThCr2Si2structure.2. Measurements of the Anisotropic In-Plane Resistivity of Underdoped FeAs-Based Pnictide SuperconductorsIn this chapter, We systematically investigated the in-plane resistivity anisotropy of electron-underdoped EuFe2-xCoxEAs2and BaFe2-xCoxAs2, and hole-underdoped Ba1-xKxFe2As2.Large in-plane resistivity anisotropy was found in the former samples, while tiny in-plane resistivity anisotropy was detected in the latter ones. When it is detected, the anisotropy starts above the structural transition temperature and increases smoothly through it. As the temperature is lowered further, the anisotropy takes a dramatic enhancement through the magnetic transition temperature. We found that the anisotropy is universally tied to the presence of T-linear behavior of resistivity. Our results demonstrate that the nematic state is caused by electronic degrees of freedom, and the microscopic orbital involvement in magnetically ordered state must be fundamentally different between the hole and electron doped materials.3. Transport and magnetic properties of La-doped CaFe2As2In this chapter, we measured the transport properties and susceptibility of sin-gle crystals Ca1-xLaxFe2As2(x=0,0.05,0.1,0.15,0.19and0.25). Large in-plane resistivity anisotropy similar to that in Co-doped122iron-pnictides is observed al-though no transition metals were introduced in the FeAs-plane. The in-plane resistiv-ity anisotropy gradually increases with La doping below TSDW, being different from the hole-doped122superconductors. The susceptibilities of the samples show that La doping leads to suppression of SDW and induces a Curie-Weiss-like behavior at low temperature, which is much stronger than the other122iron-based superconductors.4. The synthesis and physical properties of AxFe2-yCh2(A=K,Rb and Cs,Ch=S,Se) single crystalsIn this chapter, We successfully grew the high-quality single crystals of AxFe2-ySe2(A=K and Cs) by self-flux method. Sharp superconducting transition was observed for both types of crystals. The crystals show the onset superconducting transition temperatures (TC) of31K and30K for K-and Cs-compounds, respectively, with nearly100%superconductive shielding fraction. Much larger upper critical field HC2is inferred from low-temperature iso-magnetic-field magnetoresistance in these crys-tals than in FeSe. The anisotropy Hc2ab(0)/Hc2c(0) is estimated to be around3for both of the two materials. Then, we performed the high hydrostatic pressure resistivity measurements (up to1.7GPa) on the newly discovered superconductors AxFe2-ySe2(A=K and Cs) single crystals. Two batches of single crystals KxFe2-y,Se2with dif-ferent transition temperatures (Tc) were used to study the effect of pressure. The Tc of the first one gradually decreases with increasing pressure from32.6K at ambient pressure. While a dome-like behavior was observed for the crystal with TC=31.1K, and TC reaches its maximum value of32.7K at the pressure of0.48GPa. It indicates that there exists a optimal doping with maximum TC of32.7K in KxFe2-ySe2system. The behavior of TC vs. pressure for CsxFe2-ySe2also shows a dome-like behavior, and Tc reaches its maximum value of31.1K at the pressure of0.82GPa. The hump observed in temperature dependence of resistivity for all the samples tends to shift to high temperature with increasing pressure. The resistivity hump could arise from the vacancy of Fe or Se. In addition, we successfully synthesized two new compounds RbxFe2-yS2and CsxFe2-yS2which were isostructural with KxFe2-ySe2superconduc-tor. We systematically investigated the resistivity, magnetism and thermoelectric power of AxFe2-yS2(A=K, Rb and Cs) single crystals. High temperature resistivity and magnetic measurements show anomalies above500K depending on A which are similar to AxFe2-ySe2. Discrepancy between ZFC and FC curves was observed in KxFe2-yS2and RbxFe2-yS2, while it disappears in CsxFe2-yS2. Our results indicate the similar magnetism between AxFe2-yS2and AxFe2-ySe2at high temperature.5. Electron spin resonance in EuFe2-xCoxAs2single crystalsIn this chapter, the temperature dependence of electron spin resonance (ESR) was studied in EuFe2-xCoxAs2(x=0.0,0.067,0.1,0.2,0.25,0.275,0.285,0.35,0.4and0.5). The ESR spectrum of all the samples indicates that the linewidth strongly depend on the temperature. Moreover, the linewidth shows the Korringa behavior, indicating an exchange coupling between the conduction electrons and the Eu2+ions. The linewidth, g factor and the integrate ESR intensity show anomalies at the tem-perature of the spin-density-wave (SDW). The linewidth below the SDW transition does not rely on the temperature. This gives the evidence of the gap opening at the TSDW-The slope of the linewidth is closely associated to TSDW and TC. This exotic behavior may be related to the nesting of the Fermi surface.6. The magnetic properties and phase diagram of Ca1-xSrxCo2As2single crystalsIn this chapter, we report the magnetic and transport measurements of CaCo2As2and Ca0.9Sr0.1Co2As2single crystals. Antiferromagnetic transition was observed at about70K and90K for CaCo2As2and Ca0.9Sr0.1Co2As2, respectively. Magnetic and magnetoresistance measurements reveal metamagnetic transition from an an-tiferromagnetic state to a ferromagnetic state with the critical field of3.5T and1.5T for these two samples respectively along c-axis at low temperature. For the field applied along ab-plane, spins can be fully polarized above the field of4.5T for . While for CaCo2As2, spins can not be fully polarized up to7T. We proposed the cobalt moments of these two materials should be ordered ferromagnet-ically within the ab-plane but antiferromagnetically along the c-axis(A-type AFM). Then, we report the magnetic phase diagram of Ca1-xSrxCo2As2system by measur-ing the transport and magnetic properties of the single crystals. Its ground state evolves from A-type antiferromagnetism (AFM) with large anisotropy for CaCo2As2to ferromagnetism with spins aligned along c-axis for Ca0.8Sr0.2C02As2. With further increasing the Sr content, the system would become AFM again but with spins aligned along ab-plane. For the Sr content above0.4, the system shows no magnetic order and changes to uncollapsed tetragonal phase. Our results indicate that the magnetic property in this system is strongly correlated with the interlayer spacing. |
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