Superconductivity In Iron-based Superconductors Tb_1-xTh_xFeAsO And BaFe_2-xM_xAs₂(M=Co,Ni)

Since the discovery of high-Tc superconducting cuprates, iron-based superconductor is a new type of high Tc superconductors which also containing transition metals. Because of the similarities to the cuprates in layered structure, electronic phase diagram, and very high critical transition temperature, they have attracted lots of attention and become the focus of research since last year. Exploring new iron-based superconductors and achieving higher critical transition temperature Tc are two important topics. Element substituting on different sites in iron-based compounds is an effective way to induce carrier doping and thus induce superconductivity with even higher Tc. Meanwhile, investigating the doping effect in different sites and establishing the electronic phase diagram is helpful for understanding mechanism of superconductivity.We have successfully fabricated single phase TbFeAsO parent compound under ambient pressure and reported its transport and magnetic properties for the first time. Then we discovered superconductivity with Tc=52.5 K in Th4+doped Tb1-xThxFeAsO which was also fabricated under ambient pressure. In contrast, F doped TbFeAsO1-xFx superconductors which can be synthesized only under high pressure have Tc around 46 K. Furthermore, we found out that the Fe-As-Fe angle was close to the ideal tetrahedron angle 109.47°in our samples. With increasing Th4+doping, the absolute value of thermopower increases and reaches an maximal value of 100 uV/K. We concluded that there is a close relationship between the enhancement of thermopower and superconductivity.We have successfully grown 122 phase single crystals of BaFe2As2 and its cobalt or nickel doped single crystals by self flux method. In contrast, to the doping in CuO2 layer in cuprates, doping of cobalt or nickel in Fe2As2 layer also induce superconductivity. We firstly reported superconductivity with Tc=20.5 K in nickel doped BaFe2-xNixAs2 crystals and its phase diagram. SDW and/or structural transition is remarkably suppressed by Co/Ni doping and superconductivity emerges. SDW coexists with superconductivity in underdoped region. The fact that the optimal doping content of Ni is only about half of that of Co can be understand by that each Ni dopant induces two extra itinerant electrons while each Co dopant induces only one extra itinerant electron. The study of thermopower confirm again that there is a close relationship between the enhancement of thermopower and superconductivity. Finally, we investigated properties of charge density wave (CDW) in 2H-NbSe2 single crystals and properties of Fe intercalated 2H-NbSe2 single crystals. Through exploring transport properties of samples with different quality, we found that the strong temperature dependent behavior below 60 K and sign change at 25 K of Hall coefficients could be explained by the change of electronic mean free path caused by the occurrence of charge density wave. This explanation is in accordance with the theory of Van Hove singularity driven CDW. We also explored the ground states of Fe intercalated FexNbSe2 crystals. Spin glass state was found and characterized by a.c. magnetic susceptibility measurements in Fe0.15NbSe2 crystals while in higher Fe content samples antiferromagnetic states were discovered. Phase diagram of ground states varying with Fe content was given.