| The exotic phenomenon of superconductivity has attracted great interest of physicists since it was discovered in 1911. From then on, physicists began a long journey to find higher Tc superconductors, until maybe one day in the future the room-temperature superconductors are found. People can imagine what the daily life will be at that time. Great amount of energy will be saved since there is no energy loss in the transport of electricity through a superconductor.During the last 100 years, scientists have made great approach about superconductors which are mainly focused on two topics:unveiling the mechanism of superconductivity and trying to find the higher Tc superconductors. In 1957, BCS theory gave the first microscopic explanation of the superconductivity in the materials which are later named conventional superconductors, and also predicted the highest Tc limit, which is only a little higher than 30 K. People were depressed about the results. Until nearly 20 years later, the found of cuprate superconductors brought their confidence back again. In 2008, another 20 years had passed, however, the raising of Tc stopped there and the mechanism of cuprates remained unclear. Just at that time, LaO1-xFxFeAs (Tc= 26 K) was discovered by Japanese scientists and the so-called iron-based superconductors started a new era of research.Charge and heat transport properties in ultra-low temperature are sensitive to the superconducting gap and the low-temperature excitations. In this thesis, first of all. an introduction will be given on the research motivation and the theory background, including the research results on the iron-based superconductors especially the 122-system, the theories on charge and heat conductivity measurements in low temperature, and the corresponding experimental facilities. Then, the results of the resistivity and the thermal conductivity measured on the heavily electron-doped and extremity hole-doped samples of 122-system will be reported. They are:1. The in-plane thermal conductivityκof overdoped FeAs-based superconductor BaFe1.7Coo.3As2 (Tc= 8.1 K) single crystal was measured down to 80 mK. In zero field, the residual linear termκ0/T is negligible, suggesting nodeless superconducting gap in the ab plane. In magnetic field, ko/T increases rapidly, very different from that of conventional s-wave superconductors. This anomalousκ0(H)/T may reveal an exotic superconducting gap structure in overdoped BaFe1.7Co0.3As2:the vanishing hole (β) pocket has a much larger gap than the electron (y andδ) pockets which contain most of the carriers. Such an exotic gap structure is an evidence for superconducting state induced by interband interactions, in which the band with the smaller density of states has a larger gap.2. The in-plane resistivity p and thermal conductivityκof the FeAs-based superconductor KFe2As2 single crystal were measured down to 50 mK. We observe non-Fermi-liquid behaviorρ(T)～T1.5 at Hc2= 5T, and the development of a Fermi liquid state withρ(T)～T2 when further increasing the field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field Hc2. In zero field, there is a large residual linear termκ0/T, and the field dependence ofκ0/T mimics that in d-wave cuprate superconductors. This indicates that the superconducting gaps in KFe2As2 have nodes, likely d-wave symmetry. Such a nodal superconductivity is attributed to the antiferromagnetic spin fluctuations near the quantum critical point. |
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