| The discovery of superconductivity with Tc=26K in LaFeAsO1-xFx in2008attracted fully attention. On the one hand, the iron-based superconductors pro-vide a new approach to the unclear mechanism of high-Tc superconductivity in cuprates. On the other hand, the emergence of high-Tc superconductivity in com-pounds with Fe, which is usually considered as a magnetic element, breaks the normal regulations. Compared with Iron Pnictides, the Iron Chalcogenides with-out toxic dement As have simpler structure and composition, which make them more ideal platform to investigate the underlying physics. Moreover, the Iron Chalcogenides have some different properties from Iron Pnictides. In this paper, the relation between superconductivity, magnetism and microstructure in iron-chalcogenides is investigated through modulating the magnetism and microstruc-ture by methods such as applying high pressure, annealing in different atmosphere and doping elements into Fe site.In the first two chapters, the exploration of superconducting materials and physical properties of iron-chalcogenide superconductors are introduced.The third chapter, the pressure effect on superconductivity of FeTe1-xSx s-ingle crystals. With increasing pressure to10kbar, the superconductivity of homogenous sample is suppressed gradually, however Tc of inhomogeneous sam-ple can be raised up to21K. The increase in Tc is believed as stabilizing the structure by strain effect in inhomogeneous sample, thereby avoiding structural transition under pressure. Our results imply that higher Tc could be expected in iron chalcogenides if this structural instability could be suppressed.The fourth chapter, annealing effects on superconductivity and magnetism in FeTe1-xSx single crystals. It is found that the superconductivity is improved not only by annealing in air but also by annealing in vacuum or argon. In the air annealing case, results show that oxygen is incorporated into the system and the long-range magnetic order is partially suppressed. In contrast, for vacuum or argon annealing, no change of composition is detected and no other ions are incorporated, and the enhancement of superconductivity clearly indicates that the modulation of the microstructure plays an important role in the annealing effect on superconductivity and magnetism.The fifth chapter, single crystals with nominal composition M0.05Fe0.95Te0.8Se0.2(M=Cr, Mn, Co, Ni, Cu, and Zn) have been grown, through which the doping ef-fect on magnetism and superconductivity is studied. Elementary analysis reveals that Cu, Co, and Ni, with smaller ionic radii for valence state2+can substitute effectively for Fe with doping levels near5%. In contrast, the solid solution of Cr, Mn, and Zn in the host system is low. Magnetic and electronic investigation-s show that the substitution of Co, Ni, or Cu for Fe leads to the formation of spin-glass state and suppression of superconductivity. The superconductivity is partly suppressed by Co doping, while completely destroyed by Ni and Cu doping. Compared with Cu-and Ni-doped samples, the Co-doped sample has the smallest lattice constant, indicating that the superconductivity might be also modulated by the changes of microstructure.The sixth chapter, the effects of Co and Cu doping on superconductivity in Fe1+yTe0.6Se0.4single crystals. Resistivity and magnetic susceptibility measure-ments show that the Cu doping suppresses the superconductivity more remark-ably than Co doping, but the Co doping has a stronger influence on the magnetic properties. Heat capacity results reveal that for Co-doped samples a Schottky heat capacity anomaly at low temperatures is observed, but for Cu-doped sam-ples no Schottky anomaly is found. Analysis based on these phenomena suggests that the Co substitution for Fe induces paramagnetic moments, whereas the Cu substitution is non-magnetic. The severe suppression of superconductivity by non-magnetic impurities supports the s±pairing symmetry. |
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